Metal vapor synthesis of ultrasmall Pd nanoparticles functionalized with N-heterocyclic carbenes

Article abstract of DOI:10.1039/c8dt02535e

The synthesis of N-heterocyclic carbene (NHC)-stabilized palladium nanoparticles (PdNPs) by an entirely new strategy comprising the NHC functionalization of ligand-free PdNPs obtained by metal vapor synthesis is described. Detailed characterization confirms the formation of very small monodisperse PdNPs (2.3 nm) and the presence of the NHC ligand on the Pd surface. The stable NHC-functionalized PdNPs dispersed onto a carbon support showed high activity in the hydrogenation of limonene with enhanced regioselectivity in comparison to bare PdNPs on carbon.

Full text of DOI:10.1039/c8dt02535e

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2018, DOI: 10.1039/C8DT02535E.
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DOI: 10.1039/C8DT02535E
Journal Name
COMMUNICATION
Metal vapor synthesis of ultrasmall Pd nanoparticles
functionalized with N-heterocyclic carbenes
Patricia Tegeder,a Marcello Marelli,b Matthias Freitag,a Laura Polito,b Sebastian Lamping,a Rinaldo
Received 00th January 20xx,
Accepted 00th January 20xx
Psaro,b Frank Glorius,a Bart Jan Ravoo,a*and Claudio Evangelistib*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The synthesis of N-heterocyclic carbene (NHC)-stabilized palladium
nanoparticles (PdNPs) by an entirely new strategy comprising the
NHC functionalization of ligand-free PdNPs obtained by metal
vapor synthesis is described. Detailed characterization confirms the
formation of very small monodisperse PdNPs (2.3 nm) and the
presence of the NHC ligand onto the Pd surface. The stable NHC-
functionalized PdNPs dispersed onto carbon support showed high
activity in the hydrogenation of limonene with enhanced
regioselectivity in comparison to bare PdNPs on carbon.
MNP formation in the presence ofNHC
1) NHC complex reduction
With preformed MNPs
This work: NHC immobilization
R
R
MLx
R'
R'
R'
Deprotonation
Complexation
R'
R'
N
N
H
Mx+ L-x
N
R
N
R
N
N
R
R'
R'
R'
R
M
Reduction
N
N
R
R
2) Azolium salt deprotonation
and reduction
MNP
R'
R'
R'
R'
M
N
N
R
N
N
R
R
R
Ligand
NHC@MNP
H2
M
M0 Ln
Ligand
3) Metal(0) complex decomposition
Ligand@MNP
4) Ligand exchange
Ligand stabilized metal nanoparticles (MNPs) attract a large
interest as alternative to classical homogeneous and
heterogeneous transition metal catalyst due to their unique
activity and selectivity.1–3 The surface chemistry of the MNPs
plays a crucial role in their performance as catalyst. On the one
hand, surface ligands can activate the metal they are binding to
by electronic effects. On the other hand, the adsorbed ligands
can influence the number and location of active sites on the
MNP, which can have in turn a huge impact on the selectivity.
In the recent years, N-heterocyclic carbenes (NHCs) have
attracted increasing attention as a new type of MNP ligands.4,5
Similar to thiols, they form strong covalent bonds to many
transition metals, but contrary to thiols, do not tend to inhibit
the catalytic properties of the metal. Moreover, they are
structurally very versatile, feature a facile synthesis and can
induce catalytic activity by their outstanding donor ability.6
NHC-stabilized MNPs have been so far synthesized by four
different strategies: 1) reduction of an NHC metal complex,7–12
2) deprotonation of an azolium metalate and subsequent
reduction,13,14 3) degradation of a metal(0) complex in the
presence of the free NHC15–17 and 4) ligand exchange with
Scheme 1: Synthetic approaches to NHC-stabilized MNPs.
