Soluble Platinum Nanoparticles Ligated by Long-Chain N-Heterocyclic Carbenes as Catalysts

Article abstract of DOI:10.1002/chem.201702288

Soluble platinum nanoparticles (Pt NPs) ligated by two different long-chain N-heterocyclic carbenes (LC-IPr and LC-IMe) were synthesized and fully characterized by TEM, high-resolution TEM, wide-angle X-ray scattering (WAXS), X-ray photoelectron spectrosc

Full text of DOI:10.1002/chem.201702288

A Journal of
Accepted Article
Title: Soluble Platinum Nanoparticles ligated by Long-Chain N-
Heterocyclic Carbenes as Catalysts
Authors: Luis M. Martínez-Prieto, Lena Rakers, Angela M. López-
Vinasco, Israel Cano, Yannick Coppel, Karine Philippot, Frank
Glorius, Bruno Chaudret, and Piet W. N. M. van Leeuwen
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To be cited as: Chem. Eur. J. 10.1002/chem.201702288
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Soluble Platinum Nanoparticles Ligated by Long-Chain N-
Heterocyclic Carbenes as Catalysts
Luis M. Martínez-Prieto,[a]† Lena Rakers,[b]† Angela M. López-Vinasco,[a] Israel Cano,[a] Yannick
Coppel,[c] Karine Philippot,[c] Frank Glorius,*[b] Bruno Chaudret,*[a] and Piet W. N. M. van Leeuwen*[a]
Abstract: Soluble platinum nanoparticles (Pt NPs) ligated by two
different long-chain N-Heterocyclic Carbenes (LC-IPr and LC-IMe)
were synthesized and fully characterized by TEM, HRTEM, WAXS,
XPS and solution NMR. The surface chemistry of these NPs (Pt@LC-
IPr and Pt@LC-IMe) was investigated by FT-IR and solid state NMR
using CO as a probe molecule. A clear influence of the bulkiness of
the N-substituents was observed on the size, surface state and
catalytic activity of these Pt NPs. While Pt@LC-IMe showed no
activity in the hydroboration of phenylacetylene, Pt@LC-IPr revealed
good selectivity for the trans-isomer which might be supported by a
homogeneous species. The latter is the first example of hydroboration
of acetylenes catalyzed by non-supported Pt NPs.
activity of MNPs. Recently, it was observed that NHCs with long
aliphatic chains in their backbone increase the stability and
solubility of Pd and Ru NPs.[5] In particular, Ru NPs stabilized with
long-chain NHCs (LC-NHCs) demonstrated to be a versatile and
robust catalyst in both hydrogenation and oxidation reactions and,
more significantly, in a novel one-pot oxidation/hydrogenation
process.[5b] Inspired by these works, we synthesized two types of
Pt NPs ligated with LC-NHCs, namely LC-IPr (1,3-bis(2,6-
diisopropylphenyl)- 4,5-diundecyl imidazoline) and LC-IMe (1,3-
dimethyl- 4,5-diundecyl imidazoline) (Figure 1). The long aliphatic
chains may render the Pt NPs soluble in organic solvents and
would also lead to an increased stability. Thus, the substituents
on the nitrogen can be varied to create ligand-tuned catalysts.
Introduction
In the course of the last two decades, metal nanoparticles
(MNPs) have demonstrated to be excellent catalysts in terms of
activity and selectivity in numerous organic transformations, such
as hydrogenation, oxidation and C-C coupling reactions.[1] Their
value as catalyst lies in the elevated surface/volume ratio, which
provides a large number of active sites in comparison with
heterogeneous catalysts. The stabilizing ligands are able to
modify the size, stability, and catalytic activity of MNPs, as well as
their surface properties.[2] N-heterocyclic carbenes (NHCs)
became prominent examples as ligands for transition-metal
catalysts due to their extraordinary electron-donor properties that
result in strong metal-ligand bonds.[3] This characteristic does not
only make them suitable for the generation of homogeneous
catalysts, but also allows their application in MNP-preparation.[4]
Therefore, NHCs act as bifunctional ligands that on the one hand
are capable of stabilizing MNPs and on the other hand modifying
the electron density of the metal surface to enhance the catalytic
Figure 1. Schematic representation of long-chain carbene ligands LC-IPr and
LC-IMe.
