Palladium Nanoparticle Loaded Bifunctional Silica Hybrid Material: Preparation and Applications as Catalyst in Hydrogenation Reactions

Article abstract of DOI:10.1002/chem.201604508

Bifunctional mesoporous silica was prepared by co-condensation of tetraethyl orthosilicate (TEOS) with functionalized organosilanes containing azides or alkoxyamines. Orthogonal functional groups at the particles were selectively addressed in subsequent chemical modifications through “click”-chemistry (“click to ligand” strategy) and radical nitroxide exchange. Palladation with PdCl₂ delivered Pd nanoparticle-loaded silica material bearing sulfoxides and additional aminoamides as stabilizing ligands by means of in situ reduction of the Pd^II-salt. These functional particles were successfully applied to the hydrogenation of alkynes and alkenes. Aldehyde hydrodeoxygenation and benzyl ether cleavage were achieved with these hybrid catalysts under mild conditions. Particles were analyzed by IR, TEM/STEM, EDX, and solid-state NMR spectroscopy.

Full text of DOI:10.1002/chem.201604508

DOI: 10.1002/chem.201604508
Full Paper
&
Palladium Nanoparticles
Palladium Nanoparticle Loaded Bifunctional Silica Hybrid
Material: Preparation and Applications as Catalyst in
Hydrogenation Reactions
Sabrina K. Surmiak,[a] Carsten Doerenkamp,[b] Philipp Selter,[b] Martin Peterlechner,[c]
Andreas H. Schꢀfer,[d] Hellmut Eckert,*[b, e] and Armido Studer*[a]
Abstract: Bifunctional mesoporous silica was prepared by
co-condensation of tetraethyl orthosilicate (TEOS) with func-
tionalized organosilanes containing azides or alkoxyamines.
Orthogonal functional groups at the particles were selective-
ly addressed in subsequent chemical modifications through
“click”-chemistry (“click to ligand” strategy) and radical nitro-
xide exchange. Palladation with PdCl2 delivered Pd nanopar-
ticle-loaded silica material bearing sulfoxides and additional
aminoamides as stabilizing ligands by means of in situ re-
duction of the PdII-salt. These functional particles were suc-
cessfully applied to the hydrogenation of alkynes and al-
kenes. Aldehyde hydrodeoxygenation and benzyl ether
cleavage were achieved with these hybrid catalysts under
mild conditions. Particles were analyzed by IR, TEM/STEM,
EDX, and solid-state NMR spectroscopy.
Introduction
introduced a “late-stage modification” strategy for the prepara-
tion of small libraries of hybrid catalyst systems. Different or-
thogonal functionalities were incorporated into silica “mother”-
particles that were subsequently orthogonally addressed for
the synthesis of bifunctional inorganic–organic hybrid materi-
als (Scheme 1).[11] We have also shown that (bi-)pyridyltriazole
ligands can be readily conjugated to the silica surface next to
basic amine functionalities using a “click to ligand” approach.
The chemically modified silica material has been successfully
used to build up bifunctional heterogeneous palladium cata-
lyst systems applicable to the Tsuji–Trost and the Suzuki reac-
tion.[12,13] Activity of the hybrid catalysts could be tuned as
a function of the basic additive covalently bound to the sur-
face.
The application of functionalized mesoporous silica in acid–
base- as well as in metal-catalyzed processes has recently
gained increased attention in organic synthesis.[1–4] In the field
of metal catalysis, these materials proved to be ideal supports
for immobilization of various metals and metal catalysts. This is
due to the large surface area and well-defined structure of
silica materials.[5,6] By applying co-condensation[7,8] or post-syn-
thetic grafting[9] methods it is now possible to introduce li-
gands and additives selectively to the silica surface, thereby
creating valuable multifunctional catalyst systems. Along these
lines, “click”-chemistry such as, for example, the 1,3-dipolar
alkyne/azide cycloaddition, has become a powerful tool for
chemical surface modification.[10] In previous studies we have
[a] S. K. Surmiak, Prof. Dr. A. Studer
Westfꢀlische Wilhelms Universitꢀt, Organisch-Chemisches Institut
Corrensstrasse 40, 48149 Mꢁnster (Germany)
Fax: (+49)251-833-6523
E-mail: Studer@uni-muenster.de
[b] C. Doerenkamp, P. Selter, Prof. Dr. H. Eckert
Westfꢀlische Wilhelms Universitꢀt, Physikalisch-Chemisches Institut
Corrensstrasse 28/30, 48149 Mꢁnster (Germany)
E-mail: Eckerth@uni-muenster.de
Scheme 1. Synthesis of silica “mother”-particles followed by orthogonal
chemical modification for the preparation of bifunctional silica supported
catalysts.
[c] Dr. M. Peterlechner
Westfꢀlische Wilhelms Universitꢀt Institut fꢁr Materialphysik
Wilhelm-Klemm-Str. 10, 48149 Mꢁnster (Germany)
[d] Dr. A. H. Schꢀfer
nanoAnalytics GmbH, Heisenbergstraße 11, 48149 Mꢁnster (Germany)
The Lindlar catalyst[14] is generally used for semihydrogena-
tion of internal alkynes to obtain the sterically more demand-
ing Z alkenes. However, the use of toxic lead and the need to
suppress over-hydrogenation are serious drawbacks of this
method. Recently, Copꢁret et al. disclosed NHC-palladium com-
[e] Prof. Dr. H. Eckert
Instituto de Fꢂsica em Sao Paulo, Universidade de Sao Paulo
Av. Trabalhador Saocarlense 400, Sao Carlos, S.P. 13560-590 (Brazil)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201604508.
