Surprisingly high sensitivity of copper nanoparticles toward coordinating ligands: Consequences for the hydride reduction of benzaldehyde

Article abstract of DOI:10.1039/c8cy01516c

Functionalized copper nanoparticles are widely used as catalysts; however, identification of the catalytically active species remains a challenge. In this study we investigate hydride-assisted reduction reactions with special focus on the structural evolution of copper nanoparticles in the presence of phosphine and nitrogen-based ligands. These compounds are commonly used as stabilizing agents, and therefore are important in the context of other catalytic reactions as well. In particular, we investigate the case of tri-n-butylphosphine. Our findings show that ultrasmall nano-objects are formed as key intermediates to produce catalytically active species in the hydrosilylation of benzaldehyde into the corresponding silyl alcohol. Moreover, we found that the strength of the hydride donor is essential for the formation of the active catalysts. Therefore, this work unveils the previously overlooked high sensitivity of copper nanoparticles toward coordinating ligands in the context of catalysis.

Full text of DOI:10.1039/c8cy01516c

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ISSN 2044-4753
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
rsc.li/catalysis

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Surprisingly high sensitivity of copper nanoparticles toward
coordinating ligands: consequences for the hydride reduction of
benzaldehyde
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
c
c
a,b
DOI: 10.1039/x0xx00000x
Xavier Frogneux, Ferenc Borondics, Stéphane Lefrançois, Florian D’Accriscio, Clément
a,b
a,b,
Sanchez, Sophie Carenco
*
www.rsc.org/
Functionalized copper nanoparticles are widely used as catalysts; however, identification of the catalytically active species
remains a challenge. In this study we investigate hydride-assisted reduction reactions with special focus on the structural
evolution of copper nanoparticles in the presence of phosphine and nitrogen-based ligands. These compounds are
commonly used as stabilizing agents, therefore are important in the context of other catalytic reactions as well. In
particular, we investigate the case of tri-n-butylphosphine. Our findings show that ultrasmall nano-objects are formed as
key intermediates to produce catalytically active species in the hydrosilylation of benzaldehyde into the corresponding silyl
alcohol. Moreover, we found that the strength of the hydride donor is essential for the formation of the active catalysts.
Therefore, this work unveils the previously overlooked high sensitivity of copper nanoparticles toward coordinating ligands
in the context of catalysis.
the case of Cu-based NPs, leaching has been observed in some
reactions but, so far, leached species were not considered as
Introduction
2
6–29
potential catalytic species.
Among the variety of metallic nanoparticles (NPs), copper NPs
are made from a cheap and abundant metal, allowing scale-up
in industry. They feature specific properties such as a plasmon
band in the infrared range, high electrical and thermal
conductivity and interesting catalytic performances, e.g. for
In the present study, Cu NPs were produced by colloidal
synthesis in oleylamine, following a previously reported
1
5
procedure. Their surface was characterized under air-free
atmosphere by FTIR, revealing the presence of unexpected N-
oleylacetamide moieties at the nanoparticles surface.
Hydrosilylative reduction of benzaldehyde was performed with
the Cu NPs as catalysts and the formation of products was
monitored in situ using an attenuated total reflectance
infrared cell especially designed for air-sensitive nanoparticles.
We showed that the addition of a second ligand (tri-n-
1
methanol production. In particular, they catalyze the chemical
reduction of various moieties (carbonyl derivatives, alkynes,
2
alkenes, nitro-compounds).
–11
Nonetheless, due to their high
sensitivity to oxidation, fabrication routes for Cu NPs are not as
well-documented as for Ag or Au NPs. In order to obtain pure
Cu(0) nanoparticles, colloidal syntheses are of interest as they
allow working in an organic solvent with a large variety of
ligands, enabling the tuning of the capping species at the
butylphosphine,
1,8-diazabicyclo[5.4.0]undec-7-ene
or
oleylamine) was modulating the catalytic activity of the
nanoparticles. While all these tested ligands induced a
significant leaching of copper species into the solution from
the nanoparticles, only the tri-n-butylphosphine provided
active catalytic species. Moreover, such species were only
formed with strong enough hydride donors such as
phenylsilane. Leaching of copper from the nanoparticles was
not sufficient by itself to allow the catalytic reduction of
benzaldehyde. Rather, formation of active species demanded
an adequate hydride source. This may explain why the facile
leaching of copper in the presence of coordinating ligands was
overlooked so far in many works using copper nanoparticles as
catalysts.
1
2–17
surface of the nanoparticles.
nanoparticles can be controlled by the interaction between
The reactivity of the
1
8–22
the ligand and the metallic center.
In some cases, ligands
can be detrimental to the integrity of the NPs due to the
leaching of metal-ligand complexes into the medium, as it has
2
3–25
been thoroughly studied for Pd-based catalytic systems.