preformed ligand stabilized MNPs.18–21 By using the first three
approaches, the particles are formed in presence of the NHC
ligand, which consequently governs the size of the MNPs. Since
the catalytic performance of MNPs is strongly influenced by the
particle diameter and the ligand structure,2 it is highly desirable
to tune both parameters individually. To the best of our
knowledge, the ligand exchange procedure is the only method
that allows the synthesis of MNPs with defined size and shape
independent from the used NHC ligand. However, the
disadvantages of this approach are the excess of NHC needed
to replace the precursor ligand, the elaborate and time-
consuming work-up to purify the particles and the instable
mixed ligand shell during the exchange process which can lead
to irreversible aggregation.22 Therefore, it would be highly
beneficial to develop a synthetic protocol in which the MNPs are
formed with defined properties and further functionalized with
the NHC ligand avoiding the use of additional ligands.
a.Organic Chemistry Institute, Westfälische Wilhelms-Universität Münster,
Corrensstrasse 40, 48149 Münster, Germany. Email: b.j.ravoo@uni-muenster.de.
b.CNR, Institute of Molecular Science and Technologies, Via C. Golgi 19, 20133
Milano, Italy. Email: claudio.evangelisti@istm.cnr.it.
Electronic Supplementary Information (ESI) available: Experimental details and
spectral data. See DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C8DT02535E
1) Metal vapor synthesis (MVS)
1-hexene / mesitylene
(1:1)
Resistive heating
Pd
Frozen matrix
1)
2)
°
C)
(-196
°
Melting
C)
(-20
Pd atoms
(vapor)
Bulk Pd
PdNPs
2) Ligand immobilization
I
KOtBu
mesitylene
10
10
N
N
rt
0.5
h,
H
LC-NHC . HI
10
10
N
N
LC-NHC
N
10
1-hexene / mesitylene
rt
N
10
n
18
h,
LC-NHC@Pd
Scheme 2: PdNPs synthesis by metal vapor synthesis (1) and subsequent NHC
immobilization (2).
Figure 1: 1H (top) and 13C NMR (bottom) spectra of LC-NHC@Pd measured in benzenze-
d6.
The metal vapor synthesis (MVS) potentially enables this
approach.23 By using MVS, very small (< 5 nm) solvent-stabilized
MNPs of a variety of transition metals can be generated.24-26
MNPs are weakly stabilized by solvent molecules, thus the NHC
coordination is effective and an excess of NHC is not necessary
for surface functionalization. Since only the solvent and the
pure metals are used in the MVS method, the number of
were re-dispersed in toluene and were stable at room
temperature for a very long time (> 6 months).
Transmission electron microscopy (TEM) analysis revealed the
presence of very small NPs with a size distribution ranging from
1.5 to 3.0 nm (mean diameter of 2.3 nm ± 0.4 nm) (Fig. S1). High
resolution TEM micrographs (Fig. S2) confirmed the presence of
crystalline lattice planes exhibiting spots in the fast Fourier
transform pattern at 2.3 Å, which are ascribed to the spacing of
{111} planes of face centered cubic (fcc) structure of metallic Pd.
The presence of the NHC ligand on the particle surface was
possible impurities is reduced to
subsequent purification is facilitated.
a minimum and the
We describe in this communication the synthesis of ultrasmall
NHC-stabilized PdNPs by the MVS technique. To the best of our
knowledge this is the first report on the immobilization of NHC
ligands on preformed, ligand-free MNPs. We selected PdNPs as
a first test system because their functionalization with NHC
ligands by means of other methods is already known. As ligand,
we selected a NHC with long alkyl chains in the backbone of the
heterocycle (LC-NHC), which forms a protective monolayer
around the MNPs and was successfully used before for MNP
stabilization.27,28
The solvent-stabilized PdNPs were synthesized by the MVS
technique according to a previously reported procedure (see
ESI).29-31 Briefly, Pd vapor generated in high-vacuum by resistive
heating of the pure metal was co-condensed with a 1:1 mixture
of mesitylene and 1-hexene in a liquid nitrogen cooled glass
reactor. The melting of the solid matrix afforded a brown
solution of PdNPs which was handled and stored at low
temperature (−20°C) under argon atmosphere. For the surface
modification with LC-NHC, the free carbene was first generated
in situ by deprotonation of the corresponding imidazolium salt
with KOtBu in mesitylene. The PdNPs were added to the NHC
and the solution was stirred at room temperature. After 18 h,
the absence of any precipitate indicated the successful
coordination of LC-NHC on the particle surface (LC-NHC@Pd).