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plexes immobilized on mesoporous silica for selective alkyne
semihydrogenation.[15] Kaneda et al. utilized core-shell struc-
tured nanocomposites, comprising a silica-supported palladi-
um nanoparticle core surrounded by an organic alkyl sulfoxide
network, for alkyne hydrogenation.[16] In this respect it is
known that sulfoxide ligands incorporated in hybrid materials
are able to strongly coordinate palladium, and semihydrogena-
tion of alkynes is feasible because bulky sulfoxide ligands at Pd
efficiently suppress coordination of alkenes to the metal,
whereas interaction with the sterically less demanding alkynes
is possible. Additionally, the size of the Pd nanoparticles seems
to influence the semihydrogenation selectivity.[17a] Lipshutz
showed that a palladium nanoparticle–nanomicelle combina-
tion is a very valuable catalyst system for the semihydrogena-
tion of various alkynes in water.[17b] Boron nitride as well as
iron oxide materials were also found to be efficient supports
for palladium nanoparticle-based semihydrogenation reac-
tions.[18,19] However, the design of efficient catalysts that sup-
press over-hydrogenation and provide high Z/E-selectivity in
alkyne semihydrogenation is still challenging.[20,21]
(energy-dispersive X-ray spectroscopy), and XPS (X-ray photo-
electron spectroscopy) analysis, and results on the use of these
novel materials as efficient hydrogenation catalysts are report-
ed.
Results and Discussion
Preparation and analysis of palladium nanoparticle hybrids
“Mother”-particle 1 was prepared according to our previously
published procedure by reacting tetraethoxysilane (TEOS) with
an organotriethoxysilane bearing an azide functionality and
a second organotriethoxysilane conjugated with an alkoxya-
mine moiety (see the Supporting Information).[10]
The mesoporous silica 1 was further functionalized through
dipolar alkyne/azide cycloaddition.[10] To this end, racemic sulf-
oxides 6a and 6b were prepared (see the Supporting Informa-
tion). “Click to ligand” reaction[22] of these alkynes with parti-
cles 1 was readily achieved by using typical conditions for on-
surface copper-catalyzed dipolar alkyne/azide cycloaddition
(CuSO4, EtOH/H2O, 908C) to provide mesoporous silica particles
7 and 8 (Scheme 3). Reactions, which occurred quantitatively,
In this full paper we report the synthesis of bifunctional
mesoporous silica hybrid materials 2–5 bearing sulfoxide li-
gands. They are able to form complexes with Pd nanoparticles,
possess basic aminoamide functionalities, and were synthe-
sized through late-stage chemical modification of “mother”-
particle 1 and subsequent Pd nanoparticle (PdNP) synthesis at
the organic–inorganic hybrid material (Scheme 2). The hybrid
materials were characterized by solid-state NMR, IR, TEM, EDX
Scheme 3. “Click to ligand” reaction.
were readily monitored by IR spectroscopy (see below). Amino-
amide functionalities were introduced by thermal radical nitro-
xide exchange reactions of the TEMPO (2,2,6,6-tetramethylpi-
peridin-1-yl)oxyl) moiety in 7 and 8 by using functionalized ni-
troxides 9a and 9b.[23] Reactions were performed by suspend-
ing 7 or 8 in dichloroethane (DCE) in the presence of an
excess of the nitroxide 9a or 9b followed by heating to 1258C
for 24 h to provide bifunctional mesoporous silica particles 10–
13 (Scheme 4). Finally, these sulfoxide-conjugated particles
were suspended in EtOH/water. An excess of palladium(II) chlo-
ride was added, and the suspension was stirred at room tem-
perature for two hours. Silica particles 10–13, bearing immobi-
lized Pd nanoparticles (PdNP), were eventually isolated by fil-
tration followed by several washing cycles with MeOH, di-
chloromethane, and Et2O as solvents. The amine functionalities
at the particles and/or ethanol can act as reducing reagents
for generation of the PdNP from the PdII-salt.
Scheme 2. Schematic presentation of palladium nanoparticle catalysts 2–5
stabilized by different organic functionalities that are introduced through
late-stage chemical modification of 1.
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We further analyzed the structure of the hybrid particles by
1
using H, 13C, and 29Si solid-state NMR spectroscopy. For exam-
1
ple, each step of the synthesis was examined by H MAS-NMR
spectroscopy from 1 on the way to catalyst 4 (Figure 2). As the
spectrum of the “mother”-particles looks rather complex and
could not be assigned completely (possibly containing signals
from surface water, solvent, and residual silanol groups), we
focus on the changes introduced by further functionalization
1
of the “mother”-particles. Overall, the H MAS-NMR linewidths
are greatly increased, suggesting that the functionalization
strongly reduces the molecular mobility of the surface species,
leading to pronounced dipolar broadening. In the spectrum of
the sulfoxide-functionalized material 8 obtained after azide/
alkyne click reaction, the broad resonance in the aromatic
region (7.3 ppm) is greatly increased in intensity, confirming
that the attachment of the naphthaline residue was successful.