In
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-1
H) at 1558 cm ) could not be found on the nanoparticles
Results and Discussion
spectrum; and no –OH group was detected in the 3000-
-1
000 cm area (see SI, Figure S4). Moreover, the alkenes
Surface capping agent of copper nanoparticles prepared with
oleylamine
4
-1
) at 3005 cm and ν(C=C) at
absorption bands (ν(C=
1
C
–
H
-
Copper nanoparticles were prepared by reducing [Cu(acac)
2
] in 1650 cm ) were also not observed on the spectrum.
oleylamine (OAm) at 250 °C for 2 h under inert atmosphere Noticeably, the spectrum of the NPs layer showed an
1
5
-1
(
see experimental section). The NPs were isolated with four absorption band at 1728 cm that could indicate the presence
cycles of washing and centrifugation and dried under a N
2
of a carbonyl moiety.
1
flow. X-ray diffraction on powder (XRD) confirmed the fcc- As a complementary analysis, H NMR analysis was performed
Cu(0) phase with a crystallite average size of 13 nm according on the supernatant of the reaction (prior to washing steps)
to the Debye-Scherrer formula. Transmission Electron (see SI, Figure S2). A multiplet at 5.35 ppm was detected,
Microscopy (TEM) showed the presence of polydisperse corresponding to alkene moieties. The absence of the
nanoparticles of 35 ± 10 nm in diameter (Figure 1A,B and S1 in corresponding IR band on the NPs could indicate either that
SI).
the ligands coordinated to the nanoparticles did not contain
alkene moieties, or that these moieties were in strong
1
interaction with the surface. Moreover, based on the H NMR
spectrum, it is clear that the amino group of oleylamine, which
should have appeared as a triplet at 2.64 ppm (N–CH2) and a
signal at 1.1 ppm (N–H2), is completely absent. Further, the
presence of a singlet at 1.97 ppm indicated a possible
condensation of oleylamine with an acetylacetonate fragment,
consistent with the presence of a ν(C=O) band in FTIR. The
supernatant of the reaction was also analyzed by gas
chromatography coupled to a mass spectrometer (GC-MS). It
-
1
showed the presence of a major product of mass 309 g.mol .
In relation with the bands detected in the FTIR spectrum, this
was attributed to N-oleylacetamide (CH –CO–NH–R, with R as
3
the alkyl chain of oleylamine), formed during the heating
process of the NPs synthesis from the condensation of
oleylamine with an acetyl fragment coming from the
3
0,31
decomposition of acetylacetonate.
H from this N–H moiety
1
was detected as a “free” species by H NMR of the supernatant
by a broad signal at 2.60 ppm (Figure S2), although the N–H
vibration was not observed by FTIR for the species coordinated
to the surface.
It should be noted that this product was also detected in the
analogous synthesis of nickel nanoparticles, although in that
3
1,32
case it was not the major product.
Altogether, an
Figure 1 – (A) TEM of the Cu NPs. (B) Histogram of diameters for Cu NPs. (C) FTIR unexpected surface ligand, N-oleylacetamide, formed during
spectrum of Cu NPs layer (top) and oleylamine (bottom) measured under inert
atmosphere.
the synthesis of the nanoparticles and acted as the main native
capping ligand on the surface of the nanoparticles.
The identification of surface species stabilizing the copper
nanoparticles was performed by Fourier-transformed infrared Influence of tri-n-butylphosphine on the reactivity of copper
spectroscopy (FTIR) in a tailor-made air-tight Teflon cell nanoparticles for the reduction of benzaldehyde
designed for measurement in attenuated total reflection (ATR)
The copper-catalyzed reduction of benzaldehyde was
mode (see experimental section). In the glovebox, a solution of
investigated in solution, without and with additional tri-n-
the nanoparticles was drop-casted on a silicon single-crystal
butylphosphine. Tri-n-butylphosphine was selected, among
wafer. The wafer was inserted and sealed in the cell, allowing
commercial phosphines, as a coordinating ligand, expected to
measurements under inert conditions.
improve the colloidal stability of the nanoparticles in solution,
33–35
Figure 1C shows FTIR of the copper nanoparticles compared
with pure oleylamine, which was expected to be the main
capping ligand around the nanoparticles. As expected, alkyl
chains were detected on the nanoparticles through the C-H
as observed with other metal nanoparticles,
and also for
its ability to modify the electronic properties of the copper
nanoparticles.