As a matter of fact, the solvent-stabilized PdNPs are not stable
in air and precipitation of Pd powder takes places after few
hours (ca. 2 h) at 25°C even under inert atmosphere. The
particles were then purified in air by precipitation in acetonitrile
and subsequent centrifugation. The thus prepared LC-NHC@Pd
1
investigated by NMR measurements. In the H NMR spectrum
of LC-NHC@Pd, the expected proton signals of the alkyl chains
are visible in the aliphatic region (Fig. 1 top). The signals are
broadened due to the immobilization on the particle surface, as
expected for ligands anchored on NPs.32 The signal of the
methyl groups at 4.1 ppm is particularly weak due to the close
distance of the protons to the particle surface. The formation of
the carbene species is attested by the absence of a signal at
10.1 ppm corresponding to the proton at the C2 position of the
imidazolium salt. In the 13C NMR spectrum only the signals of
the aliphatic alkyl chains can be observed (Fig. 1 bottom). The
carbon signals of the heterocycle are not visible in the spectrum
which is in agreement with our previous report on NPs
functionalized with LC-NHC 28
For the determination of the
.
metal-to-ligand ratio, the PdNPs were investigated by
thermogravimetric analysis (TGA) (Fig. S3). Taking the residual
toluene in the sample into account, the particles contain 38%
Pd and 62% ligand corresponding to a metal-to-ligand ratio of
2.3 : 1. Elemental analysis carried out on LC-NHC@Pd revealed
a carbon-to-nitrogen atomic ratio of 17.9 in good agreement
with the theoretical value of the free NHC ligand (C/N atomic
ratio = 16.1).
2 | J. Name., 2012, 00, 1-3
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In order to effectively use the NPs as heterogeneous catalyst, partially hydrogenated p-1-menthene wasVieweAvrtaiclleuOantleinde
DOI: 10.1039/C8DT02535E
LC-NHC@Pd was quantitatively deposited on a carbon support considering the large applicability of this product for the
(Vulcan® XC72) by simple impregnation, yielding LC- synthesis of bulk and fine chemicals.36,37
NHC@Pd/C. For comparison, the unmodified PdNPs
synthesized by the same MVS procedure were also deposited
on the carbon material, denoted as Pd/C (Fig. S4).
30°C
[Pd/C],
+
+
H
bar)
2(g) (4
HAADF-STEM and TEM analysis of the NHC-functionalized
PdNPs deposited onto the support (Fig. 2a, Fig. S5a) revealed
the presence of MNPs with identical size and shape compared
to the unsupported MNPs, proving that no reconstruction of the
metal core or coalescence takes place during the immobilization
step. Likewise, the MVS-derived supported unfunctionalized
PdNPs (Pd/C) have a similar size of 2.1 ± 0.4 nm indicating that
the NHC ligand does not affect the particle dimensions (Fig. 2b,
Fig. S5b).
2
1
3
4
T
(min)
8
15
8
Conv.
(%)
68.3
86.4
67.5
85.4
Sel. 1
TOF
(h-1)b
Catalyst
(%)a
65.5
66.5
47.6
42.9
2.6∙104
LC-NHC@Pd/C
-
2.5∙104
Pd/C
15
-
Figure 3. Reaction conditions: (R)-(+)-Limonene (1.1 mL), Pd catalyst 1
wt.% (35 mg), Sub/Pd molar ratio = 2000, p(H2) = 4 bar; T = 30°C.
a Selectivity (Sel 1) calculated as mol of p-(1)-menthene/ mol limonene
b
conv. Normalized TOF (i.e. mmol substrate (converted) x (mmol
Pd(surface) x h)-1.