Scheme 4. Radical nitroxide exchange reaction with subsequent palladation.
In order to prove successful functionalization of the inorgan-
ic–organic hybrid silica material, IR- and MAS NMR-spectra
were recorded. The complete set of spectra is depicted in the
Supporting Information. IR measurements were performed on
all systems after each chemical functionalization step. Success-
ful alkyne/azide cycloaddition on particles 1 resulted in com-
plete loss of the characteristic azide band at around
2100 cmÀ1, showing that cycloaddition occurred in a quantita-
tive yield within the error limit of IR spectroscopy. This is exem-
plarily shown for particles 8 in Figure 1 (see inset). As expect-
ed, nitroxide exchange and palladation did not reveal any sig-
nificant characteristic changes on the spectra, as shown for the
IR spectra of particles 10 and 2 in Figure 1.
Figure 2. 1H MAS-NMR spectra of a) palladated particles 4; b) sulfoxide-
amine functionalized particles 12; c) sulfoxide functionalized particles 8;
d) “mother”-particles 1.
Figure 1. IR spectra of particles 1, 8, 10, and 2.
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In addition, a new signal near 2.7 ppm is observed, which may
be assigned to the methyl group next to the sulfoxide S-atom.
Nitroxide exchange to give 12 and also the palladation step to
provide 4 did not significantly alter the H MAS-NMR spectra as
expected.
cles 12 is also clearly detected by a broadened signal near
165 ppm corresponding to the amide carbonyl species. Finally,
the ultimate palladation step indicates no further obvious
changes in the NMR spectra, aside from a general loss in reso-
lution, which indicates an overall increase of disorder. The ab-
sence of chemical shift changes observed for any of these res-
onances suggests that the Pd2+ species is not coordinated to
any particular part of the molecule. This conclusion is consis-
tent with the formation of dispersed metallic Pd particles with-
out any specific coordination.
1
13C CP/MAS-NMR spectra were also recorded for the particles
1, 8, 12, and 4 (see below, Figure 4). For comparison, the
13C NMR signals of the functionalized triethoxysilanes used for
preparation of the “mother”-particles and their assignments
are summarized in Table 1. Detailed peak deconvolutions and
assignments are given in the Supporting Information. All the
predicted characteristic resonances can be observed in the
spectrum of “mother”-particles 1. The spectrum suggests that
the ratio of azide to alkoxyamine residues deviates from the
1:1 ratio intended in the synthesis. This result is also reflected
in the chemical analysis data: The experimental C/N ratio indi-
cates that the molar azide to alkoxyamine residue ratio is
about 4:1, which is consistent with the intensity ratio observed
in the CP/MAS-NMR spectrum. Sulfoxide functionalization
through azide/alkyne click reaction to yield particle 8 results in
a disappearance of the 52 ppm signal caused by the C-atom in
a-position to the azide moiety. Furthermore, the signals of the
triazole ring (60 and 72 ppm) and of the sulfur-bound methyl
group (42 ppm) are clearly identified. In addition, there is a sig-
nificant increase in the signal area observed in the aromatic
region, as expected from the presence of the naphthyl group.
The aminoamide functionalization of particles 8 to yield parti-
Figure 3. a) 29Si MAS-NMR spectrum of “mother”-particle 1.
In addition, solid-state 29Si NMR spectra for particles 1 were
recorded in order to characterize the silica scaffold (Figure 3).
Five signals were observed in the spectrum that can be as-
signed to different Si groups in the material. The Q4 group
[Si(OSi)4] shows a resonance at around À109 ppm, the Q3
group [Si(OSi)3OH] at À101 ppm. Additionally, a small reso-
nance for the Q2 [Si(OSi)2(OH)2] group is observed at À91 ppm.
The T3 [Si(OSi)3R] and T2 [Si(OSi)2OHR] groups appear at À65
and À57 ppm, respectively. They form 4 (T3) and 8% (T2) of the
material, respectively, and their low intensity is consistent with
the eightfold excess of TEOS over organotriethoxysilane used
in the synthesis of “mother”-particle 1.
Table 1. Chemical shifts of the 13C NMR signals and their assignments of
the parent alkoxysilanes.
Signal assignment
[ppm]
12
b, b’
16
a, a’
19
o
22
p, l
29
c, k, n, c’
39
n
52
m, d’
Based on the NMR results we conclude that the PdCl2 added
in the palladation step is not ligated to the sulfoxide and
amine ligands, and hence is not present as surface bound PdII-
complexes. Rather, the PdCl2 gets reduced to palladium nano-
particles (PdNP) under the applied conditions, as also indicated
by the black color observed for the catalyst systems. To ad-
dress this point and to measure the size of these particles, we
performed TEM analysis on particles 2–5 (Figure 6). For com-
parison, TEM images were also recorded on “mother”-particles
1 (Figure 5, particles dispersed on TEM carbon substrate). The
“mother”-particles 1 appear as spherical structures and display
a relatively homogeneous size distribution with a mean diame-
ter of around 285 nm (see also Figure 5b for the size distribu-
tion).
72
93
129
136
139
d, e
j
g, h
f
i
The TEM-images of the palladated particles (Figure 6) reveal
the presence of palladium nanoparticles (PdNPs) adsorbed to
the surface of the spherical particles, confirming that the
added PdCl2 is reduced during the palladation step. For cata-
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Figure 7. a) particles 2, bulky sulfoxide–small amide; b) particle 4, small sulf-
oxide–small amide; c) particle 5, small sulfoxide–bulky amide; d) particles 3,
bulky sulfoxide–bulky amide.
lyst 2, bearing the bulky isopropyl sulfoxide ligand, the mean
diameter of Pd particles formed is around 40 nm (Figures 6a
and 7a). These PdNPs are well dispersed at the SiO2 surface,
and they were formed with a rather narrow size distribution
(30–55 nm, Figure 7a). Individual (unbound) Pd nanoparticles
or Pd agglomerates were not observed. All PdNPs identified
were adsorbed to the silica surface. This observation supports
our assumption that the PdII precursor coordinates to the
ligand at the surface and is subsequently reduced to Pd0,
thereby initiating NP growth at the surface. Therefore, we as-
sumed that the steric demand of the ligands at the hybrid ma-
terial should influence the palladium nanoparticle growth and
their stabilization, and therefore size and distribution might
change as a function of the surface-bound ligands. This in turn
might also change their catalytic activity (see below).[24] More-
over, along with ethanol the ligands can actively participate in
the Pd-salt reduction step. To discuss that point, the different
ligand couples used in this study were categorized based on
their steric demand with respect to the interaction with metal
atoms. Particles 2 are derived from a material that contains
bulky sulfoxide ligands along with small amides. Accordingly,
particles 3 are composed of bulky sulfoxide moieties and bulky
amides; particles 4 contain small sulfoxides and also small
amides; particles 5 are conjugated with small sulfoxides and
bulky amides.
Figure 4. 13C{1H} CP/MAS-NMR spectra of a) “mother”-particle 1; b) sulfoxide
modified particle 8; c) sulfoxide-amine functionalized particles 12; d) palla-
dated particle 4. Spinning sidebands are indicated by asterisks.
Figure 5. a) TEM bright-field image of an ensemble of “mother”-particles 1;
b) size distribution of particles 1.
By comparing the size and shape of PdNPs in hybrids 4
(small sulfoxide, small amide) and 2 (bulky sulfoxide, small
amide), it becomes apparent that the PdNP size decreases as
the steric demand of the sulfoxide ligand is decreased (Figure-
s 6a,b and 7a,b). The mean diameter for the PdNPs in 4 lies at
around 30 nm and size distribution is slightly broader. The size
of the amide ligand apparently also influences the PdNP
growth. The smallest PdNPs were formed on hybrids 3 bearing
bulky amine and bulky sulfoxide ligands (Figures 6d and 7d).
For system 5 we noted a bimodal distribution (Figures 6c and
7c).
In order to further evaluate the impact of the ligands on NP
formation, unfunctionalized silica and “mother”-particles
1 were tested for PdNP formation. Inorganic silica particles and
1 were palladated under the identical conditions used for pal-
ladation of the bifunctional hybrid systems. Compared to the
chemically postmodified materials, the palladated “mother”-
Figure 6. TEM bright field images of a) particles 2; b) particles 4; c) particles
5; d) particles 3; e) Pd@“mother”-particles, Pd agglomerate (left arrow)/
“mother”-particle without Pd (right arrow); f) Pd agglomerate (left arrow)/
SiO2 (right arrow).
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particles, lacking the amine and sulfoxide moieties (Pd@1, Fig-
ure 6e), and also the palladium-loaded pure silica (Pd/SiO2, Fig-
ure 6 f), revealed a high tendency to form Pd agglomerates.
Only very few surface-bound PdNP were found on Pd@1. In
the case of the Pd/SiO2 sample, only isolated Pd agglomerates
were observed, showing the importance of the amine and sulf-
oxide ligands for controlled growth and stabilization of PdNPs
at the silica surface. We also studied the different particles by
EDX, as exemplarily presented for hybrid 2 and Pd@SiO2. EDX
mapping of Pd by means of the Pd-L2,3 (3330 and 3173 eV)
edges showed that for system 2 the well-dispersed PdNPs are
located at the surface of the SiO2 core particles (Figure 8), in
agreement with the TEM bright-field images discussed above.
blocked during the palladation process by coordination of the
Pd species and subsequent particle growth of up to 40 nm at
the pore entrances. This finding is in agreement with the TEM
images and EDX analysis of particles 4, which show that the
palladium nanoparticles are found at the surface and not
inside the core of the silica particles.
Hydrogenation of 4-ethynylbenzaldehyde
We continued our investigations by using the novel hybrids as
catalysts for various hydrogenation reactions. As a first sub-
strate, we tested hydrogenation of 4-ethynylbenzaldehyde
with catalysts 2–5. Initial reductions were conducted at 2 bar
H2 pressure in a closed over, pressure-resistant Schlenk tube at
408C by using 1 mol% of the catalyst in dichloromethane
(DCM). Pd loading in the hybrid material was estimated based
on elemental analysis and XPS analysis (see the Supporting In-
formation). With catalyst 2 after 24 h reaction time, we identi-
fied the semihydrogenation product 14 (67%) as a major com-
pound, along with 4-ethylbenzaldehyde 15 (Table 2, entry 1).
Selectivity towards formation of 14 was increased upon reduc-
ing reaction time to 6 h (entry 2). Catalyst 3, containing the
smallest PdNPs, showed a lower activity and a higher selectivi-
ty towards formation of the semihydrogenation product 14
Figure 8. EDX mapping: HAADF image of 2 with NPs (left); corresponding
EDX map using Pd-L edges (right, scale in counts over background).
The chemical sensitive HAADF (high-angle annular dark-field)
image shows bright contrast for the NPs compared to the
“mother”-particle, in agreement with the EDX data showing
the heavy element Pd on these surface particles. In contrast,
for the Pd/SiO2 particles, Pd was not bound to the SiO2 core
material. Instead, it is found next to the SiO2 core as agglomer-
ated Pd (see the Supporting Information). Moreover, the over-
all amount of generated Pd was smaller in the latter case. In
order to determine the oxidation state of the Pd present in the
different samples, XPS was carried out. Comparison of the re-
sults with reference spectra of PdO, Pd0 in the presence of
DMSO, and related literature indicates that the Pd on the surfa-
ces of the NPs appears in a mixture of oxidation states Pd0 and
PdII. For all systems, the presence of Pd0 on the surface is indi-
cated by doublets at binding energies of 336 (Pd 3d5/2) and
341 eV (Pd 3d3/2). In addition, peaks at 338 (Pd 3d5/2) and
343 eV (Pd 3d3/2) can be attributed to PdII species, indicating
a partial oxidation of the PdNPs, at least at a depth of approxi-
mately 5 nm of the sample.[25]
Table 2. Hydrogenation of 4-ethynylbenzaldehyde with various palladium
catalysts.
Entry Cat.
T
t
Solvent p(H2)
Yield [%][a]
14 15
Conv.
16 17 [%]
[8C] [h]
1
2
3
4
5
6
7
8
2
2
3
4
4
4
40 24 DCM
2 bar 67 33
2 bar 85
2 bar 83 17
–
–
–
–
9
–
–
–
–
–
–
>99
94
>99
40 DCM
6
9
40 24 DCM
40 24 DCM
2 bar
2 bar
atm. 55
–
3
–
71
–
99 >99
17 >99
–
–
40
RT
6
2
DCM
DCM
55
>99
4[b]
PdCl2
5
RT 24 DCM
RT 24 DCM
40 24 DCM
atm.
atm.
2 bar
–
–
–
99
–
37
99 >99
9
39 24 >99
10
11
12
13
14
15
16
17
18
19
20
21
22
23
5
Pd/C
40
6
DCM
2 bar 53 45
2 bar 31 69
–
–
–
–
–
58
–
56
99
99
2
–
–
–
>99
>99
>99
72
40 24 DCM
Pd/SiO2 40 24 DCM
2
2
3
4
5
Pd/C
Pd/C
2
2 bar
–
>99
RT
6
MeOH[c] atm. 65
7
Finally, the pore-size distribution of our particles was deter-
mined by the BJH-method[26] on the basis of nitrogen adsorp-
tion–desorption (BET) experiments (see the Supporting Infor-
mation for BET isotherms and BJH-plot). For the “mother”-parti-
cles 1 isotherms of type H4 were observed. There is a distribu-
tion of pore diameters around a central value of 330 pm. The
BET surface area of particle 1 amounts to 224 m2gÀ1. The mi-
cropore area as well as the external surface area are found to
reach 206.5 and 17.5 m2gÀ1, respectively, as calculated from
the t-plot. In the palladium-loaded bifunctionalized particles 4
the BET curves indicate no porosity (BET surface area:
20.9 m2gÀ1). This is likely due to the fact that the pores get
RT 24 MeOH[c] atm.
RT 24 MeOH[c] atm.
RT 24 MeOH[c] atm.
–
–
–
–
42
–
99 99
–
99 >99
6
–
–
>99
RT 24 MeOH[c] atm. 19 19
40 24 MeOH[c] atm.
RT 24 MeOH[c] atm.
RT 24 EtOAc
40 EtOAc
40 24 THF
40 24 THF
>99
>99
99
–
–
–
–
–
10
2 bar
70 20 >99
2
2
Pd/C
6
2 bar 56 38
2 bar 49 42
2 bar 32 68
–
–
–
–
9
–
94
>99
>99
[a] Yield determined by GC-MS analysis; [b] catalyst was removed after
2 h and results provided represent reduction with leached out Pd as cata-
lyst; [c] 2,2,2-trifluoroethanol was added.
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after 24 h at 408C (83%, entry 3). Catalyst 4 showed the high-
est activity. Under standard conditions, complete reduction
along with deoxygenation was observed, and 4-ethyl-toluene
(17) was quantitatively formed (entry 4). Even after 6 h reaction
time, a substantial amount of fully reduced and deoxygenated
17 was formed when 4 was used as a catalyst (entry 5). A
slightly lower activity was noted for hybrid system 5. Hence,
running hydrogenation with 5 at 408C for 24 h provided the
deoxygenated compound 17 as a minor product along with
15 and small amounts of 16 (entry 9). Stopping the reaction
after 6 h did not lead to any deoxygenation product 17
(entry 10). As a control experiment, hydrogenation was repeat-
ed using commercially available Pd/C, and 4-ethylbenzalde-
hyde (15) was formed as the major product along with the
semihydrogenation product 14 (entry 11). With Pd/SiO2, in
which the PdNPs are present as agglomerates and are not
bound to SiO2, 4-ethylbenzaldehyde (15) was quantitatively
formed (entry 12).
22 h in the absence of 4. It was observed that the semihydro-
genation product was formed in 55% yield after 2 h at an
overall conversion of 55% (Table 2, entry 6). However, even
after the removal of the catalyst the reaction proceeded to de-
liver product 15 in a quantitative yield after 24 h, clearly reveal-
ing that Pd leached out of 4 into the solution during the initial
2 h reaction period and that the leached Pd species is active
(entry 7). Leaching was also noted for hybrid 3 where total re-
flection X-ray fluorescence analysis (TXRF)[27] revealed that a sig-
nificant amount of palladium leached out from the surface
into the solution within 2 h (462.4 mgLÀ1, see the Supporting
Information for detailed analysis). A further control experiment
was conducted by using the precursor salt (neat PdCl2) as the
catalyst. In the course of the reaction, the yellow solution
turned black, indicating a reduction of the palladium salt to
Pd0 under the applied reaction conditions. Not surprisingly, it
was observed that the fully reduced and deoxygenated prod-
uct 17 was formed after 24 under these conditions (entry 8).
This result showed that Pd nanoparticles are formed from Pd
salts under the applied reducing conditions. However, the reac-
tivity of these PdNPs is different than the reactivity of the NPs
derived from hybrid 4.
Because one of the main advantages of heterogeneous cat-
alysis is the recyclability of the catalyst system, recycling ex-
periments were conducted. We tried several methods, includ-
ing filtration and centrifugation, to recover the silica particles
after successful hydrogenation in DCM. The method of choice
for catalyst separation and recovery turned out to be centrifug-
ing of the reaction mixture using Teflonꢃ vials. By using this
technique, we were able to recycle catalyst 2 four times (see
the experimental section below).
From these studies we can conclude that the highest activity
and hence the lowest selectivity for semihydrogenation is ach-
ieved for catalyst systems 4 and 5 (bearing methyl sulfoxide li-
gands), leading to aldehyde reduction and also deoxygenation.
Notably, in these two hybrid materials the mean diameter of
the PdNPs lies at around 30 nm. The least active catalyst 2 is
composed of PdNPs with the largest mean diameter (around
40 nm). To our surprise, hybrid 3 with PdNPs showing the
smallest mean diameter of around 25 nm turned out to be less
active than systems 4 and 5.
Next we investigated solvent effects on the hydrogenation
of 4-vinylbenzaldehyde. To this end, reductions were repeated
in MeOH as a solvent. We found that activity of all catalyst sys-
tems was higher in this polar protic solvent, and therefore hy-
drogenations were conducted under atmospheric pressure of
H2 at room temperature. After 6 h reaction time with catalyst
2, 65% of the semihydrogenation product 14, along with 15
(7%) and unreacted starting material was observed (Table 2,
entry 13). However, extending reaction time to 24 h provided
17 quantitatively (entry 14). Note that catalyst 2 in DCM did
not provide any aldehyde reduction and deoxygenation (com-
pare with entry 1).
Hydrogenation of external and internal alkynes
To investigate substrate scope, various alkynes were hydrogen-
ated. Experiments were conducted in MeOH and in DCM. One
major point of interest was the capability of our particles to
semihydrogenate internal alkynes with ideally high Z/E-selectiv-
ity. Results obtained for the reduction of alkynes, which show
different steric and electronic properties, are depicted in
Table 3.
Again 3 showed lower activity than 2 (Table 2, entry 15), and
the highest activity was noted for system 4 (entry 16). As in
DCM, activity of 5 was lower than that of 4 in MeOH (entry 17).
Interestingly, hydrogenation with Pd/C as the catalyst provided
benzyl alcohol 16 quantitatively (entries 18 and 19). We then
switched to polar aprotic solvents such as EtOAc and THF. The
hydrogenation proceeded faster in EtOAc than in THF (en-
tries 20–22). With Pd/C as a catalyst no significant activity dif-
ference was observed in DCM and THF (entries 11 and 23).
Hence, the order of activity of our hybrid systems with respect
to the tested solvents is as follows: MeOH>EtOAc>THF>
DCM.
Hydrogenation of 1-phenyl-1-propyne in DCM provided fully
reduced propylbenzene quantitatively even with the least reac-
tive catalyst 2 (Table 3, entry 1), and with Pd/C the same result
was obtained (entry 2). Therefore, we tested the sterically more
hindered diphenylacetylene as a substrate. In order to obtain
better conversion, MeOH was chosen as solvent and reactions
were conducted with the more reactive catalysts 3–5, which
showed similar results. Semihydrogenation with high Z selec-
tivity was achieved in these transformations (entries 3–5). Pd/C
was significantly less reactive than the hybrid catalysts 3–5
(entry 6). Ethyl 3-phenyl-propiolate is a reactive substrate, and
hence reductions were performed in DCM with the least reac-
tive catalyst 2. We were pleased to find that only the corre-
sponding semihydrogenation product was identified with com-
plete Z selectivity and good conversion (entry 7). In contrast,
Pd/C as a catalyst in the control experiment under the same
In order to estimate leaching effects, the most active catalyst
4 was employed for 2 h at an atmospheric pressure of hydro-
gen, and the reaction was analyzed. Hybrid catalyst 4 was then
removed by simple filtration, the filtrate was analyzed, and the
hydrogenation of the filtrate was continued for an additional
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Table 3. Hydrogenation of alkynes at 2 bar H2 pressure, 408C, for 24 h.
Cat.
Solvent
DCM
Substrate
Products[a]
Conv. [%]
1
2
2
>99
>99
Pd/C
DCM
3
4
5
6
3
MeOH
41
46
56
19
4
MeOH
MeOH
MeOH
5
Pd/C
7
8
2
DCM
DCM
47
Pd/C
>99
9
2
EtOAc
EtOAc
>99
>99
10
Pd/C
[a] Yield determined by GC-MS analysis.
conditions delivered the fully hydrogenated ethyl 3-phenylpro-
panolate in quantitative yield (entry 8). Surprisingly, hydroge-
nation of 3-pentyl-propargyl alcohol with hybrid catalyst 2 af-
forded the fully hydrogenated and deoxygenated octane in
36% yield as major product along with 36% of 1-octanol
(entry 9). Pd/C showed far lower activity for this transforma-
tion, and octane was observed only in 27% yield (entry 10).
tatively cleaved by using catalyst 2, as shown for the quantita-
tive transformation of benzyl 4-hydroxyphenyl ether to toluene
(entry 6).
Summary and Conclusion
Applying our modular synthesis approach for construction of
functionalized mesoporous silica particles, various bifunctional
organic–inorganic hybrid materials were successfully prepared
using late stage chemical modification. A “click to ligand” strat-
egy was applied to attach bidentate triazole-sulfoxide ligands
to the silica surface. Reactions were readily analyzed by IR and
MAS-NMR spectroscopy. Along with the sulfoxide entities, ami-
noamide functionalities were conjugated to the surface by
thermal radical nitroxide exchange reactions. Palladation with
PdCl2 in EtOH provided a quick and simple synthesis route to
hybrid materials with ligand-induced size-tuned PdNPs ad-
sorbed at the surface of the mesoporous silica, without the
need of an external reducing agent.[28,29] The mean diameter of
the PdNPs varied from 25 to 40 nm as a function of the organ-
Hydrogenation of external and internal alkenes, and benzyl
ether cleavage
Finally, we investigated hydrogenation of differently substitut-
ed alkenes. Z-b-methylstyrene and ethyl b-methylcinnamate
were quantitatively hydrogenated with the least active catalyst
2 (Table 4, entries 1 and 3), and the same results were achieved
with Pd/C (entries 2 and 4). Allyl phenyl ether was hydrogenat-
ed without concomitant ether cleavage (entry 5). For this sub-
strate, hydrogenation is faster than allyl ether cleavage, and
the phenyl alkyl ether is resistant towards CÀO bond cleavage.
Along these lines, we found that benzyl ethers can be quanti-
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Typical nitroxide exchange reaction protocol
The sulfoxide-functionalized particles or
Table 4. Hydrogenation of alkenes and benzyl ether cleavage at 2 bar H2
pressure, 408C, and 24 h reaction time.
7
8 (200 mg, ca.
1.51 mmolgÀ1, ca. 0.30 mmol) were suspended in DCE (5 mL), and
the TEMPO-derivative 9a or 9b (200 mg) was added. After heating
the reaction mixture for 24 h at 1258C, the particles were filtered
and washed with H2O, MeOH, DCM, and Et2O (30 mL each). Drying
of the particles at 500 mbar and 408C afforded the bifunctional
MSNs 10–13.
Entry Cat. Solvent Substrate
Products[a]
Conv. [%]
1
2
2
EtOAc
>99
Pd/C EtOAc
>99
>99
General procedure for the hydrogenation reactions
3
4
2
EtOAc
Hydrogenation reactions were carried out in a pressure-resistant
Schlenk tube. The palladium catalyst (1 mol%) was suspended in
the corresponding solvent, and the substrate (0.25 mmol) was
added. The tube was purged with hydrogen gas (ten times), and
the reaction mixture was heated to 408C (or stirred at RT) under
2 bar H2-atmosphere for the time specified in the tables. The cata-
lyst was then filtered off, and the filtrate was concentrated in
Pd/C EtOAc
>99
5
6
2
2
EtOAc
75
1
vacuo. GC-MS as well as H NMR techniques were used to deter-
MeOH[b]
>99
mine conversion and yield.
[a] Yield determined by GC-MS analysis; [b] 2,2,2-trifluoroethanol was
added.
General procedure for the recovery of the catalyst 2
After completion of the hydrogenation reaction, the reaction mix-
ture was transferred into a Teflonꢃ vial and subsequently centri-
fuged at 6000 min in a Minicentrifuge Mini G by ROTH. The solu-
tion was separated with a syringe, the remaining particles 2 were
again suspended in DCM, and this procedure was repeated five
times. After this cleaning process, the particles were dried in vacuo
and employed in the next reaction cycle.
ic moieties of the mesoporous silica, as analysed by TEM. The
PdNPs are well dispersed (EDX mapping and TEM) at the sur-
face of the silica core beads, which themselves have a mean
diameter of around 285 nm. Importantly, PdNP growth on
SiO2-core material lacking the sulfoxide and amino amide func-
tionalities was not possible, clearly showing the importance of
these surface bound ligands for controlled growth and stabili-
zation of the PdNPs. The novel Pd catalyst systems were tested
in various hydrogenation reactions. It was found that the
hybrid materials containing PdNPs of around 30 nm mean di-
ameter reveal the highest activity. The least reactive system
(PdNP diameter around 40 nm), which is less active than Pd/C,
showed good selectivity for semihydrogenation of some al-
kynes. The most reactive catalyst showed higher activity than
Pd/C and is able to reduce and deoxygenate aldehydes. The
hydrogenation of internal alkynes is also possible with high Z
selectivity under mild conditions, and benzyl ether cleavage
can also be achieved with these hybrid systems. Importantly,
catalysts can be recovered and reused up to four times with-
out any significant loss in activity.
General procedure for deoxygenation and ether cleavage
Hydrogenation reactions were carried out in a pressure-resistant
Schlenk tube. The palladium catalyst (10 mg, ca. 0.24 mmolgÀ1, ca.
0.24 mmol, ca. 1 mol%) was suspended in the corresponding sol-
vent, and the substrate (0.25 mmol) was added. The tube was
purged with hydrogen gas (ten times), connected to a H2-filled bal-
loon, and the reaction mixture was heated to 408C (or stirred at
RT). The catalyst was then filtered off, and the filtrate was concen-
trated in vacuo. GC-MS and 1H NMR techniques were used to
follow the conversion and to determine the yield.
NMR spectroscopy
Solid-state NMR measurements were carried out on Bruker Avance
III 300 and DSX 400 and 500 spectrometers equipped with 2.5 and
4 mm single and double resonance NMR probes. The resonance
frequencies were 75.433 and 100.61 MHz for 13C at 7.04 and 9.39 T,
respectively, and 99.325 MHz for 29Si at 11.7 T. Chemical shifts are
reported relative to TMS (tetramethylsilane), using adamantane
(1.78 ppm), adamantane (38.56 ppm for the methine resonance),
and TTMS (tetrakis(trimethylsilyl)silane) (À9.8 ppm) as secondary
references for 1H, 13C, and 29Si, respectively. 1H MAS-NMR spectra
were recorded by means of rotor-synchronized Hahn echo experi-
ments at a spinning frequency of 30.0 kHz. The 908 pulse length
was 3.0 ms, and a recycle delay of 5 s was used. Ramped 13C{1H} CP/
MAS experiments were measured at 10 kHz spinning frequency,
with typical pulse lengths of 5.0 ms and contact times of 5 ms,
TPPM-15 proton decoupled acquisition and recycle delays of 10 s
were used. Single-pulse 29Si MAS-NMR spectra were obtained at
10.0 kHz spinning frequency, using 908 pulses of 5.0 ms length and
relaxation delays of 180 s.
Experimental Section
Typical alkyne/azide click reaction protocol
Azide/alkoxyamine-functionalized
MSNs
1
(350 mg,
ca.
2.58 mmolgÀ1, ca. 0.90 mmol) were reacted with CuSO4 (105 mg,
0.66 mmol), sodium ascorbate (123 mg, 0.62 mmol), and sulfoxides
6a or 6b (350 mg) overnight at 908C in EtOH/H2O (2:1–10 mL). Fil-
tration and washing with MeOH (50 mL), DCM (50 mL), as well as
Et2O (20 mL) afforded the functionalized MSNs 7 and 8 (ca.
500 mg) as white solids.
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[12] A. T. Dickschat, F. Behrends, S. Surmiak, M. Weiß, H. Eckert, A. Studer,
Chem. Commun. 2013, 49, 2195.
Acknowledgements
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Keywords: click chemistry
·
cooperative catalysis
·
hydrogenation · mesoporous silica · palladium nanoparticles
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Published online on && &&, 0000
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FULL PAPER
Click-chemistry: Various palladium
nanoparticle-loaded recyclable bifunc-
tional sulfoxide–aminoamide silica parti-
cles were prepared and applied to the
semihydrogenation of internal as well as
external alkynes. Alkene hydrogenation
and deoxygenation of activated alcohols
can also be achieved by using these
novel functionalized mesoporous cata-
lysts.
&
Palladium Nanoparticles
S. K. Surmiak, C. Doerenkamp, P. Selter,
M. Peterlechner, A. H. Schꢀfer, H. Eckert,*
A. Studer*
&& – &&
Palladium Nanoparticle Loaded
Bifunctional Silica Hybrid Material:
Preparation and Applications as
Catalyst in Hydrogenation Reactions
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