Weakly-coordinated stable platinum nanocrystals

Article abstract of DOI:10.1039/c2ce25904d

Stable platinum nanocrystals with small diameters (1.0-2.3 nm), that are stable for a long time, with narrow size distributions were easily and reproducibly prepared without any additional stabilizers in glycol, glycerol, the ionic liquids (ILs) 1-n-butyl-3-methyl-imidazolium tetrafluoroborate [BMIm][BF₄] and N-butyl-N-trimethyl-ammonium bis(trifluoromethylsulfonyl)imide [N₄₁₁₁][NTf₂] or diphenylmethane (CH₂Ph₂) by thermal, photolytic or microwave assisted decomposition of the organometallic precursor methylcyclopentadienyl-trimethylplatinum(iv), (MeCp)PtMe₃. Decomposition of the easily dispensible, air and moisture stable organometallic Pt(iv) precursor (MeCp)PtMe₃ leads to well defined, small, crystalline and longterm stable Pt-nanoparticle (Pt-NP) dispersions without any additional surface-capping ligands. The Pt-NP/IL dispersion was shown to be a highly active catalyst (TOF 96000 h^-1 at 0.0125 mol% Pt and quantitative conversion) for the biphasic hydrosilylation of phenylacetylene with triethylsilane, to the distal and proximal products triethyl(2- and1-phenylvinyl)silane.

Full text of DOI:10.1039/c2ce25904d

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PAPER
Weakly-coordinated stable platinum nanocrystals{
Dorothea Marquardt,a Juri Barthel,b Markus Braun,a Christian Gantera and Christoph Janiak*a
Received 6th June 2012, Accepted 20th August 2012
DOI: 10.1039/c2ce25904d
Stable platinum nanocrystals with small diameters (1.0–2.3 nm), that are stable for a long time, with
narrow size distributions were easily and reproducibly prepared without any additional stabilizers in
glycol, glycerol, the ionic liquids (ILs) 1-n-butyl-3-methyl-imidazolium tetrafluoroborate
[BMIm][BF4] and N-butyl-N-trimethyl-ammonium bis(trifluoromethylsulfonyl)imide [N4111][NTf2] or
diphenylmethane (CH2Ph2) by thermal, photolytic or microwave assisted decomposition of the
organometallic precursor methylcyclopentadienyl-trimethylplatinum(IV), (MeCp)PtMe3.
Decomposition of the easily dispensible, air and moisture stable organometallic Pt(IV) precursor
(MeCp)PtMe3 leads to well defined, small, crystalline and longterm stable Pt-nanoparticle (Pt-NP)
dispersions without any additional surface-capping ligands. The Pt-NP/IL dispersion was shown to be
a highly active catalyst (TOF 96 000 h21 at 0.0125 mol% Pt and quantitative conversion) for the
biphasic hydrosilylation of phenylacetylene with triethylsilane, to the distal and proximal products
triethyl(2- and1-phenylvinyl)silane.
Pt(hfa)2.13 Reducing agents, capping ligands or surfactants for
Introduction
aqueous systems are borohydrides, hydrazine, dihydrogen, citrate
Platinum-based nanocrystals, including in the form of alloys and
and ascorbic acid. Polyols, diols or amines are used in organic
solvents.13
core-shell composite nanoparticles, are widely used in chemical
syntheses, magnetic, catalytic, and biomedical applications and
in energy conversion devices.1,2 Many different reactions, e.g.
Organometallic hydrocarbyl precursors are favored metal
sources for chemical vapor deposition (CVD) processes growing
thin films of metals18,20,21 and also became important in metal
oxidations,3 oxygen reduction,4,5 hydrogenation,6,7 dehydro-
genation, hydrosilylation8–11 and C–C coupling reactions that
nanoparticle chemistry. The advantage of hydrocarbyl precur-
are essential in industrial processes are catalyzed by Pt and its
sors containing alkyl, allyl, olefin or cyclopentadienyl groups is
alloys.12,13 Pt-based nanoparticles (Pt-NPs) are used as electro
the satisfactory thermal, photolytic or sonolytic decomposition
catalysts4,5 on carbon or SiO2 supports in electrode layers in
to give bare metal atoms condensing in the reaction medium to
almost all proton exchange membrane fuel cell devices.1,13
The catalytic potential and selectivities of nano catalysts
depend on the sizes and shapes of the nanoparticles.14–16 Metal
alloy, core-shell and yolk-shell17 nanoparticles can be prepared
using colloid solution techniques8,14 from metal salts or
organometallic precursors with reducing agents at high tempera-
tures, by photolysis or sonolysis.12,18 Reaction conditions
including temperature, concentration or gas pressure affect the
resulting nanostructures.13
nanoparticles with a contaminant-free surface.18,22 This synthetic
approach is notably capable for the syntheses of precious metal-
nanoparticles.18 For platinum films from CVD processes the
following organometallic complexes are favored because of their
volatility, thermal stabilities and clean decomposition routes: cis-
PtCl2(CO)2 (1), (g5-C5H5)PtMe(CO) (2) or Pt(PF3)4 (3), Pt-
complexes containing allyl- and (di)olefin ligands (4–6),
CpPtMe3 and its air and moisture stable derivates (7).21
Room temperature ionic liquids (ILs) are known to stabilize
metal nanoparticles without additional capping ligands.