The obtained Pt NPs (Pt@LC-IPr and Pt@LC-IMe) were
fully characterized by the typical material science techniques:
Transmission Electron Microscopy (TEM), High resolution TEM
(HRTEM), Wide-angle X-ray scattering (WAXS) and X-Ray
Photoelectron Spectroscopy (XPS). In addition, thanks to their
high solubility in organic solvents it was possible to characterize
them by solution NMR (1H and DOSY). The surface chemistry of
Pt@LC-NHCs was investigated by MAS-NMR and FT-IR after
reaction with CO. Pt@LC-NHCs displayed different sizes and
surface states depending on the N-substituent of the ligand. An
important difference in activity was also observed in
hydroboration reactions: Pt@LC-IMe demonstrated to be inactive,
whereas Pt@LC-IPr showed full conversion and high selectivity
in the hydroboration of phenylacetylene. The use of Pt NPs in
hydroboration or diboration reactions is limited to heterogeneous
systems supported on titania, magnesia or onium salts.[6] Herein
we present the first example of hydroboration of acetylenes
catalyzed by non-supported Pt NPs.
[a]
Dr. L. M. Martínez-Prieto, Dr. A. M. López-Vinasco, Dr. I. Cano,
Prof. B. Chaudret and Prof. Piet W. N. M. van Leeuwen
LPCNO, Laboratoire de Physique et Chimie des Nano-Objets,
UMR5215 INSA-CNRS UPS, Institut des Sciences appliquées, 135,
Avenue de Rangueil, F-31077 Toulouse, France.
E-mails: chaudret@insa-toulouse.fr; vanleeuw@insa-toulouse.fr
L. Rakers, Prof. F. Glorius
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität
Münster, Corrensstraße 40, 48149 Münster, Germany.
Dr. Y. Coppel, Dr. K. Philippot
[b]
[c]
LCC, Laboratoire de Chimie de Coordination; CNRS, UPS, 205,
Route de Narbonne, F-31077, Toulouse, France.
Email: glorius@uni-muenster.de
†
These authors contributed equally to this work
Results and Discussion
Supporting information for this article is given via a link at the end of
the document.
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Synthesis, characterization and surface studies
cubic (fcc) structure of bulk Pt. Wide-Angle X-ray Scattering
(WAXS) analysis of Pt@LC-NHC NPs shows crystalline Pt NPs
with a pattern consistent with face centered cubic (fcc) structure
and a coherence length of 1.2 nm for Pt@LC-IPr and 2.1 nm for
Pt@LC-IMe, (Figures S5 and S6) which are in agreement with
HRTEM observations. These analyses also reveal a single
domain and do not indicate any oxidation. When these Pt NPs
were exposed to open air during two weeks at room temperature
(r.t.), the NPs were hardly oxidized presenting only a very slight
decrease of their crystallinity. This demonstrates the high stability
of these NPs to oxidation when exposed to ambient air at r.t.
(Figures S7 and S8).
Thermogravimetric Analyses (TGA) show a Pt content
around 60 %, which is consistent with a platinum–ligand ratio of
5.5:1 for Pt@LC-IPr and 3.7:1 for Pt@LC-IMe (Table 1). It is worth
mentioning that the Pt/L ratio is in principle sufficiently high to
enable the coordination of all NHC molecules, apart from steric
factors, since the calculated number of Pt surface atoms exceeds
the estimated number of ligands (up to 4 times for Pt@LC-IPr and
2 times for Pt@LC-IMe).
Combining a new methodology recently developed for the
synthesis of Ru NPs stabilized with non-isolable NHCs [7] and the
use
of
the
novel
organometallic
precursor
tris(2-
norbornene)platinum(0) [Pt(NBE)3],[8] we synthesized two long-
chain NHC-ligated Pt NPs, namely Pt@LC-IPr and Pt@LC-IMe.
Colloidal solutions of Pt NPs were easily obtained by reduction of
[Pt(NBE)3] in THF under 3 bar of dihydrogen (H2) and in the
presence of 0.2 equivalents (equiv.) of the ligand (LC-IPr or LC-
IMe), which was previously generated in solution from its
corresponding imidazolium salt by adding a slight excess of
KOtBu as base (Scheme 1). After washing with MeOH, the
purified Pt NPs were re-dispersed in THF and examined by
Transmission Electronic Microscopy (TEM). TEM analyses of
Pt@LC-IPr and Pt@LC-IMe revealed monodisperse and well-
distributed NPs with a mean diameter of 1.2 (0.2) and 1.9 (0.3)
nm, respectively. TEM micrographs and their corresponding size
histograms are shown in Figure 2. High resolution TEM (HRTEM)
images of Pt@LC-IPr and Pt@LC-IMe (see SI, Figures S3 and
S4) present crystalline nanoparticles retaining the face centered
Table 1. Composition of Pt@LC-IPr and Pt@LC-IMe NPs.
[a]
[b]
Pt NP[a]
Size (nm)
% Pt
Pt/L Ratio
Ptx/Ly
Ns
47
141
Pt@LC-IPr
Pt@LC-IMe
1.2 (0.2)
1.9 (0.3)
60.6
64.0
5.5: 1
65/12
3.7 : 1
261/70
[a] The approximate composition is based on the Pt/L ratio and the mean
diameter measured by TEM. [b] Number of surface atoms. Approximate
values obtained from the graphs of Van Hardeveld and Hartog.[9]
Recently, X-ray photoelectron spectroscopy (XPS) has
demonstrated to be an appropriate technique to study the binding
mode of coordinating ligands on
a
metal surface.[8,10]
Scheme 1. Synthesis of Pt@LC-IPr and Pt@LC-IMe.
Consequently, the coordination of the long-chain NHC ligand in
our Pt NPs was investigated by XPS. The N(1s) signals for
Pt@LC-IPr and Pt@LC-IMe showed binding energies (BE) of
400.5 and 400.2 eV respectively [Figure 3, a) and b)], which are
in agreement with reported values of coordinated NHC ligands on
Pd NPs.[10a] These values are lower than those of the imidazolium
salts used as precursors [LC-IPr•HBr (401.5 eV) and LC-IMe•HI
(401.2 eV)], as in these cationic molecules the electrons are
strongly bound. This suggests that LC-NHC ligands bind to the
metal nanoparticle through a covalent bond with the carbenic
carbon. The BE of Pt 4f7/2 in the XPS spectra is 71.6 eV for
Pt@LC-IPr and 71.2 eV for Pt@LC-IMe [Figure 3, c) and d)]. Here
the main contributions are located between 71.5 and 71.1 eV,
which is characteristic of Pt(0). [11] Another contribution at 72.6-
72.7 eV is assigned to surface atoms bonded to LC-NHCs
(Pt(δ+)) and some PtO2 formed during the during the preparation
of the samples for XPS (under air).
In addition to the above measurements, Pt@LC-IPr and
Pt@LC-IMe were characterized by infrared spectroscopy (FT-IR).
Coordination of CO was used to investigate their surface
chemistry through the localization of the actives sites. It is known
Figure 2. TEM micrographs and the corresponding size histograms of Pt@LC-
IPr (left) and Pt@LC-IMe (right).
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during 5 min). In both cases, the coordination of CO in terminal
mode was evidenced through the presence of strong bands at
2035 cm-1 for Pt@LC-IPr and 2030 cm-1 for Pt@LC-IMe. The
bridging CO appeared as a broad and weak band centered at
1820 and 1800 cm-1 for Pt@LC-IPr and Pt@LC-IMe, respectively.
The ratio of COb/COt is higher for Pt@LC-IMe than for Pt@LC-
IPr, which can be explained by the larger size of Pt@LC-IMe (1.9
nm), exhibiting more available faces for the coordination of CO in
a bridging mode. Also, the intensity of the CO bands in
comparison with the aliphatic chain ones (2956-2853 cm-1) is
lower for Pt@LC-IMe, indicating less sites for CO coordination,
and probably the presence of an inferior number of active sites in
comparison with Pt@LC-IPr.
Solid state 13C MAS NMR spectra of Pt@LC-IPr and
Pt@LC-IMe samples confirm the presence of LC-NHCs on the Pt
surface (Figures 5a and 5b). 13C spectra of Pt@LC-IPr present a
broad signal at 125 ppm which belongs to the aromatic rings. The
13C signal of imidazolium backbones of both ligands were not
detected (structural and electronic disorder at the metallic surface
and the potential presence of conduction electrons often hampers
the observation of NMR resonances of nuclei close to metallic NP
surfaces). The intense peaks between 29 and 22 ppm correspond
to the long alkyl chains of LC-NHCs as well as the alkyl groups
from the nitrogen-substituents of LC-IPr. The signal at 13 ppm for
Pt@LC-IPr and for Pt@LC-IMe is attributed to the methyl groups
placed at the end of the long hydrocarbon chain. In 13C CP-MAS
NMR spectra (bottom of Figures 5a and 5b), the intensity of these
signals decreases, most likely due to the mobility of CH3 groups
at the end of the tail. When Pt@LC-IMe NPs (1.9 nm) are
pressurized with 1 bar of 13CO (r.t., 20 h), 13C MAS NMR spectra,
with or without CPMG detection scheme,[14] show a very broad
peak centered around 380 ppm (bottom of Figure 5d and top of
Figure 6). This peak is characteristic of a Knight-shifted 13CO
signal due to the presence of conduction electrons at the surface.