The nanoparticles were engaged in the reduction of
benzaldehyde by hydrosilylation using phenylsilane in
tetrahydrofuran (THF) (Scheme 1). Hydrosilanes represent a
very convenient hydride source for the reduction of carbonyl
-1
vibration bands at 2922 and 2852 cm and the C–H
-1
-1
deformation mode at 1465 cm slightly shifted at 1455 cm
for the copper nanoparticles. The characteristic signals of the
-1
free amino group of oleylamine (ν(N–H) at 3318 cm and δ(N–
2
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compounds since they can be used in very mild conditions,
limiting the formation of over-reduction products and allowing
3
6
chemoselective and/or asymmetric reactions. The silicon
moiety can then be easily removed in the presence of fluoride
3
7
to recover the alcohol. Among them, phenylsilane is
considered a strong hydride donor, and was selected for the
first series of experiment.
First attempts using the nanoparticles as prepared showed no
3
catalytic activities. Tri-n-butylphosphine (P(n-Bu) , 1 equiv.
vs. Cu) was added as a co-catalyst: a dark-reddish suspension
formed, suggesting an improved colloidal stability of the
nanoparticles. As a result the complete reduction of
benzaldehyde was observed within 1 h at room temperature
1
RT), using 0.6 mol% of copper vs. benzaldehyde. The H NMR
(
spectrum of an aliquot of the mixture revealed the complete
conversion of benzaldehyde into the corresponding silylated
alcohol (Table 1, entry 1, vide supra). Noticeably, upon the
addition of phenylsilane and benzaldehyde to suspension of
Cu NPs/P(n-Bu) , strong bubbling and heating of the solvent
3
was immediately observed. Bubbles came from the
dehydrogenation of the phenylsilane (producing H , detected
2
1
by H NMR), which is a competitive reaction.
Cu NPs (0.6 mol%)
H
H
H
Ph
+
O
SiH
3
P(n-Bu)3 (0.6 mol%)
Si
Si
Ph
Ph
O
Ph
Ph
O
Ph
+
THF (5 mL), RT, 1 h
3
2
0
.4 equiv.
Scheme 1 – Cu-catalyzed hydrosilylation of benzaldehyde with phenylsilane
Figure 2 – Reduction of benzaldehyde with phenylsilane. (A) Series of FTIR
spectra (B) Integration of the carbonyl (blue) and hydride (red) absorption vs.
time. Dotted line indicates the addition of the solution.
In order to understand the role of the phosphine, the reaction
was reproduced in the air-tight FTIR cell: the nanoparticles
were drop-casted on the silicon wafer as before, and a THF
As a partial conclusion, the combination of both the Cu NPs
3
solution containing benzaldehyde, phenylsilane and P(n-Bu) in
and P(n-Bu)
, entries 1-4). The absence of an induction time at a 10 s time
scale, as well as the immediate release of H observed
3
was essential to observe a catalytic activity (Table
molar ratio 1:0.4:0.006 (identical to the ratio used in the
standard protocol) was added onto the nanoparticles layer.
Figure 2A shows the series of spectra collected immediately
before (first spectrum) and after addition of the solution (ca.
1
2
experimentally upon addition of the benzaldehyde into the
nanoparticles suspension, indicated that the active catalytic
0
.1 mL), over 2000 s. Several bands were observed on the
species were formed from Cu NPs and P(n-Bu)
addition of the reactants. This phosphine effect is not specific
to P(n-Bu) as other phosphines (tricyclohexylphosphine,
triphenylphosphine) or the diphosphine (1,2-
bis(diphenylphosphino)benzene showed comparable
3
prior the
-
1
-1
initial spectrum, coming from THF (1969 cm , 1460 cm ,
-1
-1
365 cm , 1289 cm ), benzaldehyde (1704 cm , two bands at
-1
1
3
-1
-1
597 and 1585 cm , 1310 cm ; blue dots) and phenylsilane
1
-1
(2157 cm , red dot). After addition of the reaction mixture in
the cell, the FTIR spectra were dominated by the absorption
peaks of the solvent and reactant. Product formation was
reactivities after 15 min of reaction time (see ESI, Table S1).
The more sterically hindered tri-tert-butylphosphine and the
-
detected through the appearance of bands at 1594 cm ,
1
tridentate
phosphine
1,1,1-
-
1
-1
497 cm and 1379 cm (Figure 2A, green dots). Figure 2B
1
tris(diphenylphosphinomethyl)ethan proved to be very poor
ligands for this catalytic reaction.
shows the time evolution of the integrated intensities of the
-1
-1
bands at 1704 cm (ν(C=O)) and 2156 cm (ν(Si–H)). Their fast
decrease indicated that the reaction in the cell was completed
within 500 s. Moreover, the monotonic decrease of the
benzaldehyde and phenylsilane concentration (Figure 2B)
suggested no induction time at the time-scale of a few tens of
seconds. As a control experiment, injection of the reaction
mixture deprived of P(n-Bu)
3
lead – as expected – to no
partial
reduction of the benzaldehyde, although
a
decomposition of the silane was observed (see SI, Figure S5).