Agglomeration of particles is prevented by weakly-coordinating
IL-anions and -cations forming protective shells surrounding the
particles.23–28 The IL appears to limit the diffusion and growth
of nanoparticles so that small particles form.
Classical platinum precursors can be chosen from hexachloro-
platinic acid, H2PtCl6 potassium tetrachloroplatinate, K2PtCl4,
platinum bis-dibenzylidene acetone, Pt2(dba)3,6,19 platinum acet-
ylacetonate, Pt(acac)2 or platinum hexafluoroacetylacetonate,
aInstitut fu¨r Anorganische Chemie und Strukturchemie, Heinrich-Heine
Universita¨t Du¨sseldorf, Universita¨tsstrabe 1, D-40225 Du¨sseldorf,
Germany. E-mail: janiak@uni-duesseldorf.de; Fax: +49-(0)211-8112287
bGemeinschaftslabor fu¨r Elektronenmikroskopie RWTH-Aachen, Ernst
Ruska-Centrum fu¨r Mikroskopie und Spektroskopie mit Elektronen,
D-52425, Ju¨lich, Germany
Here we describe the decomposition of the precursor
methylcyclopentadienyl-trimethylplatinum(IV), (MeCp)PtMe3
for Pt-NP generation (Scheme 1). The easily accessible, storable,
short-term air and moisture stable (MeCp)PtMe3 complex was
used as a precursor for CVD processes21 with Pt metal
deposition29 and as a highly efficient organometallic photo-
initiator for hydrosilylation.10,11 We decomposed the precursor
{ Electronic supplementary information (ESI) available: 1H-NMR
spectra of the hydrosilylation reaction for entry 3 in Table 2. See DOI:
10.1039/c2ce25904d
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are absorbed by the solvent molecules. Once metal particles have
started to form, the microwaves will couple into these nucleation
seeds to form hot spots where further decomposition and particle
growth will occur. Thus, energy saving microwave irradiation
accelerates the process of decomposition.31 Microwave-assisted
heating was successfully used for the microwave-active ionic liquids
1-n-butyl-3-methyl-imidazolium tetrafluoroborate [BMIm][BF4] and
N-butyl-N-trimethyl-ammonium bis(trifluoromethylsulfonyl)imide
[N4111][NTf2]. For comparison UV decomposition of (MeCp)PtMe3
was carried out in [BMIm][BF4] and [N4111][NTf2].
Decomposition of (MeCp)PtMe3 gave dispersions of nano-
crystalline Pt-NPs with small average diameters that were stable
for up to 11 months, typically between 1.0–2.3 nm in diameter
and with narrow size distributions. Sizes depended somewhat
on the dispersive medium glycol, glycerol, ionic liquids
[BMIm][BF4] and [N4111][NTf2] or diphenylmethane (CH2Ph2)
and on the means of decomposition. The stability of the
dispersions explicitly includes ligand-free Pt-NPs in ILs and in
CH2Ph2.
Scheme 1 Chemical structures of Pt precursors for CVD.
in glycol, glycerol, ionic liquids and diphenylmethane without
any additional stabilizers (Scheme 2). Each of the chosen
reaction media represent a different stabilizing principle, notably
strong coordinating ligands in glycol and glycerol, weakly-
coordinating anions and cations in ILs and phenyl-p-metal
interactions30 in diphenylmethane (CH2Ph2). In particular we
focused on the synthesis of stable platinum nanocrystals in ionic
liquids (Pt-NP/IL).
Pt-NP dispersions were analyzed by high-resolution transmis-
sion electron microscopy (HRTEM) and high-angle annular
dark-field-scanning transmission electron microscopy (HAADF-
STEM). Particle diameters were determined from HAADF-
STEM images. TEMs were carried out both on freshly prepared
samples and on (the same) samples after several months. Particle
sizes were measured from HAADF-STEM micrographs because
this mode offers advantages over the HRTEM mode concerning
the recognition of metal nanoparticles below 2 nm. The
HAADF-STEM mode allows the identification of a higher
number of particles. This leads to a smoother distribution of
particle sizes displayed in a histogram to be approximately fitted
by a Gaussian function.32 Table 1 provides a listing of the
diameters and their distributions of the obtained Pt-NPs
according to solvent, method of decomposition and sample age.
Results and discussion
The decomposition of the platinum hydrocarbyl complex
(MeCp)PtMe3 in different solvents with different stabilizing
effects leads to dispersions of small and well defined Pt-NPs. For
the desired Pt-NP dispersions of 0.5–2 mass% a defined amount
of the complex (MeCp)PtMe3 was suspended in the solvent
under gentle warming (see Experimental section). Then, the
Pt(IV) complex was decomposed by reduction to Pt(0) under
microwave assisted heating, under conventional thermal treat-
ment or under UV irradiation. The advantage of such hydro-
carbyl precursors is their clean decomposition releasing volatile
small organic fragments of the ligands which can be easily
removed under reduced pressure. The decomposition in glycol,
glycerol, ionic liquids and diphenylmethane reproducibly leads
to small platinum nanoparticles of high crystallinity.
Pt-NPs in glycol and glycerol
The analysis of a 0.5 mass% dispersion of Pt-NPs prepared from
(MeCp)PtMe3 in glycol or glycerol by conventional heating up to
150 uC–200 uC for 1.5 h reproducibly gave particle diameters
between 0.3 and 3.6 nm with an average diameter of 1.7 ¡
0.5 nm (Table 1, entry 1) in glycol (Fig. 1) and 1.5 ¡ 0.5 nm
(Table 1, entry 4) in glycerol (Fig. 2) in an aged sample.
The microwave-assisted decomposition of the organometallic
Pt precursor in glycol (Fig. 3) led to particle diameters of 2.3 ¡
0.7 nm (Table 1, entry 2) for a freshly prepared sample and 1.7 ¡
0.4 nm (Table 1, entry 2) for a 67 days old sample.
The decomposition of (MeCp)PtMe3 can be initiated under
photolytic and thermal conditions. Thermal decomposition by
conventional heating was used for the solvent diphenylmethane
with microwave-inactive molecules and for glycerol and glycol in
comparison to microwave-assisted decomposition of the com-
plex. Photolytic decomposition is a fast and energy-saving
method, comparable to microwave-assisted heating. For micro-
wave active solvents it is possible to provide fast and efficient
thermal heating by microwave irradiation. Initially the microwaves
Photolytic decomposition of (MeCp)PtMe3 led to stable
nanoparticle dispersions of 0.5 mass% in glycol (Fig. 4) with
an average particle diameter of 1.9 ¡ 0.7 nm (Table 1, entry 3).
Resulting particle diameters were smaller in comparison to
particles generated by microwave techniques (2.3 ¡ 0.7 nm). All
three methods reliably afford Pt-NP dispersions of small
diameters in glycol as a stabilizing solvent.
Pt-NPs in [BMIm][BF4] and [N4111][NTf2]
Pt-NP dispersions in the ionic liquids [BMIm][BF4] and
[N4111][NTf2] were reproducibly synthesized by conventional
heating, fast (minutes), hence, energy saving microwave-assisted
Scheme 2 Decomposition of (MeCp)PtMe3 under different conditions
(MW microwave irradiation; hn photolysis; D conventional thermal
heating) and in different media.
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Table 1 Pt-NPs from (MeCp)PtMe3 in different solvents and from
different decomposition methodsa
Particle diameter and
standard deviation s/[nm]
Fresh
Older
Decomposition
methodb
Entry Solvent
(1 day)
(57–331 days)
1
2
3
4
5
6a
glycol
glycol
glycol
glycerol
[BMIm][BF4]
D
—
1.7 ¡ 0.6c
1.7 ¡ 0.4e
1.9 ¡ 0.6g
1.5 ¡ 0.5h
—
1.5 ¡ 0.3
(57 days)j
1.5 ¡ 0.5
(228 days)k
1.7 ¡ 0.1m
—
MW
hn
D
2.3 ¡ 0.7d
1.9 ¡ 0.7f
—
Fig. 2 Example of HAADF-STEM image of Pt-NPs prepared from
(MeCp)PtMe3 in glycerol under thermal decomposition (entry 4 in
Table 1, 114 days old sample). Particle diameter distribution in
histogram.
D
3.9 ¡ 1.5i
—
[BMIm][BF4] MW
[BMIm][BF4] MW
[BMIm][BF4] hn
6b
—
typical small particle diameters of 1.4 ¡ 0.4 nm (Table 1, entry 8,
Fig. 6) similar to diameters from microwave or photolytic
decomposition.
7
8
9
1.0 ¡ 0.3l
1.4 ¡ 0.4n
1.4 ¡ 0.3o
1.1 ¡ 0.5p
—
[N4111][NTf2]j
D
[N4111][NTf2]j MW
—
[N4111][NTf2]j hn
1.2 ¡ 0.4q
1.6 ¡ 0.6r
2.3 ¡ 0.6t
b
HRTEM images and HAADF-STEM images of Pt/
[BMIm][BF4] dispersions from microwave syntheses were taken
from a Pt-NP dispersion 57 days after synthesis and the same
sample was analyzed again by HAADF-STEM after 228 days
(more than seven months) with no apparent change in particle
diameter and distribution (Table 1, entries 6a and 6b; Fig. 8).
The particle diameter distribution of the 57 days old sample was
1.5 ¡ 0.3 nm (0.8–5.1 nm) and the analysis of the same sample
five months later gave very similar diameters from 0.8–5.1 nm
with a distribution of 1.5 ¡ 0.5 nm. These results show that Pt-
NP dispersions in the IL [BMIm][BF4] were stable over several
months without increase in average particle diameter or
distributions.
10
11
12
a
CH2Ph2
CH2Ph2
D
hn
2.3 ¡ 0.7s
Dispersion contained 0.5 mass% Pt unless noted otherwise.
D
conventional thermal heating; MW microwave irradiation; hn
photolysis. 719 particles, sample age 122 days. 257 particles. 311
c
d
e
f
g
particles, sample age 67 days. 86 particles. 198 particles, sample age
h
67 days. 648 particles, sample age 114 days. 474 particles. 505
i
j
k
particles, sample age 57 days. 219 particles, sample age 228 days.
l
m
n
o
194 particles. 230 particles, sample age 67 day. 310 particles. 92
p
q
particles.
2 mass% Pt 703 particles. 824 particles, sample age
s t
331 days. 518 particles, sample age 57 days. 412 particles. 345
particles, sample age 67 days.
r
decomposition or by photolysis of (MeCp)PtMe3. Complete
1
decomposition was verified by H-NMR spectroscopy through
The analysis of
a freshly prepared Pt-NP/[N4111][NTf2]
dispersion from microwave irradiation gave a particle diameter
and distribution of 1.4 ¡ 0.3 nm (Table 1,entry 9, Fig. 9).