By contrast, the smaller Pt@LC-IPr NPs (1.2 nm) exhibit in their
13C MAS NMR spectra (bottom of Figure 5c and bottom of Figure
6) after pressurizing with 13CO a resonance at δ ~ 180 ppm,
assigned to diamagnetic 13COt, and broad overlapping
resonances from 200 to 400 ppm assigned to diamagnetic 13COb
and also Knight-shifted 13CO signals. The correlation between the
NP size and the presence of a Knight shift for surface CO signals
on Pt NPs was recently investigated by some of us,[8] and it was
shown, as in the present case, that when the size of the NPs
decreases, the Knight shift of the CO signal also decreases, being
almost zero at 1 nm. Indeed, it is practically suppressed for the
1.2 nm Pt@LC-IPr NPs, and the observation of partial Knight-
shifted CO signals (centered at 380 ppm) is probably due to the
presence of some aggregated (or coalesced) NPs. In the 13C CP
MAS NMR spectrum of Pt@LC-IPr the intensity of COb and COt
resonances decreases (compared to the ligand signals) as a
result of their longer distance to H nuclei, but possibly also due to
stronger local mobilities. For the Knight-shifted CO resonances of
Pt@LC-IMe and Pt@LC-IPr, the intensity decrease of the CP
signals can also be associated to additional relaxation pathways
related to the presence of surface electrons. Nevertheless, these
results confirm the presence of various types of active sites on the
platinum surface, necessary for catalytic reactions.
Figure 3. Top: X-ray photoelectron spectroscopy (XPS) of the N(1s) signals of
LC-IPr•HBr (a, red), LC-IMe•HI (b, red), Pt@LC-IPr (a, blue) and Pt@LC-IMe (b,
blue). Bottom: Pt 4f5/2 and 4f7/2 XPS spectrum of Pt@LC-IPr (c) and Pt@LC-IMe
(d).
Figure 4. ATR FT-IR spectra of Pt@LC-IPr (a) and Pt@LC-IMe (b) before (blue)
and after (red) CO adsorption.
that CO coordinates in a bridging (COb) or in a terminal
(COt)mode on the platinum surface.[12] Normally, COb are located
on the faces of MNPs and COt on their apexes and edges.[13]
Attenuated total reflectance (ATR) FT-IR spectra of Pt@LC-IPr
and Pt@LC-IMe (Figures 4) present the characteristic absorption
bands of COt (2030 - 2035 cm-1) and COb (1871-1802 cm-1) after
reaction with carbon monoxide (bubbling CO to a THF solution
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Figure 5. Top: 13C MAS (blue) and CP-MAS (red) NMR spectra of Pt@LC-IPr (a) and Pt@LC-IMe (b). Bottom: 13C MAS (blue) and CP-MAS (red) NMR spectra of
Pt@LC-IPr (c) and Pt@LC-IMe (d) after exposure to 13CO (1 bar, 20 h, r.t.).
Solution NMR Studies
The soluble nature of the Pt@LC-NHCs NPs allowed us to
characterize them by solution NMR and to discover interesting
clues about their size distribution and the interaction between the
1
LC-NHC ligand and the Pt NP surface. H NMR of Pt@LC-IPr in
THF(d8) showed that the resonances of chemical groups
connected to the imidazolium ring (Harom, CH(IPr) and CH2(C=C))
are significantly broader than those of the free ligand. It suggests
that they possess short T2 relaxation times associated to a strong
reduction of their mobility caused by the proximity of the platinum
surface (top of Figure 7a). In the same way, the resonances
attributed to N-CH3 and CH2(C=C) in LC-IMe•HI are very broad
and barely visible after coordination to the NP surface (top of
Figure 7b).
Diffusion-ordered spectroscopy (DOSY) NMR enabled us to
distinguish different species according to their diffusion coefficient.
Figure 6. 13C CPMG MAS NMR spectra of Pt@LC-IPr (bottom) and Pt@LC-
IMe (top) after exposure to 13CO (1 bar, 20 h, r.t.).
The DOSY spectra of LC-IPr and Pt@LC-IPr reveal diffusion
coefficients of 9.5±0.5 x 10-10 and 4.8±1.0 x 10-10 m2.s-1
respectively (Figure 7c). This means that the ligands
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Figure 7. Top: 1H NMR spectra of LC-IPr•HBr (a, blue), LC-IMe•HI (b, blue), Pt@LC-IPr (a, red) and Pt@LC-IMe (b, red) in THF (d8). The peaks with the asterisk
(*) correspond to residual solvent signals and the ones with hash (#) are due to water. Bottom: 1H DOSY NMR spectra of LC-IPr•HBr (c, red), LC-IMe•HI (d, red),
Pt@LC-IPr (c, black) and Pt@LC-IMe (d, black) in THF (d8). 1H NMR spectrum with diffusion filter of Pt@LC-IPr (c, blue) and Pt@LC-IMe (d, blue).
coordinated to the Pt surface diffuse more slowly than the free
ones, as is to be expected. Moreover, only one diffusion
coefficient value is evidenced for Pt@LC-IPr, indicating a narrow
size distribution. The hydrodynamic diameter determined from the
diffusion coefficient[15] of Pt@LC-IPr is 2.0±0.4 nm, in good
agreement with the Pt NP core size (1.2 nm) surrounded by a
monolayer of ligands. On the other hand, the DOSY spectrum of
Pt@LC-IMe (Figure 7d) shows several different diffusion
coefficients, smaller than the ones of Pt@LC-IPr (from 2.0 to 4.6
x 10-10 m2.s-1; hydrodynamic diameter from 2.0 to 4.5 nm). These
values are in agreement with the bigger size of Pt@LC-IMe NPs
and their higher size disparity. The diffusion coefficient of LC-IMe
is 7.6±0.5 x 10-10 m2.s-1, which is in the same range of that of LC-
IPr (its slightly slower diffusion is probably due to some molecular
interaction like π–π stacking).
potential catalytic reactions. Especially, the particles Pt@LC-IPr
with approximate composition of Pt65L12 are suited as stable
catalysts as they have, despite the high coverage, still surface
sites available and more than half the atoms are surface atoms.
Catalytic Studies
To validate the differences in catalytic activity, we
investigated the catalytic behaviour of Pt@LC-IPr and Pt@LC-
IMe in the hydroboration of alkynes as a less explored reaction for
MNPs. The catalytic hydroboration of alkynes is an effective
methodology to produce vinylboranes.[16] These compounds are
valuable starting materials for a variety of transformations,
including the formation of carbon-carbon and carbon-heteroatom
bonds.[17] In spite of the high importance of this reaction, there are
Thus, we have described and characterized new Pt NPs that
fulfil our requirements as regards size, stability, well-established
surface ligand modification, and solubility in organic solvents for
only a few examples of hydroboration using MNPs as
catalysts.[6c,18] The hydroboration of phenylacetylene with HBpin
was chosen as a model reaction (Table 2), showing a substantial
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Table 2. Hydroboration of phenylacetylene catalyzed by Pt catalysts.[a]
Entry
Catalyst
Pt@LC-IPr
Pt@LC-IMe
Pt/C
Conversion(%)[b]
Yield (%)[c]
1
2
3
4
> 99
15
3/4/5 = 81:7:8
Hydrogenation products[d]
Hydrogenation products[d]
3/4/5 = 64:3:5
28
[Pt(IPr]2] [e]
85
[a] Reaction conditions: Phenylacetylene (0.2 mmol), HBpin (0.24 mmol) and Pt-NPs (2mg, 0.006 mmol Pt assuming ~60% of Pt from
TGA ); hexane; inert atmosphere; r.t.; 2 h. [b] Conversion was determined using GC-FID analysis with mesitylene as internal standard.[c]
Yield was determined using NMR-analysis with dibromomethane as internal standard. [d] Formation of Styrene and Ethylbenzene
determined by GC-MS analysis. [e] 1 mol-% [Pt].
difference in terms of activity between the two nanoparticle
systems. While Pt@LC-IPr showed full conversion and high
selectivity to product 3 (81%, Table 2, entry 1), Pt@LC-IMe did
not lead to the desired product. Instead, hydrogenation of
phenylacetylene (15% conversion, Table 2, entry 2) to styrene
possible pathway, in which Pt@LC-IPr NPs would act as
molecular catalyst reservoir which generates active molecular
species in the conditions applied for hydroboration. ESI-MS
analysis of Pt@LC-IPr NPs in colloidal solution (Figure S11)
evidenced the presence of platinum complexes [Pt(LC-Pr)2 and
and ethylbenzene was observed (determined by GC-MS analysis). Pt2(LC-Pr)2] similar to the one used as catalyst in the control
In a recent publication, we reported a significant influence of the
N-substituents on the activity and selectivity of LC-NHC stabilized
RuNPs in oxidation/hydrogenation reactions.[5b] For the reported
Pt systems the effect of the N-substituens is even more
pronounced leading to divergent catalytic behaviour between the
two systems. We also performed the hydroboration of
phenylacetylene in a multiple addition manner, confirming that the
reactivity of Pt@LC-IPr NPs remains after the catalysis (for further
details see experimental section and Figure S12). Additionally,
we analyzed the catalytic activity of Pt@LC-IPr for a few more
substrates (Table S1). In general, all substrates were converted
to the desired trans-isomer in moderate to good yields.
experiment (Table 2, entry 4). Nevertheless, TEM-analysis of the
reaction mixture after catalysis (Figure S1), showed no significant
difference in size, distribution and dispersity. A leaching pathway
is likely to yield the catalytic active species which either readsorbs
on the nanoparticle or leaches only in very low amounts,[20] so that
the particle size is not affected significantly. However, we cannot
discard a dual catalytic system, where both Pt@LC-IPr NPs and
molecular complexes are the active species.
Conclusions
We briefly compared our systems with a heterogeneous and a
homogeneous catalyst. An analogous catalytic activity to Pt@LC-
IMe was found for Pt on carbon (Pt/C, Table 2, entry 3), which is
usually employed as catalyst in hydrogenation reactions, but has
demonstrated to be inactive in hydroboration of phenylacetylene.
To examine whether leaching is a possibility to generate a
catalytic active species, we synthesized a platinum(0) complex
containing IPr as ligand (previously reported by Nolan et al.)[19]
and tested it in the hydroboration reaction (Table 2, entry 4).
Although no full conversion of phenylacetylene was observed, the
products were obtained in a similar ratio to those generated by
using Pt@LC-IPr as catalyst. This result suggests leaching as a
Two long-chain NHCs (LC-IPr and LC-IMe) were used to
synthesize stable Pt NPs. The obtained Pt NPs were
characterized by the state-of-the-art material science techniques
(TEM, HRTEM, WAXS and XPS). Taking profit of their high
solubility in organic solvents, we also characterized them by
solution NMR and ESI-MS. Moreover, the surface chemistry of
Pt@LC-NHCs was explored by FT-IR and MAS NMR through
coordination of CO. A clear influence of the ligand, in particular of
the bulkiness of the N-substituent, was observed on the size,
surface state and catalytic activity of these Pt NPs. LC-IMe-
stabilized Pt NPs resulted in a larger diameter than Pt@LC-IPr.
The latter exhibits a high activity and selectivity in hydroboration
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dimension was processed with the Laplace inversion routine CONTIN
(Topspin software).
reactions, in contrast to Pt@LC-IMe which does not display any
remarkable activity. The catalytic behavior of Pt@LC-IMe is more
similar to heterogeneous catalysts such as Pt/C, while the
catalytic activity of Pt@LC-IPr goes parallel to that of molecular
complexes. Although this may point to reversible leaching of the
NPs, we have no definite proof for the nature of the active species.
Thus, we have succeeded in the synthesis of stable Pt NPs, of
which the size, stability and reactivity can be easily tuned by
changing the nature of the substituents present on the nitrogen of
the imidazole core. The solubility and small size of the long-chain
NHC ligated Pt NPs makes them interesting materials for future
catalysis.
Transmission Electron Microscopy (TEM) and High resolution TEM
(HRTEM). Pt NPs were observed by TEM and HRTEM after deposition of
a drop of a solution of the isolated nanoparticles after dispersion in THF
on a copper grid. TEM analyses were performed at the UMS-Castaing by
using a JEOL JEM 1011 CX-T electron microscope operating at 100 kV
with a point resolution of 4.5 Å. The approximation of the particles mean
size was made through a manual analysis of enlarged micrographs by
measuring a number of particles on a given grid. HRTEM observations
were carried out with a JEOL JEM 2010 electron microscope working at
200 kV with a resolution point of 2.35 Å. FFT treatments have been carried
out with Digital Micrograph Version 3.7.4.
X-Ray Photoelectron Spectroscopy (XPS). XPS analyses were performed
at CIRIMAT Laboratory (Toulouse) using a Thermoelectron Kalpha device.
The photoelectron emission spectra were recorded using Al-Kα radiation
(hν= 1486.6 eV) from a monochromatized source. The analyzed area was
about 0.15 mm2. The pass energy was fixed at 40 eV. The spectrometer
energy calibration was made using the Au 4f7/2 (83.9 ± 0.1 eV) and Cu2p3/2
(932.8 ± 0.1 eV) photoelectron lines. XPS spectra were recorded in direct
mode N(Ec). The background signal was removed using the Shirley
method. The atomic concentrations were determined from photoelectron
peak areas using the atomic sensitivity factors reported by Scofield, taking
into account the transmission function of the analyzer. The photoelectron
peaks were analyzed by Gaussian/ Lorentzian (G/L=50) peak fitting. The
charge reference of XPS spectra was made using C1s = 284.8 eV.
Experimental Section
General considerations and starting materials
All chemical operations were carried out using standard Schlenk tubes,
Fischer-Porter bottle techniques or in a glove-box under argon atmosphere.
Solvents were purified before use: THF (Sigma-Aldrich) by distillation
under argon atmosphere and pentane (SDS) and hexane (ACROS) by
filtration on adequate column of a purification apparatus (MBraun).
Pt(NBE)3 was purchased from Nanomeps Toulouse, CO from Air liquide,
13CO (13C, 99.14%) from Eurisotop, phenylacetylene (98%), 4-
ethynylanisole (97%), 4-ethynyl-α,α,α-trifluorotoluene (97%), 1-ethynyl-4-
fluorobenzene (99%) and 3-ethynyl-α,α,α-trifluorotoluene (97%) from
Sigma Aldrich. All reagents used for the catalysis experiments were used
without purification and purchased from TCI, Sigma-Aldrich, CombiBlocks,
Apollo or Alfa Aesar.
Infrared spectroscopy (IR). ATR IR-FT spectra were recorded on a Thermo
Scientific Nicolet 6700 spectrometer in the range 4000-600 cm-1.
Electrospray Ionization (ESI) Mass Spectroscopy (MS). ESI-MS spectra
were recorded using a Waters QTof Premier. The instrument was operated
in positive mode, direct introduction and cone voltage set to 30 V.
Thermogravimetric Analyses (TGA). TGA analyses were performed in a
TGA/DSC 1 STAR System equipped with an ultra-microbalance UMX5, a
gas switch GC200 and sensors DTA and DSC. The samples were
analyzed through a two steps oxidation/reduction method. First the sample
was heated from 25 °C to 500 °C at 10°C/min under air (2h). After cooling
down, it was heated again from 25 °C to 700 °C at 30 °C/min under a gas
mixture Ar/H2 4% (3h).
Synthesis of long-chain imidazolium salts
LC-IPr.HBr and LC-IMe.HI were synthesized according to the procedure
described in the literature.[5a]
Synthesis of Pt(IPr)2 complex.
Wide-angle X-ray scattering (WAXS). WAXS was performed at CEMES-
CNRS. Samples were sealed in 1.0 mm diameter Lindemann glass
capillaries. The samples were irradiated with graphite monochromatized
molybdenum Kα (0.071069 nm) radiation and the X-ray intensity scattered
measurements were performed using a dedicated two-axis diffractometer.
Radial distribution functions (RDF) were obtained after Fourier
transformation of the reduced intensity functions.
Pt(IPr)2 was synthesized according to the procedure described by Nolan
et al.[18]
Synthesis of Pt NPs.
Pt@LC-IPr: A Schlenk flask was charged with LC-IPr·HBr(49.0 mg, 0.063
mmol, 0.2 equiv.) and KOtBu (7.8 mg, 0.069 mmol, 0.22 equiv.). The solids
were suspended in THF (50 mL) and stirred at r.t. for 20 h. After that the
resulting suspension was filtered through dry celite (1 cm) under argon
atmosphere and added to a 250 ml Fischer-Porter bottle charged with a
cooled solution (-80 °C) of Pt(NBE)3 (150 mg, 0.315 mmol, 1 equiv.) in 50
ml of THF (previously degassed by three freeze-pump cycles). The
Fischer-Porter was pressurized with 3 bar of H2, and the solution was
allowed to reach the room temperature whilst the solution was stirred
vigorously. A black homogeneous solution was immediately formed. The
stirring was continued for 20 h at r.t.. After that, the remaining H2 pressure
was released and the solution was evaporated until dryness. The black
residue solid was re-dispersed in the minimum quantity of THF (~ 3 ml)
and 150 ml of degassed methanol was added. The resulting black
precipitate was washed twice with degassed methanol (50 mL) and dried
overnight under vacuum. The size of the NPs was measured by TEM on a
NMR spectroscopy. Solid-state NMR experiments were recorded at the
LCC (Toulouse) on a Bruker Avance 400 spectrometer equipped with 2.5
mm probe. Samples were spun between 16 to 20 kHz at the magic angle
using ZrO2 rotors. 13C MAS experiments were performed with a recycle
delay between 2 s to 80 s. 13C CP/MAS spectra were recorded with a
recycle delay of 2 s and contact times between 2 to 6 ms. 13C CPMG were
acquired with 32 echoes, a delay between train of 180° pulse of 8 rotor
periods and a recycle delay of 3s.
1H liquid state NMR experiments were recorded on a Bruker Avance 500
spectrometer equipped with a 5 mm triple resonance inverse Z-gradient
probe. Samples were prepared in THF-d8. 1H signals were assigned on the
basis of chemical shifts, spin-spin coupling constants, splitting patterns
and signal intensities. Diffusion measurements were made using the
stimulated echo pulse sequence with bipolar gradient pulses. The diffusion
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10.1002/chem.201702288
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sample of at least 100 nanoparticles, which afforded a mean value of 1.3
(0.2) nm. TGA gave the following Pt content: 60.6 % Pt.
Soc. Rev. 2011, 40, 4973-4985. g) Nanomaterials in Catalysis (Eds.: P.
Sherp, K. Philippot), Wiley-VCH Weinheim 2013.
[2]
[3]
a) K. An, G. A. Somorjai, ChemCatChem 2012, 4, 1512-1524. b) D.
González-Gálvez, P. Nolis, K. Philippot, B. Chaudret, P. W. N. M. van
Leeuwen, ACS Catal. 2012, 2, 317-321. c) L. M. Martínez-Prieto, S.
Carenco, C. H. Wu, E. Bonnefille, S. Axnanda, Z. Liu, P. F. Fazzini, K.
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Pt@LC-IMe: These Pt NPs were prepared as described for Pt@LC-IPr.
[Pt(NBE)3 (150 mg, 0.315 mmol, 1 equiv.), LC-IMe·HI (33.5 mg, 0.063
mmol, 0.2 equiv.) and KOtBu (7.8 mg, 0.069 mmol, 0.22 equiv.)]. The size
of the NPs was measured by TEM on a sample of at least 100
nanoparticles, which afforded a mean value of 1.9 (0.3) nm. TGA gave the
following Pt content: 64.0 % Pt.
a) M. N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 2014,
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Wiley-VCH, New York, 2006. c) N-Heterocyclic Carbenes in Transition
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Hydroboration reactions.
To a Schlenk-tube equipped with a magnetic stirring bar, Pt@LC-IPr
nanoparticles (2 mg, 3%) were added. The nanoparticles were dissolved
in dry n-hexane (1 mL) and phenylacetylene (0.20 mmol, 22 μL, 1.0 eq)
and HBpin (0.24 mmol, 35 μL, 1.2 eq) were added under argon. The
reaction mixture was stirred at RT for 2 h, afterwards filtered over a
whatman-filter and concentrated. The residue was dissolved in deuterated
chloroform and dibromomethane (0.20 mmol, 14 μL, 1.0 eq) was added as
internal standard. The yields of the corresponding isomers were
determined by NMR analysis.
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procedure. Conversion and product formation were determined using GC-
analysis. In case no full conversion of phenylacetylene occured,
mesitylene (0.20 mmol, 28 μL, 1.0 eq) was added as an internal standard
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According to the general procedure, the catalysis was started. Mesitylene
(0.20 mmol, 1.0 eq, 28 μL) was added as an internal standard. After 2 h,
the starting materials phenylacetylene (0.20 mmol, 22 μL, 1.0 eq) and
HBpin (0.24 mmol, 35 μL, 1.2 eq) were added under argon. This was
repeated in total 4 times. Conversions and yields (of 3) were determined
using GC-FID.
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Acknowledgements
The authors thank CNRS, UPS-Toulouse, INSA, IDEX/Chaires
d'attractivité l’Université Fédérale Toulouse Midi-Pyrénées and
the Deutsche Forschungsgemeinschaft (SFB 858), for financial
support. We also thank L. Datas for TEM facilities (UMS
Castaing ), S. Cayez for HRTEM micrographs and P. Lecante
(CEMES, CNRS) for WAXS measurements.
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Keywords: platinum nanoparticles • N-heterocyclic carbenes •
hydroboration of alkynes • influence of the ligand • surface
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푘훽
푇
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퐻
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10.1002/chem.201702288
Chemistry - A European Journal
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Luis M. Martínez-Prieto, Lena Rakers,
Angela M. López-Vinasco, Israel Cano,
Yannick Coppel, Karine Philippot, Frank
Glorius,* Bruno Chaudret,* and Piet W.
N. M. van Leeuwen*
Page No. – Page No.
Soluble platinum nanoparticles ligated by two different long-chain N-Heterocyclic
Carbenes (Pt@LC-IPr and Pt@LC-IMe) show marked differences in their catalytic
activity depending on the bulkiness of the N-substituents. While Pt@LC-IMe
behave similarly to heterogeneous catalysts, the catalytic activity of Pt@LC-IPr
parallels that of molecular complexes in the hydroboration of phenylacetylene.
Soluble Platinum Nanoparticles
ligated by Long-Chain N-Heterocyclic
Carbenes as Catalysts
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