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Table 1 – Reduction of benzaldehyde with various catalysts
both cases, PhSiH3 addition was quickly followed by the
formation of bubbles and a slight heating of the solution,
meaning that active copper complexes were still present in the
1
nanoparticle-free solution (C). After 15 min of stirring, H NMR
of both solutions showed similar conversion of benzaldehyde
into the silylated alcohol. According to TEM, the initial Cu NPs
(see Figure 1A) were already etched in numerous smaller NPs
(< 5 nm) after addition of the TMDS (Scheme 2D) and their
morphology was even farther from the starting one after
1
Entry
catalyst
Cu NPs + P(n-Bu)
none
Benzaldehyde conversion ( H NMR)
1
2
3
4
5
6
3
>99%
NR
Cu NPs
NR
P(n-Bu)
[Cu(acac)
[Cu(acac) ] + P(n-Bu)
3
NR
addition of PhSiH (Scheme 2E).
3
2
]
NR
2
3
>99%
Reaction conditions: in a 10 mL closed tube, catalyst (0.6 mol%, 0.016 mmol) in
THF (5 mL), P(n-Bu) (0.6 mol%, 0.016 mmol), PhCHO (5 mmol), hydride source
6 mmol). NR: no conversion of benzaldehyde.
3
(
In order to gain insights into the catalyst nature, the activity of
nanoparticles was compared with that of copper complexes.
The initial [Cu(acac)
synthesis proved to be an inefficient catalyst for this reaction
Table 1, entry 5). However, in the presence of 1 equiv. of P(n-
Bu) , the blue solution quickly turned yellow to dark in
2
] compound used for the nanoparticles
(
3
seconds, suggesting the formation of Cu NPs (Figure S13),
showed the same reactivity than the nanoparticles for
benzaldehyde reduction (Table 1, entry 6). The in situ
3
formation of Cu NPs from CuI and PhSiH has recently been
reported by Sharma et al. in the C–N bond reaction formation
3
of carbonyls with amines.
8
The observations described above using nanoparticles as
catalysts (e.g. the brightening of the red color at the addition
of the phenylsilane) were reminiscent of the formation of
Scheme 2 – Leaching test and TEM of the supernatants (D) and (E), black bar is
5
0 nm
3
9,40
copper hydride species stabilized by phosphine ligands.
In
Inductively Coupled Plasma Mass Spectrometry (ICP-MS) of the
supernatant (C) revealed that it contained about 5% of the
initial copper: homogeneous copper species and/or small
clusters were produced from the NPs via a leaching
fact, among the variety molecular metallic complexes known
to promote hydrosilylative reduction of carbonyl
3
6,41,42
derivatives,
potent candidates because of the formation of copper hydride
Cu(I) and Cu(II) complexes proved to be
phenomenon. It is to be noted that the system {[Cu(acac)
2
] +
43–47
species, stabilized by electron-rich phosphines.
the in situ formation of nanoparticles or clusters, such as
3
Stryker’s reagent, is rarely discussed whereas they can be
However,
P(n-Bu) } is also efficient at the loading of the detected species
3
by ICP in the supernatant (approx. 0.03 mol%). At this point,
two hypotheses can be considered: (i) Cu NPs act as a copper
source to release catalytically active copper hydride molecular
complexes/clusters or (ii) both Cu NPs and leached copper
hydride species are catalytically active.
8
1
,48,49
active in various reactions.
Thus, we investigated the
nature of the active species and the possibility of leaching in
the current reaction.
The leaching was further studied by analyzing clean
Description of the active catalytic species
Cu NPs/P(n-Bu)
3
suspensions in THF after 18 h at RT.
in a 1:1 ratio already showed a non-negligible
Leaching is a recurring issue when using NPs in catalysis, as the Cu NPs/P(n-Bu)
3
generated solvated metal complex or clusters, in our case Cu, amount of small Cu NPs compared to the initial ones (<5 nm
could as well be catalysts for the reaction. Thus, we performed versus 40 nm) (Scheme 3A). Increasing the quantity of
a leaching test, using a less reactive hydrosilane, TMDS phosphine in solution led to even more etching of the Cu NPs.
(
1,1,3,3-tetramethyldisiloxane), in order to stop the reaction at At 250 equiv. of P(n-Bu)
a partial conversion after 15 minutes (Scheme 2).