The decomposition of (MeCp)PtMe3 under UV-irradiation
was also carried out in ionic liquids. The resulting dispersions
of Pt-NPs in [BMIm][BF4] (0.5 mass%) and [N4111][NTf2]
(2 mass%) were characterized by HAADF-STEM (Fig. 10 and 11).
the absence of the characteristic signals from the Pt–Me groups
of (MeCp)PtMe3 (singlet at d = 0.84 ppm, flanked by Pt-satellites
JPt,H = 82 Hz).29
The dispersions of Pt-NPs in ILs were stable over several
months when kept under nitrogen atmosphere. The stability was
verified by HAADF-STEM and HRTEM analysis for Pt-NPs in
[BMIm][BF4] and [N4111][NTf2] for samples of different ages
synthesized by thermal decomposition (Table 1, entry 5; Fig. 5
and Table 1, entry 8; Fig. 6), microwave assisted decomposition
(Table 1, entry 6a, 6b; Fig. 7 and 8 and Table 1, entry 9; Fig. 9)
and photolytic decomposition (Table 1, entry 7; Fig. 10 and
Table 1, entry 10; Fig. 11).
Thermal decomposition of the Pt(IV) precursor by conven-
tionally heating in [BMIm][BF4] led to a somewhat larger
particle diameter distribution of 3.9 ¡ 1.5 nm (Table 1, entry 5,
Fig. 5). In the IL [N4111][NTf2] thermal decomposition gave
Fig. 3 Examples of HAADF-STEM images of Pt-NPs prepared from
(MeCp)PtMe3 in glycol under microwave-assisted decomposition
(Table 1, entry 2) top: freshly prepared sample, bottom: older sample
(67 days). Particle diameter distributions in histograms.
Fig. 1 Example of HAADF-STEM image of Pt-NPs prepared from
(MeCp)PtMe3 in glycol by thermal decomposition (Table 1, entry 1,
122 days old sample). Particle diameter distribution in histogram.
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Fig. 6 HAADF-STEM image of Pt-NPs prepared from (MeCp)PtMe3
in [N4111][NTf2] by thermal decomposition (Table 1, entry 8, 1 day
sample).
Fig. 4 Examples of HAADF-STEM images of Pt-NPs prepared from
(MeCp)PtMe3 in glycol by photolytic decomposition (Table 1, entry 3)
top: freshly prepared sample; bottom: older sample (57 days). Particle
diameter distributions in histograms.
Photolytic decomposition of (MeCp)PtMe3 in ionic liquids led
to very small Pt-NPs with diameter distributions of 1.0 ¡ 0.3 nm
or 1.1 ¡ 0.5 nm for freshly prepared samples and 1.7 ¡ 0.1 nm
or 1.2 ¡ 0.4 nm for aged samples (67 or 331 days) in
[BMIm][BF4] or [N4111][NTf2], respectively (Table 1, entry 7 or
10). Pt-NP dispersions in [N4111][NTf2] were very stable without
a significant change of particle diameter distributions. Thus, the
smallest and most homogenous dispersions of ‘‘ligand-free’’
Pt-NPs were prepared by photolytic decomposition of
(MeCp)PtMe3 in ionic liquids.
Platinum nanoparticles have also been prepared by hydrogen
reduction (4 atm, 75 uC) of Pt2(dba)3 (dba = bis-dibenzylidene
acetone) dispersed in the ionic liquids [BMIm][PF6] or
[BMIm][BF4] with mean diameters of 2.3 nm or 3.4 nm,
respectively.6,7
Fig. 7 HRTEM image of nanocrystalline Pt-NPs prepared from
(MeCp)PtMe3 in [BMIm][BF4] by microwave irradiation (Table 1, entry
6a).
Pt-NPs in CH2Ph2
Pt-NPs prepared in CH2Ph2 by conventional heating up to
200 uC for 1.5 h gave particle diameters between 0.3 nm and
3 nm with an average of 1.6 ¡ 0.6 nm (Table 1, entry 11; Fig. 12).
The fast photolytic decomposition of (MeCp)PtMe3 resulted
in a 0.5 mass% Pt-NP dispersion in diphenylmethane within ten
minutes. Particle diameters were determined from HAADF-
STEM images to 2.3 ¡ 0.7 nm both for a freshly prepared and
Fig. 8 HAADF-STEM images of Pt-NPs prepared from (MeCp)PtMe3
in [BMIm][BF4] under microwave-assisted decomposition. top: 57 days
after synthesis (Table 1, entry 6a); bottom: 228 days after synthesis
(Table 1, entry 6b). Particle diameter distribution in histogram.
Fig. 5 HAADF-STEM image of Pt-NPs prepared from (MeCp)PtMe3
in [BMIm][BF4] by thermal decomposition (Table 1, entry 5, 1 day
sample).
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Fig. 9 HAADF-STEM image of Pt-NPs prepared from (MeCp)PtMe3
in [N4111][NTf2] under microwave assisted decomposition (entry 9 in
Table 1, 1 day sample). Particle diameter distribution in histogram.
two-month aged sample (Table 1, entry 12; Fig. 13). Thus, the
average diameter and its distribution did not change over a
period of two months. These particle sizes are only slightly larger
in comparison to Pt-NP sizes from photolytic decompositions in
glycol and ionic liquids. Even though Pt-NPs were prepared in
CH2Ph2 with slightly larger diameters, the stabilizing effect of
CH2Ph2 seems to be very efficient. The stabilizing effect for Pt-
NPs in diphenylmethane in the absence of conventionally
coordinating ligands can be due to phenyl-p–Pt interactions
between the phenyl ring and Pt surface atoms.33
Fig. 11 HAADF-STEM images of Pt-NPs freshly prepared by photo-
lytic decomposition from (MeCp)PtMe3 in [N4111][NTf2]; top: freshly
prepared sample; bottom: older sample (331 days) (Table 1, entry 10).
Particle diameter distributions in histograms.
Photolysis of related CpPtMe3 in the presence of
HMe2SiOSiMe3 in cyclohexane gave Pt-NPs between 1.0–
2.5 nm.11
By comparison, the Pt-NP sizes obtained by different methods
of decomposition from (MeCp)PtMe3 in different solvents are
very similar, in particular for the aged samples (Table 1).
Noteworthy, Pt-NP dispersions in weakly-coordinating solvents
such as non-functionalized ILs or diphenylmethane contain
particles that are as small (namely around 1.5 nm) as in more
coordinating solvents like glycol and glycerol. One may have
expected that the stabilizing effect of coordinating ligands like
glycol or glycerol with their hydroxy functionalities or even the
supramolecular network of anions and cations in ILs would
stabilize more efficiently than phenyl-p-metal interactions in
Fig. 12 Example of a HAADF-STEM image of Pt-NPs prepared in
CH2Ph2 by conventional heating (150 uC) (Table 1, entry 11, 57 day old
sample). Particle diameter distribution in histogram.
Fig. 13 Examples of HAADF-STEM images of Pt-NPs prepared in
CH2Ph2 by photolytic decomposition from (MeCp)PtMe3 (Table, entry
9); top: freshly prepared sample, bottom: 67 day old sample. Particle
diameter distributions in histograms.
Fig. 10 HAADF-STEM images of Pt-NPs prepared by photolytic
decomposition from (MeCp)PtMe3 in [BMIm][BF4] (Table 1,entry 7);
top: freshly prepared sample, bottom: older sample (67 days). Particle
diameter distributions in histograms.
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CH2Ph2. Yet, the diameter of Pt-NPs in CH2Ph2 is the same for
an aged sample from conventional heating and only slightly
larger for a fresh sample from photolysis. Thus, the stability of
Pt-NP dispersions in weakly-coordinating solvents does not trail
the stability in the stronger-coordinating media. The samples of
Pt-NPs prepared in glycol, glycerol, ILs and diphenylmethane
were analyzed by HAADF-STEM for up to eleven months
(331 days) standing in their mother liquor at room temperature
under nitrogen atmosphere and showed no significant differ-
entiation in particle diameter and its distribution.
Crystallinity of the small Pt-NPs is evident from the HRTEM
in Fig. 6 which clearly shows the lattice planes. Atom lattice
parameters of nanocrystalline Pt-NPs were determined from
Fourier power spectra of HRTEM image sections. This analysis
gave an average value of interplanar d-spacing for each of the
analyzed particles. The lattice parameters matched the theore-
tical value (0.39237 ¡ 3)34 if a = 0.392 nm for the face-centered
cubic (fcc) lattice of platinum metal. A detailed analysis of
HRTEM image sections showed primarily particle orientations
with the {110} and some with the {211} zone axis facing up
(Fig. 14).
In this context we note a systematic investigation on the
structure and stability of polyhedral Pt nanocrystals by atomistic
simulations Pt nanocrystals, enclosed by high-index facets of
{310}, {311}, and {331} are suggested to possess better stability
and higher dangling bond density of surface compared with
those ones with low-index facets, such as {111}, {100} and
{110}. Octahedral nanocrystals with {111} facets, though they
have excellent structural and thermal stabilities, present the
lowest dangling bond density of the surface.35
Fig. 14 Left: HRTEM image section, cut from original image of Pt-NPs
prepared in [BMIm][BF4] by MW irradiation (cf. Fig. 7). Middle: Digital
Fourier power spectra from the cut image region. Right: Unit cell
orientation.
mixture (entries 7–9) a temperature of only 90 uC was reached
during 5 min MW irradiation and the conversion dropped to 35–
38%. To reach quantitative conversion again at 0.125 mass% Pt
in IL it was necessary to bring the amount of IL to 90 mL in
order to reach the required reaction temperature of 150 uC
within 5 min again (compare entry 9 and 10). This addition of IL
lowered the Pt mass% in IL to 0.06 mass%. At these conditions a
maximum turnover frequency (TOF) of 96 000 h21 was achieved
(entry 10).
In a recent study by Pelzer et al., platinum nanoparticles 2 nm
in diameter and containing ca. 200 atoms covered with
n-octylsilyl groups were prepared. These Pt-NPs had a cubo
octahedral shape with edges with a length of three atoms.36
Biphasic Pt-NP catalyzed hydrosilylation reactions
The Pt-NP/[BMIm][BF4] dispersions were tested as catalysts for
biphasic reactions of phenylacetylene with triethylsilane
(Scheme 3, Table 2). A low solubility of the hydrosilylation
starting materials and products with the IL led to the formation
of two phases. This reaction mixture was chosen as a model
system for the hydrosilylation of terminal acetylenes with
alkylsilanes. The catalytic reaction was initially carried out
under conventional heating in an oil bath. Performing the
reaction under MW conditions was, however, a time and energy
saving alternative to the conventional oil bath heating.
During the hydrosilylation reaction the product mixture
turned from the light yellow color of the substrate mixture to
dark yellow or orange. In turn the dark brown Pt-NP/IL turned
to light brown. These color changes of the reaction mixture
could hint to Pt leaching from the IL to the product phase, but
no traces of Pt-NP could be found in the product phase by
HAADF-STEM. At the same time, after separating the product
phase from the IL phase of the run in entry 10 and adding fresh
substrate the conversion was negligible (3%, entry 11) and no
reactivity was observed anymore when this procedure was
repeated (entry 12). Thus, we suggest catalyst poisoning, e.g.,
by in situ formed N-heterocyclic carbenes (NHCs) from the 1,4-
alkyl substituted imidazolium cation of the ionic liquid. Those
Conventional oil bath heating gave lower conversions at
longer reaction times than the same reactions under microwave
(MW) heating (compare entries 1 to 3 and 2 to 4 in Table 2).
With MW heating quantitative conversion could be reached in a
reaction time as short as 10 or even 5 min under similar
conditions (entries 5 and 6 in Table 2). At the short reaction time
of 5 min the temperature reached in the reaction mixture through
MW heating critically depended on the amount of IL or Pt-NP/
IL dispersion and varied from 150 uC to 90 uC (Table 2, entry 6–
10). Phenylacetylene and triethylsilane are MW inactive. With a
lower amount of MW-active IL (20 instead of 40 mL) or Pt-NPs
(0.25 or 0.125 mass% instead of 0.5 mass% in IL) in the reaction
Scheme 3 Biphasic hydrosilylation of phenylacetylene with triethylsilane
by Pt-NP dispersions in [BMIm][BF4] to yield distal triethyl(styryl)silane
[triethyl(2-phenylvinyl)silane] and proximal triethyl(1-phenylvinyl)silane.
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Table 2 Biphasic hydrosilylation of phenylacetylene with triethylsilane, by Pt-NP dispersions in [BMIm][BF4]a
Pt catalyst/
mmol
Molar ratio
substrate : catalyst
Mass% Pt
in IL
Reaction
time/min
Ratiod distal :
proximal
Entry
V(IL)/ml
Conversionb/%
TOFc/h21
1
2
3
4
5
6
7
8
9
1.25
12.5
12.5
1.25
2.5
4000 : 1
400 : 1
400 : 1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.25
0.125
0.06
0.06
0.06
410
40
410
20
40
40
20
40
40
90
90
90
60, D
60, D
6
20
10
5
5
5
5
5
10
50
100
90
100
100
38
35
400
200
4000
4 : 1
4 : 1
4 : 1
2 : 1
2 : 1
1.6 : 1
3.3 : 1
2.6 : 1
2 : 1
1.4 : 1
4 : 1
—
4000 : 1
2000 : 1
2000 : 1
4000 : 1
4000 : 1
8000 : 1
8000 : 1
8000 : 1
8000 : 1
10 900
11 800
24 100
18 300
16 900
34 700
96 000
2900
2.5
1.25
1.25
0.625
0.625
0.625
0.625
36
100
3
10
11e
12e
a
5
5
no reaction
—
Phenylacetylene (0.54 ml, 5.0 mmol) and triethylsilane (0.79 ml, 5.0 mmol). Microwave (MW) heating, except for entries 1 and 2 with thermal
heating (D). The pressure inside the MW reactor was set to pmax = 3.0 bar. The applied MW energy depended on the pressure and so the reaction
temperature varied from 90 uC–150 uC. The maximum reaction temperature was set to 150 uC. It was observed that the reaction temperature
b
which could be reached depended on the amount of IL or Pt-NPs in the biphasic reaction mixture. Conversion determined from crude products
by 1H-NMR spectroscopy. TOF = turnover frequency [mol(product)/mol(Pt) 6 h21]. Determined from 1H-NMR spectra of the reaction
c
d
mixture. See example spectrum for entry 3 as Fig. S1 in ESI.{ 1H-NMR signals for the mixture of distal (main product) and proximal (byproduct)
products were taken from the literature.37 Entry 11 and 12 are the second and third run, respectively, of entry 10 after separating the substrate
phase from the IL phase and adding fresh substrate.
e
The ether and aqueous phase was separated. The aqueous phase
was extracted twice with diethyl ether. The combined organic
phases were washed with brine and dried over Na2SO4. The
organic solvent was removed under reduced pressure and the
dark red colored crude product was washed with acetone. A
light-brown solid was obtained in 90% yield.
NHCs are known as strong ligands coordinating to single
transition-metal centers.38,39
Distillation of the product phase from the Pt-NP/IL dispersion
inhibited catalyst activity completely. A second run gave no
conversion.
Particle aggregation can be ruled out as a reason for
deactivation. HAADF-STEM analysis of the product mixtures
from two catalytic runs still showed Pt-NPs with unchanged
average diameters of about 1.5 nm and small distribution of
¡0.4 nm (Fig. 15 and 16).
1H-NMR (200 MHz, CDCl3): d (ppm) = 1.73 (s,2JPt–H
=
77 Hz). 13C–NMR (126 MHz, CDCl3): d (ppm) = 13.2 (s, JPt–C
= 686 Hz).
1
Synthesis of (g5-methylcyclopentadienyl)trimethylplatinum(IV)
Experimental
Methylcyclopentadienyl-dimer was cracked to the monomers by
distillation (180 uC oil bath temperature) and stored at 220 uC in
the dark. Methylcyclopentadiene monomer (0.64 g, 7.99 mmol)
was dissolved in THF (8 mL) and cooled to 278 uC.
n-Butyllithium (4.5 mL, 7.20 mmol) was added dropwise,
keeping the temperature at 278 uC. Then the cooling bath was
removed and the reaction mixture slowly warmed to room
temperature under continuous stirring. A yellow precipitate
formed. THF was added to resolve the solid and a light-yellow
solution was formed. This MeCpLi solution was added to a
dispersion of (Me3PtI)4 (2.36 g, 6.43 mmol) in THF (15 ml) and
the mixture stirred for 60 min at RT. Water (20 ml) was added
and the organic and aqueous phase separated. The aqueous
Methylcyclopentadienyl-dimer was received from Alfa Aeser,
n-butyllithium (1.6 mol L21 in hexane), methyllithium (1.6 mol L21
in diethyl ether), diphenylmethane 99%, anhydrous glycerol, 99.5%,
1,2-dibromoethane 98%, potassium hexachloroplatinate(IV) 98%
and glycol 99.5% from Sigma Aldrich, potassium iodide 99.5% from
Merck, ionic liquids 1-n-butyl-3-methyl-imidazolium tetrafluorobo-
rate [BMIm][BF4] and tri-n-butyl-methyl-ammonium bis(trifluoro-
methylsulfonyl)imide [N4111][NTf2] were purchased from IoLiTec.
The ILs were dried and degassed in vacuum (1 h, 85 uC oil bath
temperature).
(MeCp)PtMe3 was obtained through the reaction of MeCpLi
with (Me3PtI)4.40,41
Synthesis of trimethylplatinum(IV)-iodide tetramer, (Me3PtI)4
Potassium hexachloroplatinate(IV) (4.00 g, 8.23 mmol) was
dispersed in THF (8 mL) and the yellow dispersion was cooled
to 0 uC. Methyllithium (4.8 mL, 7.72 mmol) was added in 2 ml
steps over a period of 4 h at 0 uC. The cooling bath was removed
and the yellow–brown dispersion was slowly warmed to room
temperature. The resulting white slurry was again cooled to 0 uC
and 1,2-dibromoethane (3.68 mL, 42.8 mmol) was added over a
period of 60 min. For quenching an aqueous solution of
hydrochloric acid (c = 2 mol L21, 1 ml), saturated with
potassium iodide was added dropwise to the reaction mixture.
Fig. 15 HAADF-STEM example image of Pt-NPs in the product
mixture after hydrosilylation catalysis: Pt-NPs after catalysis according
to entry 1 in Table 2 (particle diameter 1.5 ¡ 0.4 nm from analysis of
194 particles). Particle diameter distribution in histogram.
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dispersion on a carbon coated copper grid lying on filter paper.
Excess dispersion was removed by covering the grid with filter
paper. After lying for a few minutes, the grid was washed with
acetone (for IL and diphenylmethane) or ethanol (for glycol and
glycerol) and it was plasma cleaned (10 s) to avoid contamina-
tion by decomposition of organic compounds during micro-
scopy.
Pt-NP synthesis-typical protocol
Fig. 16 HAADF-STEM example images of Pt-NPs in the product
mixture after hydrosilylation catalysis: Pt-NPs after catalysis according
to entry 2 in Table 2 (particle diameter 1.6 ¡ 0.4 nm from analysis of
202 particles. Particle diameter distribution in histogram.
Pt-NP dispersions with 0.5 or 2 mass% Pt (relative to solvent)
were prepared by suspending the required amount of
(MeCp)PtMe3 (Table 3) under gentle warming in glycol,
glycerol, [BMIm][BF4], [N4111][NTf2] or diphenylmethane. The
decomposition reaction was carried out either under conven-
tional thermal heating for 1.5 h in an oil bath at 150 uC (glycol)
or 200 uC (glycerol, CH2Ph2); under photolysis (lmax = 254 nm,
10 min) or under microwave irradiation (20 W, 0.5 min run time,
5.5 min hold time). The gaseous byproducts were removed under
reduced pressure.
phase was extracted with hexane and the combined organic
phases were washed with brine and dried over Na2SO4. The
organic solvent was removed under reduced pressure. The crude
product was purified by sublimation (25 uC, 2 mbar) to give a
light-yellow solid (yield 94%).
1H-NMR (200 MHz, CDCl3): d (ppm) = 5.55–5.50 (m, 2H,
3
Cp–H–a), 5.29–5.23 (m, 2H, Cp–H–b), 1.96 (s, 3H, JPt–H
=
6 Hz, Cp–CH3), 0.84 (s, 9H, 2JPt–H = 82 Hz, Pt–CH3). 13C–NMR
Hydrosilylation reactions
1
(126 MHz, CDCl3): d (ppm) = 114.61 (s, JPt–C = 14 Hz, Cp–C-
All reactions by thermal or microwave-assisted heating were
carried out using Schlenk-techniques under inert nitrogen
atmosphere in dried Schlenk-tubes. For the microwave-assisted
heating microwave vials with a Teflon septum were used. These
vials were charged with starting materials and catalysts having
the vial in a Schlenk tube under inert gas.
1
ipso), 95.91 (s, JPt–C = 4 Hz, Cp–C–a), 91.68 (s, JPt–C = 6 Hz,
1
1
Cp–C–b), 11.85 (s, Cp–CH3), –18.37 (s, JPt–C = 716 Hz,
Pt–CH3).
Instrumentation
Photolysis: Laboratory UV reactor system (150 W Hg-UV
immersion lamp TQ 150, lmax = 254 nm, Heraeus GmbH,
Hanau).
In a typical procedure, phenylacetylene (0.54 ml, 5.0 mmol)
and triethylsilane (0.79 ml, 5.0 mmol) were put in the reaction
vessel and then the required amount of Pt-NP/IL dispersion
(mostly 0.5 mass% Pt) (0.625–12.5 mmol Pt, see Table 2) was
added and a two-phase system was formed with a slightly yellow
upper substrate phase and a dark brown lower Pt/IL phase.
The reaction mixture was heated in an oil bath for 1 h at 90 uC
(oil bath temperature) (Table 2, entry 1–2) or by using a
microwave procedure (200 W, pmax was set to 3.0 bar, tmax was
set to 250 uC), reaction time, see Table 2, entries 3–12).
After cooling the reaction mixture to room temperature and
after phase separation an aliquot of the crude product phase was
assessed for conversion by 1H-NMR spectroscopy (Table 2,
entry 1–12).
Laboratory microwave system: CEM Discover microwave
reactor.
1H-NMR: Bruker Avance DRX 200 spectrometer.
13C-NMR: Bruker Avance DRX 500 spectrometer.
High resolution transmission electron microscopy (HRTEM)
HRTEM micrographs were taken from a FEI TITAN 80–
300 image CS-corrected operating at 300 kV. High-angle annular
dark-field-scanning transmission electron microscopy (HAADF-
STEM) pictures were taken from FEI TECNAI G2 F 20
HR(S)TEM and FEI TITAN 80–300 probe CS-corrected
HRSTEM.
The Pt-NP/IL catalyst was recovered by pipetting the product
phase out of the reaction vessel. To test the activity of the
recovered catalyst (Table 2 entry 11–12), the catalyst phase was
re-charged with phenylacetylene (0.54 ml, 5.0 mmol) and
triethylsilane (0.79 ml, 5.0 mmol) as above.
Particle diameters were measured from HAADF-STEM
micrographs and histograms with diameter distributions were
prepared there from. All samples for HRTEM/HAADF-STEM
were prepared by dropping a small amount of Pt-NP/solvent
a
Table 3 Amount of (MeCp)PtMe3 for the preparation of Pt-NP colloids in different solvents
m (MeCp)PtMe3/mg
n (MeCp)PtMe3/mmol
Solvent
m (solvent)/g (V = 1 ml)
for 0.5 mass% Pt
for 2 mass% Pt
for 0.5 mass% Pt
for 2 mass% Pt
glycol
1.1
1.2
1.2
1.4
1.0
8.4
9.5
9.4
10.9
7.9
—
—
37.6
43.3
—
0.026
0.030
0.029
0.034
0.025
—
—
0.118
0.136
—
glycerol
[BMIm][BF4]
[N4111][NTf2]
CH2Ph2
a
molar mass of (MeCp)PtMe3 = 319.31 g mol21; 61.1 mass% Pt in (MeCp)PtMe3.
7614 | CrystEngComm, 2012, 14, 7607–7615
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Benfield and C. Amiens, Chem. Commun., 2010, 46, 2453–2455; (f) V.
Latour, A. Maisonnat, Y. Coppel, V. Collie`re, P. Fau and B.
Chaudret, Chem. Commun., 2010, 46, 2683–2685; (g) M. Zahmakiran,
Conclusions
Platinum nanocrystals could reproducibly be prepared
from(MeCp)PtMe3 under conventional thermal heating, photo-
lysis or microwave irradiation in coordinating (glycol, glycerol)
or non-coordinating solvents (non-functionalized ILs, diphenyl-
methane). Pt-NPs were highly crystalline (from HRTEM), of
small diameters (typically between 1.0–2.3 nm) and narrow
diameter distribution (¡0.7 nm). There was no clear dependence
of the Pt-NP size and size distribution on the solvent or method
of preparation. It is noteworthy that all of these Pt-NP
dispersions were stable for several months without any sign of
aggregation. The Pt-NP/IL dispersion was verified as a highly
active catalyst for the biphasic hydrosilylation of phenylacety-
lene with triethylsilane, to the distal and proximal products
triethyl(2- and 1-phenylvinyl)silane.
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20 X.-M. Yan, C. Kim and J. M. White, Thin Solid Films, 2001, 391,
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21 C. Thurier and P. Doppelt, Coord. Chem. Rev., 2008, 252, 155–169.
22 C. Page`s, Y. Coppel, M. L. Kahn, A. Maissonat and B. Chaudret,
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Acknowledgements
27 (a) S. F. L. Mertens, C. Vollmer, A. Held, M. H. Aguirre, M. Walter,
C. Janiak and T. Wandlowski, Angew. Chem., Int. Ed., 2011, 50,
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Financial support through the DFG grant Ja466/17-1 is grate-
fully acknowledged.
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