Both MVS-derived Pd/C catalysts are very active for the
reduction of limonene to p-1-menthene (1), p-3-menthene (2)
and p-menthane (a mixture of cis- (3) and trans- (4) isomers)
with a normalized TOF [38] higher than 2∙104 h-1. Noteworthy,
LC-NHC@Pd/C led to a marked increase of the p-1-menthene
selectivity (66.5% at 86.4% of limonene conversion) respect to
the unmodified Pd/C (42.9% at 85.4 % of limonene conversion),
partially inhibiting the Pd-catalyzed isomerization of 1 to 2. The
differences in regioselectivity between the two catalysts were
found both in neat conditions and in toluene as solvent (see
Table S1 and S2). Moreover, the lower regioselectivity of
unmodified Pd/C is comparable with that found for less active
commercial and previously reported Pd/C catalysts (see Table
S1 and refs. 36 and 37). These results demonstrate that the
presence of NHC ligand influences the reactivity of the Pd
surface.
The stability of LC-NHC@Pd/C was analyzed by recyclability
studies as well as evaluating the Pd leaching in reaction
conditions. The system was effectively recycled for five reaction
runs in neat conditions with only a slightly decrease of the
catalytic activity (Conv. = 75.0 % after 15 min during the 5th
cycle) (Fig. S8). Remarkably, the selectivity towards p-1-
menthene was almost identical (67-69%) during the five runs.
Moreover, ICP-OES evidenced no Pd leaching into the reaction
mixture.
In summary, ligand-free PdNPs synthesized by the MVS method
were successfully functionalized with an aliphatic NHC ligand
leading to air-stable NPs. Characterization of the particles by
TEM, NMR, TGA, XPS and elemental analysis clearly confirm
their very small size, the presence of the NHC on the particle
surface and a high ligand density. The functionalized PdNPs can
be easily immobilized on carbon support in order to use them
as recyclable catalyst. The particles show high activity in the
hydrogenation of limonene with p-1-menthene as main
product. By comparison with the performances of
unfunctionalized Pd catalysts, an evident ligand effect was
Figure 2: Representative HAADF-STEM micrograph and corresponding histogram of
particle size distribution of LC-NHC@Pd/C (a) and Pd/C (b). MNPs can be observed as
bright spots in the micrographs.
X-ray photoelectron spectroscopy (XPS) analysis of LC-
NHC@Pd/C provided evidence that the NHC ligand is on the Pd
surface after immobilization on the carbon support, since
characteristic signals of nitrogen can be found (see Fig. S6). The
binding energy of the N1s signal at 401.2 eV matches well with
previously reported values for NHC stabilized PdNPs.22 For Pd/C,
no signal can be observed in this region. The binding energies
measured in the Pd3d region of both NHC functionalized and
unfunctionalized Pd/C compare well with previously reported
values for Pd0.33 However, small shoulders at higher binding
energies indicate partial oxidation to Pdn+ (see Fig. S7).34
The catalytic properties of LC-NHC@Pd/C were studied and
compared with those of the bare Pd/C prepared by the same
approach in the selective hydrogenation of limonene, a
monocyclic terpene, which is the main component of essential
oils derived from the rinds of various citrus fruits (Fig. 3 and
Table S1 and S2).35 In particular, the selectivity towards the
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chapter 32, S. 437.
observed on the regioselectivity, probably due to electronic
modification of the Pd surface and/or steric effects by ligand
binding. Finally, we expect that the metal vapor synthesis
approach to obtain NHC-immobilized PdNPs can be easily
extended to other homo- and heterometallic MNPs as well as
different NHC ligands.
We are grateful for financial support by the Deutsche
Forschungsgemeinschaft (DFG SFB 858). We thank Dr. Vassilios
Siozios from the MEET (WWU Münster) for TGA measurements.
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There are no conflicts to declare.
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