5 nm were found on the TEM grid. Noticeably, the same effect
Benzaldehyde was reacted with TMDS in the presence of Cu occurred by replacing the phosphine by a typical ligand for
NPs/P(n-Bu)
as catalyst for 15 min (Scheme 2A). Then, half of copper complexes, DBU (1,8-diazabicyclo[5.4.0]undec-7-
the crude (B) was centrifuged at 10,000 g for 25 min whereas ene),
3
, only nanoscaled species smaller than
3
5
1–53
13
or even by oleylamine (Scheme 3B). However,
the other half (dark red) was kept intact (D). The colorless neither of these suspensions of leached copper nanoparticles
5
0
supernatant (C) was carefully collected from (B) and PhSiH
was added, yielding a light yellow solution. Similarly, PhSiH
3
showed catalytic property for the reduction of benzaldehyde
with phenylsilane. This means that the formation of leached
3
was added to flask (D), yielding a bright red solution (E). In nanoscaled species was not sufficient to produce catalytically
4
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active species. Rather, the phosphine was required as a co-
catalyst.
Scheme 3 – Formation of smaller Cu NPs in THF due to the addition of coordinating ligands. (A) Influence of the quantity of P(n-Bu)
3
on Cu NPs. From left to right: no
3
P(n-Bu) ; 1 equiv. per Cu atom; 8 equiv. per Cu atom; 250 equiv. per Cu atom; (B) Influence of two other ligands. Left: 1 equiv. of DBU per Cu atom; right: 1 equiv. of
OAm per Cu atom. Scale bars are 50 nm. Red circles are a guide to the eye, indicating the newly-formed species.
Addition of hydride to f orm the active
catalytic species
Ligand-induced leaching of the Cu NPs
L= PR
3
and "bad H-"
-
L = DBU, OAm and any "H "
3
2
M = SiR , BR or Na
inactive
species
H
H
-
Ph
OM
"
H " source
ligand L
Cu
active catalytic
species
smaller nanoparticles
nanoclusters
H
L = PR
and "adequate H-"
3
Ph
O
Scheme 4 – Influence of the hydride source on the catalytic activity
To identify more precisely the active catalytic species, copper SI, Figure S19), indicating a possible coordination of the
species resulting from the phosphine presence were analyzed phosphine in cluster-type complexes. The control reactions
by electrospray-ionization mass spectroscopy (ESI-MS). These were performed without Cu foil or without P(n-Bu)
species were produced by stirring Cu NPs (3 mg) for 24 h in in no conversion of benzaldehyde. At this point, it appeared
pure P(n-Bu) (3 mL). After washing cycles with THF and that the source of copper was not critical regarding the
3
, resulting
3
centrifugation, the powder was redispersed in THF as a red existence of leaching.
suspension. Copper species with nuclearity of 1 to 5 stabilized
by P(n-Bu)
PBu Cu (CN)
4
3
were detected: e.g. Cu(PBu
3 2
) (m/z = 469), The generation of copper-hydride species is a key step in the
(
3
)
2
5
(m/z = 825) (see SI, Figure S14,15). Larger copper-catalyzed reduction of carbonyl compounds (Scheme
4
3,54
species could not be detected by our experimental setup but
were observed by TEM (Scheme 3A).
4
)
and appears to be related to the nature of the
coordinating ligands (P(n-Bu) , OAm, DBU). So far, we
3
In order to determine if the leaching was a cause of the nano- demonstrated that several coordinating ligands were able to
scale of initial copper nanoparticles, the reaction was also generate leached species from the Cu NPs but only phosphine-
explored with an ultrapure copper foil (purity >99.99%) as the stabilized copper species were able to act as catalysts for the
copper species. The copper foil alone (ca. 50 mg) was not able hydrosilylative reduction of benzaldehyde (Scheme 4, right).
to catalyze the reduction of benzaldehyde with phenylsilane at
2
0 °C after 18 h. However, in the presence of P(n-Bu) in a Copper-hydride species require stabilization, which is usually
3
4
8,55
catalytic amount (0.6 mol%), the colorless solution turned light done with phosphines or N-heterocyclic carbenes.
Among
yellow and the septum of the flask was swollen, indicating the the three ligands studied here, we propose that only P(n-Bu)3
1
formation of gaseous H . The H NMR analysis revealed that stabilizes them properly. To test this hypothesis, different
2
ca. 20% of benzaldehyde was converted into various silylated hydride sources were investigated in the presence of P(n-Bu)3
3
species. Additionally, the P{ H} NMR showed drastic decrease (Scheme 4).
1
1
and broadening of the signal of free P(n-Bu) (at -32 ppm) (see
3
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Influence of the hydride source on the activity of the copper
catalyst
phosphines with planar copper surfaces (e.g. single-crystals)
may inform on the elementary steps of the leaching process.
Various hydride sources were tested for the reduction of
benzaldehyde, in similar conditions as previously (0.6 mol% of
Table 2 – Reactivity of selected hydrides for the reduction of benzaldehyde and
Cu NPs/P(n-Bu
by family and strength of hydrides. Benzaldehyde was fully
reduced using Ph SiH or (EtO) SiH (entries 2, 3) and only
partially with TMDS (31%, entry 4).
3
), RT, 18 h). Table 2 shows the results ordered
their effects on the Cu NPs.
2
2
3
Hydrosilanes with a lower hydride donating ability proved to
be unreactive in these conditions (entries 5-8). NaH acted as a
catalyst for the disproportionation of benzaldehyde into
(a)
Entry
Hydride source
Conversion
5
6
benzyl benzoate, according to the Tishchenko reaction
entry 9), which occurs quicker than the direct reduction of
6
3
Hydrosilanes, from stronger to softer
(
(b)
1
2
3
4
5
6
7
8
PhSiH
Ph SiH
(EtO) SiH
TMDS
3
>99%
benzaldehyde.
Boron-based hydride donor such as NaBH
4
or 9-
2
2
>99%
>99%
31%
n.d.
borabicyclo[3.3.1]nonane (9-BBN-H) were also highly reactive
for the reduction of benzaldehyde (entries 10, 11).
Interestingly, ammonium formate was not able to reduce
3
5
7–59
PhMe
2
SiH
benzaldehyde by hydrogenation (entry 12).
Et
3
SiH
n.d.
Compared to the other reductants, ammonium formate was
also more detrimental to the integrity of the nanoparticles,
according to TEM observation after the catalytic test (SI, Table
S2).
Ph
3
SiH
n.d.
(
c)
PMHS
n.d.
Other hydride sources
Finally, bubbling H
2
in the system did not result in any
(d)
9
NaH
>99%
>99%
>99%
n.d.
conversion of benzaldehyde (entry 13). Noticeably, the
reduction of benzaldehyde was highly dependent on the
hydride choice. This means that the formation of phosphine-
stabilized copper species was not sufficient to obtain
catalytically active species. Thus, the adequate combination
between the right ligand and a strong enough hydride donor
seems mandatory to reach a catalytic activity.
1
1
0
1
NaBH4
(9-BBN-H)
HCO NH
H2
2
12
13
2
4
n.d.
Reaction conditions: in a 10 mL closed tube, Cu NPs (0.6 mol%, 0.016 mmol of Cu)
dispersed in THF (5 mL), P(n-Bu) (0.6 mol%, 0.016 mmol), PhCHO (5 mmol),
3
1
hydride source (6 mmol). (a) H NMR yield given for the conversion of
benzaldehyde to the corresponding alcohol; n.d. = not detected (b) Complete
after 20 min. (c) PMHS = polymethylhydrosiloxane. (d) benzaldehyde was fully
converted into benzyl benzoate.
Conclusions
This work highlights the ligand sensitivity of the applications of
Cu NPs in catalytic reduction reactions and reactivates the
question of the homogeneous or heterogeneous state of the
catalytically active species, as it has been the case for Pt-based
Conflicts of interest
In the presence There are no conflicts to declare.
6
0–62
catalysts for the hydrosilylation of olefins.
of a phosphine (such as P(n-Bu )), these NPs proved to be
3
active for the reduction of benzaldehyde in the presence of a
strong enough hydride source.
Acknowledgements
The activity of this system is based on the rapid partial
leaching of the NPs, which act as a copper reservoir, releasing
phosphine-stabilized copper species that seem to be the
catalytically active species. Leaching was not systematically
providing active catalytic species: rather, these should present
a suitable ligand (phosphine) in combination with a suitable
hydride source (silylated or not).
This work was performed in the framework of Idex ANR-10-
IDEX-0001-02 PSL*. The “Cellule Energie” of CNRS provided
financial support for IR instrumentation. Sorbonne Université,
CNRS and Collège de France are acknowledged for financial
support. The synchrotron SOLEIL is acknowledged for
beamtime allocation on beamline SMIS (under proposal
2
0160209 and 20170383) and for the use of chemistry
We propose this as the reason why many catalysis studies on
ligand-stabilized copper nanoparticles do not report leached
molecular species as the active catalytic species, although
these may form in the reaction medium. From a more
fundamental perspective, model studies of the interaction of
laboratory facilities onsite. We are very thankful to Christophe
Sandt and Paul Dumas for helpful discussions about ATR
measurements. Denis Lesage (Sorbonne University), Domitille
Giaume and Claudine Keil (IRCP, Chimie Paris) are
6
| J. Name., 2012, 00, 1-3
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acknowledged for their expertise in ESI-MS, ICP-MS and GC- 23
MS, respectively.
D. Astruc, F. Lu and J. R. Aranzaes, Angew. Chemie - Int. Ed.,
2005, 44, 7852–7872.
2
4
B. H. Lipshutz, S. Tasler, W. Chrisman, B. Spliethoff and B.
Tesche, J. Org. Chem., 2003, 68, 1177–1189.
Notes and references
2
5
M. Diéguez, O. Pàmies, Y. Mata, E. Teuma, M. Gómez, F.
Ribaudo and P. W. N. M. Van Leeuwen, Adv. Synth. Catal.,
1
M. B. Gawande, A. Goswami, F.-X. Felpin, T. Asefa, X.
2
008, 350, 2583–2598.
Huang, R. Silva, X. Zou, R. Zboril and R. S. Varma, Chem.
Rev., 2016, 116, 3722–3811.
2
2
2
2
3
3
6
7
8
9
0
1
J. Zhang, C. Yu, S. Wang, C. Wan and Z. Wang, Chem.
Commun., 2010, 46, 5244.
2
3
Z. Duan, G. Ma and W. Zhang, Bull. Korean Chem. Soc.,
M. J. Albaladejo, F. Alonso and M. Yus, Chem. - A Eur. J.,
2012, 33, 10–13.
2
013, 19, 5242–5245.
X. Yang, H. Zhong, Y. Zhu, H. Jiang, J. Shen, J. Huang and C.
W. Zhang, F. Guo, F. Wang, N. Zhao, L. Liu, J. Li and Z.
Wang, Org. Biomol. Chem., 2014, 12, 5752–5756.
S. K. Movahed, M. Dabiri and A. Bazgir, Appl. Catal. A Gen.,
Li, J. Mater. Chem. A Mater. Energy Sustain., 2014, 2,
9
040–9047.
A. K. Sasmal, S. Dutta and T. Pal, Dalt. Trans., 2016, 45,
139–3150.
4
5
6
7
8
9
2
014, 481, 79–88.
3
S. Wang, Y. Yu, X. Chen, H. Zhu, P. Du, G. Liu, L. Lou, H. Li
and W. Wang, Tetrahedron Lett., 2015, 56, 3093–3096.
S. Carenco, S. Labouille, S. Bouchonnet, C. Boissière, X. F.
Le Goff, C. Sanchez and N. Mézailles, Chem. - A Eur. J.,
A. Dhakshinamoorthy, S. Navalon, D. Sempere, M. Alvaro
and H. Garcia, Chem. Commun., 2013, 49, 49–51.
T. Subramanian and K. Pitchumani, Catal. Sci. Technol.,
2012, 2, 296–300.
2
012, 18, 14165–14173.
F. Arena, K. Barbera, G. Italiano, G. Bonura, L. Spadaro and
F. Frusteri, J. Catal., 2007, 249, 185–194.
3
3
3
3
2
3
4
5
K. X. Bhattacharyya, C. Pradel, P. Lecante and N. Mézailles,
Chem. - A Eur. J., 2017, 23, 9352–9361.
C. S. Chen, J. H. Wu and T. W. Lai, J. Phys. Chem. C, 2010,
J. Tang, F. Redl, Y. Zhu, T. Siegrist, L. E. Brus and M. L.
Steigerwald, Nano Lett., 2005, 5, 543–548.
114, 15021–15028.
S. Natesakhawat, J. W. Lekse, J. P. Baltrus, P. R. Ohodnicki,
J. Y. Choi, S. J. Lee, W. S. Seo and H. Song, CrystEngComm,
B. H. Howard, X. Deng and C. Matranga, ACS Catal., 2012,
2
016, 18, 6069–6075.
2
, 1667–1676.
A. Fedorov, H. Liu, H. Lo and C. Copéret, J. Am. Chem. Soc.,
016, 138, 6–11.
J. Park, E. Kang, S. U. Son, H. M. Park, M. K. Lee, J. Kim, K.
W. Kim, H. J. Noh, J. H. Park, C. J. Bae, J. G. Park and T.
Hyeon, Adv. Mater., 2005, 17, 429–434.
1
0
1
2
1
O. G. Salnikov, H. Liu, A. Fedorov, D. B. Burueva, K. V.
3
3
3
6
7
8
S. Díez-González and S. P. Nolan, Acc. Chem. Res., 2008, 41,
Kovtunov, C. Copéret and I. V Koptyug, Chem. Sci., 2017, 8,
3
49–358.
A. M. Dilauro, W. Seo and S. T. Phillips, J. Org. Chem., 2011,
6, 7352–7358.
2426–2430.
12
13
14
C. Barrière, K. Piettre, V. Latour, O. Margeat, C.-O. Turrin,
B. Chaudret and P. Fau, J. Mater. Chem., 2012, 22, 2279.
S. Duman and Ö. Saim, ChemCatChem, 2017, 9, 2588–
7
M. Kumar, V. Bhatt, O. S. Nayal, S. Sharma, V. Kumar, M. S.
Thakur, N. Kumar, R. Bal, B. Singh and U. Sharma, Catal.
Sci. Technol., 2017, 7, 2857–2864.
2598.
M. A. Ben Aissa, B. Tremblay, A. Andrieux-Ledier, E.
3
4
4
4
4
4
4
4
9
0
1
2
3
4
5
6
W. S. Mahoney, D. M. Brestensky and J. M. Stryker, J. Am.
Chem. Soc., 1988, 110, 291–293.
Maisonhaute, N. Raouafi and A. Courty, Nanoscale, 2015,
7, 3189–3195.
B. A. Baker, Ž. V. Bošković and B. H. Lipshutz, Org. Lett.,
1
5
S. Carenco, Y. Hu, I. Florea, O. Ersen, C. Boissière, N.
2
007, 10, 289–292.
Mézailles and C. Sanchez, Chem. Mater., 2012, 24, 4134–
W. Sattler, S. Ruccolo, M. Rostami Chaijan, T. Nasr Allah
and G. Parkin, Organometallics, 2015, 34, 4717–4731.
A. Rit, A. Zanardi, T. P. Spaniol, L. Maron and J. Okuda,
Angew. Chemie - Int. Ed., 2014, 53, 13273–13277.
C. Deutsch, N. Krause and B. H. Lipshutz, Chem. Rev., 2008,
4
145.
M. Salavati-niasari, Z. Fereshteh and F. Davar, Polyhedron,
009, 28, 126–130.
1
6
7
2
1
D. Adner, S. Möckel, M. Korb, R. Buschbeck, T. Rüffer, S.
Schulze, L. Mertens, M. Hietschold, M. Mehring and H.
Lang, Dalt. Trans., 2013, 42, 15599–15609.
1
08, 2916–2927.
K. Motokura, D. Kashiwame, A. Miyaji and T. Baba, Org.
Lett., 2012, 14, 2642–2645.
18
I. Cano, M. a Huertos, A. M. Chapman, G. Buntkowsky, T.
Gutmann, P. B. Groszewicz and P. W. N. M. van Leeuwen, J.
Am. Chem. Soc., 2015, 137, 7718–27.
K. Motokura, N. Takahashi, D. Kashiwame, A. Miyaji and T.
Baba, Catal. Sci. Technol., 2013, 3, 2392–2396.
A. Y. Houghton, V. A. Karttunen, E. Piers and H. M.
Tuononen, Chem. Commun., 2014, 50, 1295–1298.
L. Zhang and Z. Hou, Chem. Sci., 2013, 4, 3395–3403.
R. S. Dhayal, W. E. van Zyl and C. W. Liu, Acc. Chem. Res.,
19
20
21
22
N. Almora-barrios, I. Cano and P. W. N. M. Van Leeuwen,
ACS Catal., 2017, 7, 3949–3954.
K. R. Kahsar, D. K. Schwartz and J. W. Medlin, J. Am. Chem.
Soc., 2014, 136, 520–526.
4
4
7
8
I. Schrader, J. Warneke, J. Backenko and S. Kunz, J. Am.
Chem. Soc., 2014, 137, 905–912.
2
016, 49, 86–95.
4
9
A. W. Cook, Z. R. Jones, G. Wu, S. L. Scott and T. W. Hayton,
I. Cano, A. M. Chapman, A. Urakawa and P. W. N. M. Van
Leeuwen, J. Am. Chem. Soc., 2014, 136, 2520–2528.
J. Am. Chem. Soc., 2018, 140, 394–400.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
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Page 8 of 9
View Article Online
DOI: 10.1039/C8CY01516C
ARTICLE
Journal Name
5
0
1
J. A. Widegren and R. G. Finke, J. Mol. Catal. A. Chem.,
003, 198, 317–341.
2
5
R. Watari, Y. Kayaki, S. Hirano, N. Matsumoto and T.
Ikariya, 2015, 1369–1373.
5
2
3
B. F. Thakur and G. Sekar, Synthesis, 2009, 16, 2785–2789.
R. Watari, N. Matsumoto, S. Kuwata and Y. Kayaki,
ChemCatChem, 2017, 9, 4501–4507.
5
54
J.-T. Issenhuth, F.-P. Notter, S. Dagorne, A. Dedieu and S.
Bellemin-Laponnaz, European J. Org. Chem., 2010, 529–
541.
55
56
57
58
K. Nakamae, M. Tanaka, B. Kure, T. Nakajima and Y. Ura,
Chem. - A Eur. J., 2017, 23, 9457–9461.
T. Werner and J. Koch, European J. