Strawberry-like SiO2@Pd and Pt nanomaterials

Article abstract of DOI:10.1039/c4nj01019a

Based on a synthesis strategy of silica nanoparticles (NPs) in non-alcoholic medium, a straightforward protocol leading to strawberry-like SiO₂@M nanomaterials (M = Pt, Pd) has been developed. Pd and Pt NPs were immobilised onto surface modified silica NPs, by in situ decomposition of organometallic precursors, Pd₂(dba)₃ and Pt(dba)₂ (dba = dibenzylideneacetone). The surface of silica was functionalised by aminopropyltriethoxysilane (APTES) or 2-(diphenylphosphino)ethyltriethoxysilane (PhPETES). The co-grafting of propyltriethoxysilane (PTES) and APTES was investigated to avoid the classical interfering cohydrolysis and polycondensation reactions which can lead to agglomeration phenomena between the silica NPs. It appeared that a APTES/PTES ratio of 1/3 insured a dense and homogeneous deposition of the metallic NPs onto silica. Depending on the ligand, the diameter of the metallic NPs varies from 3.6 to 6.0 nm for Pd and from 1.5 to 3.1 nm for Pt NPs. The metal loading is adjustable, at least up to 6.91 wt%, as exemplified with Pd. Preliminary catalytic tests of the SiO₂@Pd nanomaterial are also presented.

Full text of DOI:10.1039/c4nj01019a

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PAPER
Strawberry-like SiO @Pd and Pt nanomaterials†
2
ab
a
b
b
Alfonso Cornejo, Gad Fuks, Vıctor Martınez-Merino, ´I nigo Sarobe,
´
´
˜
Cite this:DOI: 10.1039/c4nj01019a
b
cd
a
a
Maria Jose Gil, Karine Philippot, Bruno Chaudret, Fabien Delpech* and
´
a
Celine Nayral*
´
Based on a synthesis strategy of silica nanoparticles (NPs) in non-alcoholic medium, a straightforward
protocol leading to strawberry-like SiO
NPs were immobilised onto surface modified silica NPs, by in situ decomposition of organometallic
precursors, Pd (dba) and Pt(dba) (dba = dibenzylideneacetone). The surface of silica was functionalised
2
@M nanomaterials (M = Pt, Pd) has been developed. Pd and Pt
2
3
2
by aminopropyltriethoxysilane (APTES) or 2-(diphenylphosphino)ethyltriethoxysilane (PhPETES). The co-grafting
of propyltriethoxysilane (PTES) and APTES was investigated to avoid the classical interfering cohydrolysis and
polycondensation reactions which can lead to agglomeration phenomena between the silica NPs. It appeared
that a APTES/PTES ratio of 1/3 insured a dense and homogeneous deposition of the metallic NPs onto
silica. Depending on the ligand, the diameter of the metallic NPs varies from 3.6 to 6.0 nm for Pd and
from 1.5 to 3.1 nm for Pt NPs. The metal loading is adjustable, at least up to 6.91 wt%, as exemplified
Received (in Montpellier, France)
2
0th June 2014,
Accepted 29th September 2014
DOI: 10.1039/c4nj01019a
www.rsc.org/njc
2
with Pd. Preliminary catalytic tests of the SiO @Pd nanomaterial are also presented.
recovery and recyclability (which are still challenging for homo-
geneous catalysis processes), the catalytic NPs often have to be
Introduction
2
–4
After a first phase of ever-increasing interest for the synthesis of supported. Typical supports consist of mesoporous materials
NPs exhibiting controlled size, shape and composition, the for which diffusion is limited by the pores that can induce a
5
development of complex nanomaterials associating different decrease of the catalytic activity. This drawback has triggered
NPs is today one of the main goals in nanoscience. The ability the synthesis of soluble but isolable substrates such as magnetic
2
,3,6,7
to selectively arrange nanosized domains of different materials or centrifugable particles
that overcome diffusional limita-
into a single hybrid NP is a key to enlarge the potentialities of tions. If silica (embedding or not a magnetic core) appears as a
8
these objects in terms of multiple functionalities, of enhanced good candidate to play the role of a support, the achievement
properties or of compatibility with applications. In this context, of reproducible syntheses of supported metal NPs, showing well-
the synthesis of hybrid metal–silica nanomaterials appears defined composition, small diameters (o10 nm) and narrow
strategic to take advantage of the stability, biocompatibility, size distribution, is not trivial. A first synthetic strategy consists
n+
accessibility and easy surface functionalization of silica, combined of anchoring M species on the silica surface and forming metal
9
10
11
with the properties (electronic, magnetic, optical or catalytic) of NPs in a further reductive step (e.g. for Pd, Ru, or Pt ) or
1
5,12,13
14,15
the metallic NPs.
during a catalytic cycle (e.g. for Pd,
or Rh
). A second
For example, well dispersed NPs in the liquid phase are very strategy, which offers a better control of synthesis, is based on
attractive in terms of catalytic performances due to their high the impregnation of silica NPs, functionalized by a thiol or an
surface/volume ratio. However, to satisfy the environmental amine, with preformed metallic NPs. The functional groups
requirements and work with simple protocols associating easy present at the surface of the silica allow the covalent linking of
the metal NPs during the impregnation step (exemplified by
1
6
Halas et al. for the immobilization of Au NPs, or by Gude et al.
a
Universit ´e de Toulouse, INSA, UPS, CNRS, LPCNO (Laboratoire de Physique et
Chimie des Nano-Objets), 135 avenue de Rangueil, F-31077 Toulouse, France.
E-mail: fdelpech@insa-toulouse.fr, cnayral@insa-toulouse.fr
1
7
in the case of Pd NPs ). A third approach is based on the
1
8
19
formation of the metal NPs (Pt or Pd ) by a direct nucleation
on the silica surface. For example, the decomposition of an
organometallic complex of Pd led to the formation of Pd NPs at
the silica surface which had been previously functionalized by
b
c
Dpto, de Qu ´ı mica Aplicada, Ed. ‘‘Los Acebos’’, Campus de Arrosadia,
Universidad P u´ blica de Navarra, E31006, Pamplona, Spain
CNRS, LCC (Laboratoire de Chimie de Coordination du CNRS),
2
05 route de Narbonne, BP 44099, F-31077 Toulouse, France
1
9
d
amino or terpyridine groups.
Universit ´e de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
In all of these cases, the synthesis of the final nanomaterial
was achieved by multi-step processes involving therefore tedious
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4nj01019a
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8
separation and purification procedures. Silica NPs (or shells) via the amine function, for metallic NPs (such as Ru). However,
are prepared using the classical water/alcohol media which it may induce agglomeration phenomena between the silica NPs,
is not directly compatible with organometallic chemistry. for which a few precedents exist in the literature and which are
Herein, we present a novel straightforward protocol for the reasonably explained by cohydrolysis and polycondensation
synthesis of a family of nanomaterials based on Pd or Pt NPs reactions, as the silica formation occurs by base-catalysed hydro-
1
7
immobilized on silica NPs, thanks to the silica surface function- lysis and condensation of TEOS. We indeed observed such
alization either by an amine ligand or a phosphine ligand. The a phenomenon, when adding APTES to the silica particles that
originality of the approach is based on a same synthetic medium coagulated then into larger objects. We could overcome this
(non-alcoholic solvent) for the formation of the metallic NPs major drawback by grafting on the silica surface a mixture of
and of the silica NPs. Indeed, the silica NPs have been prepared APTES and propyltriethoxysilane (PTES) instead of solely APTES.
according to an original method involving tetrahydrofuran or Additionally, the addition of PTES also had a beneficial effect on
2
0
dimethoxyethane that we have previously described. These the dispersion of the silica particles in apolar solvents.
conditions are perfectly compatible with the synthesis of metallic The synthesis is carried out following a one-pot reaction
NPs by decomposition of organometallic precursors which has involving three steps (Scheme 1). A solution of tetraethoxysilane
21–26
been developed by our group.
NPs have been immobilized on the silica surface by decomposition THF is stirred at 50 1C for 7 days ([TEOS] = 0.12 M) to obtain a
of Pd (dba) and Pt(dba) in THF, in the presence of the preformed suspension of silica NPs. Then, a mixture of grafting ligands
In the present paper, Pd and Pt (TEOS), butylamine (BA) and water with 1/0.5/4 molar ratios in
2
3
2
silica NPs. This method allows a simplification of the protocol and (APTES and PTES in different ratios, TEOS/(APTES + PTES) ratio
the formation of a very homogeneous distribution of the metallic of 1/0.6) is added to the raw suspension. Pd (or Pt) precursor
NPs at the surface of silica. Besides the characterization of the and hexadecylamine (HDA, which improves the dispersion of
nanomaterials by state-of-the art techniques, preliminary catalytic the silica NPs in solution) are thereafter added into the reaction
tests are presented.
medium which is then pressurized under 3 bar of dihydrogen
to provide the decomposition of the complex. No intermediate
washing steps are required.
In the case of Pd, when an APTES/PTES ratio of 1/1 is used,
agglomerates of Pd NPs are observed on the silica support
Results and discussion
Strawberry-like nano-objects based on amine surface-modified
(
(
Fig. 1). High Resolution Transmission Electron Microscopy
HRTEM) together with the fast Fourier transform (FFT) pattern
silica NPs
We first examined the direct decomposition of Pd
2 3
(dba) and revealed lattice fringes separated by distances of 0.194 nm and
Pt(dba) in THF, in the presence of the preformed silica NPs 0.224 nm, corresponding, respectively, to the (200) and (111)
2
prepared according to the procedure previously described by lattice spacings of the face-centred cubic (fcc) palladium cell
our group (in a typical experiment the silica NPs exhibit a (Fig. 2). The grafting of this pair of ligands impacts the silica NPs
2
0
diameter centred at 88 Æ 20 nm). However, our attempts of size and morphology leading to larger silica nano-objects with a
grafting metallic NPs on unfunctionalized silica particles have mean diameter of 207 Æ 78 nm (Fig. 1, left). Due to the catalytic
been unsuccessful, leading to the formation of aggregated metallic role of APTES and the absence of intermediate washing steps,
19
NPs unlinked to silica, as previously observed by Guerrero et al.
further condensation reactions (involving triethoxysilyl species)
Therefore, the silica surface has been beforehand functionalized. In and coalescence phenomena occur in solution. When the APTES/
the first part of this work, the ligand chosen for surface modifica- PTES ratio is decreased to 1/3, a homogeneous dispersion of Pd
tion is aminopropyltriethoxysilane (APTES). This is a bi-functional NPs (with a mean diameter of 3.6 (0.8) nm) on the silica surface
ligand frequently used to provide an amine function on the silica is obtained (Fig. 1), with only a slight increase of the diameter
9,10,14,15,27
surface
and which has shown to be an efficient stabilizer, and an enlargement of the size distribution of the silica NPs
Scheme 1 Synthesis of Pd NPs immobilized on amino-functionalized silica NPs.
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Fig. 1 TEM images of Pd NPs immobilised onto silica particles functionalized with APTES/PTES 1/1 (left) and APTES/PTES 1/3 (right with inserted size
distribution).
2 2
Et, or negative charge) and the T species (T : C-Si(OSi) (OR),
À60 ppm; T : C-Si(OSi) , À68 ppm) are obtained from decon-
3
3
2
9
1
volution of the Si{ H} spectrum. Two main pieces of informa-
tion can be extracted from the analysis of these NMR data.
First, the spectra are very similar except that in the case of the
2 2
APTES/PTES ratio equal to 1/1, no T or Q are detected,
indicating a higher degree of condensation. This observation
is consistent with the presence in the reaction mixture of
higher amounts of amine function which catalyzes a further
condensation reaction of the alkoxysilyl moiety. Second, the
relative percentage of all T (12%) and of all Q (88%) substruc-
ture species remains constant indicating that the sum of the
amounts of APTES and PTES that are grafted on the NPs is
the same. In other words, the degree of functionalization of the
silica NPs is constant whatever the introduced APTES/PTES
Fig. 2 HRTEM image of Pd NPs immobilised onto silica particles function- ratio, suggesting an optimal coverage of the silica surface. The
alized with APTES/PTES 1/1 and its FFT pattern (inset).
conditions to obtain a regular distribution of organic units on the
surface of bifunctional silica materials are difficult to predict,
29
but in the case of the 1/3 APTES/PTES ratio, the amine functions
are presumably more homogeneously distributed over the silica
surface, thus leading to a homogeneous dispersion of Pd NPs as
mentioned before.
Similarly, Pt NPs of a mean diameter of 1.5 (0.3) nm, and
homogeneously dispersed onto the silica surface, are formed by
decomposition of Pt(dba) under 3 bar of dihydrogen at room
2
temperature (Fig. 3). ICP-MS analysis revealed a Pt grafting
yield which is ca. 56% and a Pt content in the nanomaterial of
(
APTES being in lower quantity). The SiO @Pd nanomaterials are
2
purified by centrifugation and washing with THF. Inductively-
coupled plasma mass spectrometry (ICP-MS) shows that 65% of
the introduced Pd has been immobilised onto silica and the Pd
content on the support is then about 2.94 wt%. This Pd loading
can be increased up to 6.91 wt% (Pd NPs keeping the same size
and morphology) just by doubling the amount of Pd complex
introduced (the Pd grafting yield is then about 52%). The
versatility of this protocol enables us to vary and control the
Pd weight contents of the nanomaterials.
These nanomaterials have been characterized by magic angle
spinning (MAS) solid-state NMR which provided valuable informa-
tion about the silica structure. It allows a direct comparison between
the two samples (APTES/PTES = 1/1 or 1/3) and a rationalisation of
2.14 wt%. The HRTEM image together with the FFT pattern showed
lattice fringes separated by distances of 0.226 nm and 0.196 nm,
corresponding, respectively, to the (111) and (200) lattice spacings
of the fcc platinum (Fig. 4).
2
9
1
Strawberry-like nano-objects based on phosphine
surface-modified silica NPs
the results. In both cases, the Si{ H} NMR spectrum (Fig. S1
and S2, ESI†) displays the Q substructure silicon signals (i.e., corre-
sponding to silicon atoms forming four Si–O bonds) and T sub- In a second part of this work, we modified the interaction between
structure silicon signals (i.e., corresponding to silicon atoms forming the grafted ligand and the metallic NPs, by replacing the amino
three Si–O bonds and one Si–C) which, respectively, belong to the function by a phosphine one. Indeed, the phosphino function is
inorganic core of the silica NPs and, to APTES and PTES grafted on expected to be a stronger coordinating ligand, as previously shown
3
0,31
the surface. The proportions of the Q (Q
: Si(OSi) : Si(OSi)
OR, À101 ppm; Q
2
: Si(OSi)
2
(OR)
2
, À91 ppm; upon the stabilization of Pd or Pt NPs.
The silica NPs have been
Q
3
3
4
4
, À110 ppm with R = H, thus functionalised by 2-(diphenylphosphino)ethyltriethoxysilane
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Fig. 3 TEM image (left) and HRTEM image (right with inserted size distribution) of Pt NPs immobilised onto silica NPs functionalized with APTES/PTES 1/3.
intermediate washing step is here required prior to the addition
of the metallic precursor. Indeed, when free PhPETES ligands
remain in solution, no metallic NPs are observed, presumably
due to side-reactions at the molecular scale, leading to more
stable Pt or Pd complexes. The protocol is presented in
Scheme 2. In a first step, the PhPETES ligand is directly added
to the silica suspension, which is then washed and isolated.
Then, HDA, Pd (or Pt) precursor and PhPETES-silica NPs are
mixed into THF and pressurised under 3 bar of dihydrogen to
form the metallic NPs.
Fig. 5 shows Pd NPs with a mean diameter of 5.1 (0.9) nm,
homogeneously immobilised onto the silica surface with a Pd
loading of 4.47 wt% determined by ICP-MS (61.5% immobilised
Pd yield). The increase in the introduced amount of Pd pre-
cursor (which is doubled) allows the Pd content to be increased
to 5.95 wt% (the yield of immobilised Pd is about 55%). However,
Fig. 4 HRTEM image of Pt NPs immobilised onto silica NPs which have
been functionalized with APTES/PTES 1/3 and its FFT pattern (inset).
in that case, we observe a clear broadening of the size distribution,
the mean diameter of Pd NPs is ca. 6.0 (2.3) nm (Fig. 5).
(PhPETES), following the same procedure as that used for The HRTEM image together with the FFT pattern revealed
APTES grafting. In contrast to the case of the amino ligand, lattice fringes separated by distances of 0.225 nm and 0.194 nm,
the use of a co-ligand is not necessary as PhPETES grafting does corresponding, respectively, to the (111) and (200) lattice spacings
not induce any aggregation between silica particles, but an of the fcc palladium (Fig. 6).
Scheme 2 Synthesis of Pd NPs immobilized on phosphino-functionalized silica NPs.
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Fig. 5 TEM images of Pd NPs immobilised onto silica particles functionalized with PhPETES (left) and with a double amount of Pd (right) with inserted
size distributions.
0.196 nm corresponding, respectively, to the (111) and (200)
lattice spacings of the fcc platinum (Fig. 7).
According to MAS-NMR experiments, in contrast to PTES
and APTES, the grafting of PhPETES appears to be much
2
9
1
less efficient. The Si{ H} spectrum of PhPETES grafted NPs
Fig. S3, ESI†) clearly shows the presence of Q , Q and Q , but T
(
2
3
4
substructure silicon atoms could only be detected as a very
minor species (B1%). However, the Cross-Polarization (CP)
sequence (Fig. S4, ESI†), thanks to the enhancement of
surface-localized spins, allows the confirmation of the chemical
functionalization and the location at the surface. Consistently,
3
1
1
the P{ H} NMR spectrum (Fig. 8) of the grafted silica NPs
displays two main signals at d À9 and 37 ppm assigned,
respectively, to PhPETES grafted on silica NPs (32%) and its
3
2
corresponding phosphine oxide (64%). The oxidation results
from the reaction of the phosphine with the silanol groups
Fig. 6 HRTEM image of Pd NPs immobilised onto silica NPs function-
alized with PhPETES and its FFT pattern (inset).
3
3
of the silica NP surface as already evidenced by Bemi et al.
Last, a weak signal (4%) at d 27 may arise from hydrogen bonds
3
2
Similarly, formation of Pt NPs was successfully obtained. between phosphine oxide and surface silanol groups. In the
ICP-MS shows that 56% of the introduced Pt is immobilised presence of Pd NPs (Fig. 9), the coordination of the NPs results
onto silica, forming very homogeneously dispersed NPs with a in the shift of resonance of the phosphine moiety to a chemical
8
mean diameter of ca. 3.1 (0.7) nm (Fig. 7) with a Pt content of shift of around d 25. Interestingly, only a very minor amount
4.57 wt%. The HRTEM image together with the FFT pattern (B3%) of PhPETES grafted remains uncomplexed. This could
revealed lattice fringes separated by distances of 0.226 nm and explain why it is not possible here to increase the Pd content
Fig. 7 TEM image of Pt NPs immobilised onto silica NPs functionalized with PhPETES (left with inserted size distribution) and HRTEM image (right with
FFT pattern).
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1
Fig. 8
P{ H} MAS NMR spectrum of silica NPs functionalized with PhPETES. The spectrum was fully deconvoluted using the dmfit software.
31
1
Fig. 9
P{ H} MAS NMR spectrum of Pd NPs immobilised onto silica NPs functionalized with PhPETES. The spectrum was fully deconvoluted using the
dmfit software.
without increasing the mean diameter of the Pd NPs. There are Table 1 Characteristics of the prepared nanomaterials (mean diameter of
Pd (or Pt) NPs and metal loading)
not enough phosphino-sites at the surface to allow the formation
of more NPs, and hence their growth is favoured.
Table 1 summarizes the characteristics of the different
strawberry-like nanomaterials prepared in this article. Catalyst
Ligand
Metal
APTES/PTES 1/3
Pd Pt
PhPETES
Pd
Pt
Catalyst 1 Catalyst 2
1
and catalyst 2 refer to the nanomaterials further investigated
in catalytic applications (vide infra).
NPs diameter (nm) 3.6 (0.8) 1.5 (0.3) 5.1 (0.9) 6.0 (2.3) 3.1 (0.7)
Metal loading (wt%) 2.94–6.91 2.14 4.47 5.95 4.57
Catalytic tests of Pd-based strawberry-like nano-objects
Pd is a highly effective catalyst in the hydrogenation–dehydrogena- the Heck cross-coupling reaction of iodobenzene with butyl
3
,6
tion reactions as well as in C–C coupling reactions. In order acrylate and styrene, Suzuki coupling of phenylboronic acid
to evaluate the potentialities of our SiO @Pd nanomaterials, and iodobenzene and finally, the reduction of cyclohexene by
preliminary catalytic tests were run for different reactions: dihydrogen (Scheme 3).
2
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Scheme 3 Evaluation of the catalytic activity of catalysts 1 and 2. Reagents and conditions: (i) 0.1 mol% Pd, Et
3
N (1.5 eq.), DMF, 100 1C. (ii) 0.1 mol% Pd,
2 3 2 2
K CO (1.5 eq.), DMF/H O (10 : 1), 100 1C. (iii) 0.02 mol% Pd, H , 3.5 bar, 75 1C.
The catalytic performance of 1 was first tested in the cross- comparable, in terms of efficiency and selectivity, to the litera-
coupling reaction of butyl acrylate and iodobenzene at 0.1% mol ture reports on the activity of Pd (complexes or NPs) supported
34
5,12,35–39
Pd loading using mild conditions (Et N, DMF, 100 1C). Quantita- on silica.
3
tive conversion was obtained within the first two hours of reaction
with 100% selectivity towards butyl cinnamate (Table 2, entry 1). catalyst,
Among the studies tackling the question of the recovery of the
5
,34,36,38,40,41
Wang et al. describe a remarkable example
After catalyst separation, mother liquors of the reaction were with the immobilization of palladium NPs on silica nanospheres
analysed and only 3.6 ppm of Pd could be detected. Similarly, encapsulated with silica. This catalyst has been used in five
the catalytic activity of 1 and 2 was evaluated in the coupling of consecutive cycles in the cross coupling of styrene and iodoben-
38
styrene and iodobenzene obtaining high conversion values zene with only a slight loss in activity after the fourth run. The
within the first 4–5 h (76% and 71% for 1 and 2, respectively, recovery of catalyst 2 was here tested in the Heck reaction between
entries 2 and 3) with moderate to high selectivities towards butyl acrylate and iodobenzene. As expected, iodobenzene was
trans-stilbene (ca. 91%) against 1,1-diphenylethene (ca. 9%) and quantitatively converted within the first two hours with 100%
almost quantitative conversion when the reaction mixture was selectivity towards butyl cinnamate. The catalyst could be easily
left overnight. 1 and 2 appeared to be also active in the Suzuki recovered after centrifugation and washed with dichloromethane
coupling of phenylboronic acid and iodobenzene that proceeded in open-air. The catalyst was used during eight runs affording
smoothly affording 96% yield of biphenyl after 3.5 h in DMF– butyl cinnamate as the only reaction product (495% yield within
2
H O at 100 1C (entries 4 and 5). Finally, a preliminary test was 3 hours, Fig. 10), without significant loss of activity.
done on the hydrogenation of neat cyclohexene at 0.017 mol%
of Pd loading using 2 with quantitative conversion and 100%
selectivity towards cyclohexane after 23 h. It is worth mentioning
that the reaction could be carried out under relatively low pressure
Conclusion
(3.5 bar) of dihydrogen in neat cyclohexene while keeping good Successful immobilization of metallic nanoparticles on silica
dispersion. Considering the wide scope of published reaction has been readily achieved by the decomposition of organo-
conditions, the results of these catalytic tests are broadly metallic precursors onto silica spheres prepared in non-alcoholic
media (being thus fully compatible with the organometallic
chemistry). A family of different strawberry-like nanomaterials
has been synthesized using different functional ligands (APTES
Table 2 Screening of the catalytic activity of catalysts 1 and 2 in C–C
cross-coupling reactions of iodobenzene with different substrates
or PhPETES) on the silica and different immobilised metallic
a
b
Entry
Substrate
Catalyst
Time (h)
Yield (%)
nanoparticles (Pd or Pt). It is remarkable to mention that, in all
the cases, the metallic nanoparticles are crystalline, dense and
well-distributed on the surface of silica. They display homo-
geneous spherical morphologies and their mean diameter can
vary, according to the functional ligand on the silica spheres, from
3.6 to 6.0 nm for Pd and from 1.5 to 3.1 nm for Pt nanoparticles.
The metal loading is adjustable, at least up to 6.91 wt%, as
exemplified with Pd. The use of a mixture of APTES–PTES, to
functionalize the silica surface, advantageously prevents agglo-
meration between the silica particles by avoiding the classical
interfering cohydrolysis and polycondensation reactions. It is
noteworthy that the ratio of APTES/PTES has to be increased
1
2
1
1
2
98
c
3.7
76 (90)
25
90 (91)
71 (91)
95 (92)
3
2
5.6
7
2
4
1
2
3.4
3.4
94
5
96
a
b
0
.1 mol% of catalyst loading and temperature = 100 1C. 1,3,5-
c
Trimethoxybenzene as the internal standard. Values in parentheses
indicate selectivity towards trans stilbene.
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Fig. 10 Recovery test of 2 in the Heck coupling of iodobenzene and butyl acrylate.
from 1/1 to 1/3 to allow the homogeneity of the dispersion of the operating at 200 kV with a point resolution of 4.5 Å and on a
Pd nanoparticles at the surface. According to preliminary catalytic JEOL JEM 1011 electron microscope operating at 100 kV with a
tests, the nanomaterial constituted by Pd NPs immobilised on the point resolution of 4.5 Å. The size distributions were determined by
PhPETES-functionalized silica nanomaterial appears as a promis- measuring ca. 150 particles using the Image J software (variation
ing catalyst in C–C cross-coupling reactions.
range is given by 3s). Crystalline structures are attributed on the
The synthetic protocol reported in this article is very easy to be basis of several HRTEM images together with the FFT patterns
implemented (one pot, low temperatures). It can be adapted for revealing the same lattice fringes on different NPs. Solid-state NMR
different ligands and different metals, providing a wide versatility. experiments were performed on a Bruker Avance 400WB equipped
The examples (Pd and Pt nanoparticles) which are presented here with a 4 mm probe, the sample rotation frequency being set at
1
29
13
are far away to be exhaustive. Indeed, the controlled synthesis 12 kHz unless otherwise indicated. All H, Si, and C chemical
2
4,26,42
of different metallic nanoparticles (such as Ru, Rh or Ir)
has been described, by using the same organometallic method. methylsilane (TMS) at 0 ppm. P chemical shifts are reported and
Thus, we could expect to produce such different metal NPs on referenced to an external H PO sample. Microanalysis was per-
formed by the ‘‘Service de microanalyses du Laboratoire de Chimie
shifts are reported using the d scale and are referenced to tetra-
31
3
4
the surface of silica spheres, in the same way.
de Coordination’’ (C, H, N) or by Antellis company (Pd, Pt).
Catalytic tests were monitored by GC-FID in an Agilent 6890.
TecknoKR TRB-FFAP (25 m, 320 mm, Tecknocroma) was used as
the stationary phase and He as a carrier gas at a constant pressure
of 16 psi with a split ratio of 15 and an inlet temperature of 230 1C.
Experimental section
General procedures and materials
All manipulations were carried out under an argon atmosphere The FID detector was operated at 250 1C with a hydrogen flow of
À1
À1
À1
using a Fischer–Porter bottle and vacuum line techniques, or in 40 mL min , air flow 450 mL min and an He make-up flow of
a glovebox. Tetraethoxysilane (TEOS) was purchased from Alfa 45 mL min . The oven was operated at an initial temperature of
À1
Aesar, hexadecylamine (HDA; Z99%) and butylamine (BA; Z99.5%) 40 1C for 5 minutes followed by a ramp of 15 1C min to 200 1C
from Fluka, aminopropyltriethoxysilane (APTES; Z98%) and propyl- and then maintained for 4 minutes. Reactants and products
triethoxysilane (PTES; Z97%) from Aldrich, 2-(diphenylphos- were calibrated using commercially available standards. 1,3,5-
phino)ethyltriethoxysilane (PhPETES) from Gelest. Tetrahydrofuran Trimethoxybenzene was used as the internal standard. Retention
was purchased from SDS (99.9%), dried and distilled over sodium/ times were as follows: 8.12 min for N,N-dimethylformamide,
2 3 2
benzophenone. Pd (dba) and Pt(dba) were purchased from 15.27 min for 1,3,5-trimethoxybenzene, 10.13 min for iodobenz-
NanoMeps. All reagents and solvents were degassed before use ene, 6.83 min for styrene, 5.06 min for butyl acrylate, 16.58 min
by using three freeze–pump–thaw cycles. Samples for TEM for trans-butylcynnamate, 14.96 min for 1,1-diphenylethene,
analysis were prepared in a glovebox by slow evaporation of a 18.25 min for trans-stilbene, and 14.14 min for biphenyl. Palladium
drop of the colloidal solution deposited onto a carbon-covered contents in the mother liquors were analyzed by atomic absorption
copper grid. TEM analysis was performed at the ‘‘Service Commun on a Perkin Elmer Atomic Absorption 2100 and a Perkin Elmer
de Microscopie Electronique de l’Universit ´e Paul Sabatier’’ Atomic Absorption 3100 using a Pd lamp. Mother liquor samples
3
(TEMSCAN) on a JEOL JEM 200 CX electron microscope were concentrated prior to be digested in boiling HCl/HNO 3/1.
New J. Chem.
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Paper
Palladium contents in the solid catalyst samples were measured is centrifuged at 5000 rpm for 15 min, yielding a black solid at
by Antellis.
the bottom of the tubes. The supernatant is eliminated and the
precipitate is then washed three times with tetrahydrofuran.
A black powder is obtained after drying the precipitate under
Nanomaterials synthesis
Pd grafting on APTES/PTES 1/1 silica NPs. Tetraethoxysilane vacuum for 6 h. Elemental analysis Pd, 6.90%.
267 mL, 1.20 mmol) is added to a solution of butylamine (60 mL, Pt grafting on APTES/PTES 1/3 silica NPs. Tetraethoxysilane
.60 mmol) in tetrahydrofuran (10 mL) in a Fischer–Porter (536 mL, 2.40 mmol) is added to a solution of butylamine
(
0
bottle at room temperature under vigorous stirring. After 10 min, (119 mL, 1.20 mmol) in tetrahydrofuran (20 mL) in a Fischer–
water (86 mL, 4.8 mmol) is injected into this mixture which is Porter bottle at room temperature under vigorous stirring. After
stirred at 50 1C for seven days. The resulting milky suspension is 10 min, water (173 mL, 9.6 mmol) is injected into this mixture
allowed to cool down and then APTES (84 mL, 0.36 mmol) and which is stirred at 50 1C for seven days. The resulting milky
PTES (83 mL, 0.36 mmol) are added to the mixture that is then suspension is allowed to cool down and then APTES (84 mL,
stirred at 65 1C. After 24 h, the mixture is again allowed to cool 0.36 mmol) and PTES (250 mL, 1.08 mmol) are added to the
down and HDA (97 mg, 0.40 mmol) and Pd
.02 mmol) are added. The bottle is then pressurized under 3 bar again allowed to cool down and HDA (194 mg, 0.80 mmol) and
of dihydrogen and the resulting solution stirred for 24 h at room Pt(dba) (26.5 mg, 0.04 mmol) are added. The bottle is then
2 3
(dba) (18.3 mg, mixture that is then stirred at 65 1C. After 24 h, the mixture is
0
2
temperature. The dihydrogen is then vented and the resulting pressurized under 3 bar of dihydrogen and the resulting solution
black suspension is centrifuged at 5000 rpm for 15 min, yielding stirred for 24 h at room temperature. The dihydrogen is then vented
a black solid at the bottom of the tubes. The supernatant is and the resulting black suspension is centrifuged at 5000 rpm
eliminated and the precipitate is then washed three times with for 15 min, yielding a black solid at the bottom of the tubes. The
tetrahydrofuran. A black powder is obtained after drying the supernatant is eliminated and the precipitate is then washed
precipitate under vacuum for 6 h. Elemental analysis: C, 13.87; three times with tetrahydrofuran. A black powder is obtained
H, 2.42; N, 2.71; Pd, 2.98%.
Pd grafting on APTES/PTES 1/3 silica NPs. Tetraethoxysilane analysis: C, 8.52; H, 2.08; N, 1.88% N, Pt, 2.14%.
536 mL, 2.40 mmol) is added to a solution of butylamine Pd grafting on PhPETES silica NPs. Tetraethoxysilane
119 mL, 1.20 mmol) in tetrahydrofuran (20 mL) in a Fischer– (267 mL, 1.20 mmol) is added to a solution of butylamine (61 mL,
after drying the precipitate under vacuum for 6 h. Elemental
(
(
Porter bottle at room temperature under vigorous stirring. After 0.60 mmol) in tetrahydrofuran (10 mL) in a Fischer–Porter bottle
0 min, water (173 mL, 9.6 mmol) is injected into this mixture at room temperature under vigorous stirring. After 10 min, water
1
which is stirred at 50 1C for seven days. The resulting milky (86 mL, 4.8 mmol) is injected into this mixture which is stirred at
suspension is allowed to cool down and then APTES (84 mL, 50 1C for seven days. The resulting milky suspension is allowed
0.36 mmol) and PTES (250 mL, 1.08 mmol) are added to the to cool down and then PhPETES (258 mL, 0.72 mmol) is added to
mixture that is then stirred at 65 1C. After 24 h, the mixture is the mixture that is then stirred at 65 1C. After 24 h, the mixture
again allowed to cool down and HDA (194 mg, 0.80 mmol) and is again allowed to cool down and then the resulting milky
Pd (dba) (36.6 mg, 0.04 mmol) are added. The bottle is then suspension is centrifuged at 20 000 rpm for 20 min, yielding
2 3
pressurized under 3 bar of dihydrogen and the resulting solution particles as a white solid at the bottom of the tubes. The
stirred for 24 h at room temperature. The dihydrogen is then supernatant is eliminated and the precipitate is then washed
vented and the resulting black suspension is centrifuged at three times with tetrahydrofuran. A last addition of tetrahydro-
5000 rpm for 15 min, yielding a black solid at the bottom of furan (10 mL) allows again the dispersion the NPs and then
the tubes. The supernatant is eliminated and the precipitate is HDA (96.8 mg, 0.40 mmol) and Pd (dba) (18.3 mg, 0.02 mmol)
2
3
then washed three times with tetrahydrofuran. A black powder are added. The bottle is then pressurized under 3 bar of
is obtained after drying the precipitate under vacuum for 6 h. dihydrogen and the resulting solution stirred for 24 h at room
Elemental analysis: C, 11.61; H, 2.66; N, 1.76; Pd, 2.94%.
temperature. The dihydrogen is then vented and the resulting
Pd grafting on APTES/PTES 1/3 silica NPs doubling the black suspension is centrifuged at 5000 rpm for 15 min, yielding
amount of Pd. Tetraethoxysilane (536 mL, 2.40 mmol) is added a black solid at the bottom of the tubes. The supernatant is
to a solution of butylamine (119 mL, 1.20 mmol) in tetrahydro- eliminated and the precipitate is then washed three times with
furan (20 mL) in a Fischer–Porter bottle at room temperature tetrahydrofuran. A black powder is obtained after drying the
under vigorous stirring. After 10 min, water (173 mL, 9.6 mmol) precipitate under vacuum for 6 h. Elemental analysis: Pd, 4.11%.
is injected into this mixture which is stirred at 50 1C for seven
Pd grafting on PhPETES silica NPs doubling the amount of Pd.
days. The resulting milky suspension is allowed to cool down and Tetraethoxysilane (267 mL, 1.20 mmol) is added to a solution of
then APTES (84 mL, 0.36 mmol) and PTES (250 mL, 1.08 mmol) are butylamine (61 mL, 0.60 mmol) in tetrahydrofuran (10 mL) in a
added to the mixture that is then stirred at 65 1C. After 24 h, Fischer–Porter bottle at room temperature under vigorous stirring.
the mixture is again allowed to cool down and HDA (388 mg, After 10 min, water (86 mL, 4.8 mmol) is injected into this mixture
1
.60 mmol) and Pd (dba) (73 mg, 0.08 mmol) are added. which is stirred at 50 1C for seven days. The resulting milky
2 3
The bottle is then pressurized under 3 bar of dihydrogen and suspension is allowed to cool down and then PhPETES (258 mL,
the resulting solution stirred for 24 h at room temperature. The 0.72 mmol) is added to the mixture that is then stirred at 65 1C.
dihydrogen is then vented and the resulting black suspension After 24 h, the mixture is again allowed to cool down and then
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NJC
the resulting milky suspension is centrifuged at 20 000 rpm for
Cross-coupling reaction of styrene and iodobenzene using
2
0 min, yielding particles as a white solid at the bottom of the catalyst 2. In a conditioned Schlenk, 2 (5.0 mg, 2.80 Â
À3
tubes. The supernatant is eliminated and the precipitate is then 10 mmol Pd), anhydrous DMF (2.5 mL), a known amount
washed three times with tetrahydrofuran. A last addition of of 1,3,5-trimethoxybenzene, iodobenzene (313 mL, 2.8 mmol),
tetrahydrofuran (10 mL) allows again the dispersion the NPs styrene (480 mL, 4.2 mmol) and triethylamine (581 mL, 4.2 mmol)
and then HDA (194 mg, 0.80 mmol) and Pd
0
3
2
(dba)
.04 mmol) are added. The bottle is then pressurized under progress of the reaction monitored by GC.
bar of dihydrogen and the resulting solution stirred for 24 h Cross-coupling reaction of styrene and iodobenzene using
3
(36.6 mg, are added. The resulting solution is stirred at 100 1C and the
at room temperature. The dihydrogen is then vented and the catalyst 1. In a conditioned Schlenk, 1 (5.0 mg, 1.50 Â
À3
resulting black suspension is centrifuged at 5000 rpm for 10 mmol Pd), anhydrous DMF (2.0 mL), a known amount
15 min, yielding a black solid at the bottom of the tubes. The of 1,3,5-trimethoxybenzene, iodobenzene (168 mL, 1.5 mmol),
supernatant is eliminated and the precipitate is then washed styrene (257 mL, 2.25 mmol) and triethylamine (311 mL, 2.25 mmol)
three times with tetrahydrofuran. A black powder is obtained are added. The resulting solution is stirred at 100 1C and the
after drying the precipitate under vacuum for 6 h. Elemental progress of the reaction monitored by GC.
analysis: Pd, 5.95%.
Cross-coupling reaction of phenylboronic acid and iodobenz-
Pt grafting on PhPETES silica NPs. Tetraethoxysilane (267 mL, ene using catalyst 2. In a conditioned Schlenk, 2 (5.0 mg, 2.80 Â
À3
1
.20 mmol) is added to a solution of butylamine (61 mL, 10 mmol Pd), anhydrous DMF (4.5 mL), a known amount of
0.60 mmol) in tetrahydrofuran (10 mL) in a Fischer–Porter bottle 1,3,5-trimethoxybenzene, iodobenzene (313 mL, 2.8 mmol), phenyl-
at room temperature under vigorous stirring. After 10 min, water boronic acid (511 mg, 4.2 mmol), potassium carbonate
86 mL, 4.8 mmol) is injected into this mixture which is stirred at (580 mg, 4.2 mmol) and water (0.45 mL) are added. The
0 1C for seven days. The resulting milky suspension is then resulting solution is stirred at 100 1C and the progress of the
allowed to cool down and then PhPETES (258 mL, 0.72 mmol) is reaction monitored by GC.
(
5
added to the mixture that is then stirred at 65 1C. After 24 h, the
Cross-coupling reaction of phenylboronic acid and iodobenzene
mixture is again allowed to cool down and then the resulting using catalyst 1. In a conditioned Schlenk, 1 (3.8 mg, 1.14 Â
À3
milky suspension is centrifuged at 20 000 rpm for 20 min, yielding 10 mmol Pd), anhydrous DMF (2.5 mL), a known amount of
particles as a white solid at the bottom of the tubes. The super- 1,3,5-trimethoxybenzene, iodobenzene (128 mL, 1.14 mmol),
natant is eliminated and the precipitate is then washed three phenylboronic acid (208 mg, 1.71 mmol), potassium carbonate
times with tetrahydrofuran. A last addition of tetrahydrofuran (236 mg, 1.71 mmol) and water (0.25 mL) are added. The
(10 mL) allows again the dispersion the NPs and then HDA resulting solution is stirred at 100 1C and the progress of the
(
194 mg, 0.80 mmol) and Pt(dba) (26.5 mg, 0.02 mmol) are reaction monitored by GC.
2
added. The bottle is then pressurized under 3 bar of dihydrogen
and the resulting solution stirred for 24 h at room temperature.
The dihydrogen is then vented and the resulting black suspension
is centrifuged at 5000 rpm for 15 min, yielding a black solid at
the bottom of the tubes. The supernatant is eliminated and the
precipitate is then washed three times with tetrahydrofuran.
A black powder is obtained after drying the precipitate under
vacuum for 6 h. Elemental analysis: Pd, 4.57%.
Acknowledgements
A.C. is indebted to the Spanish Ministerio de Ciencia y Tecnolog ´ı a
for a Post-Doctoral grant I. S. is indebted to the ‘‘Departamento de
Industria de la Comunidad Foral de Navarra’’ for a grant. We thank
C ´e cile Garcia Marcelot for helpful treatment of HRTEM images.
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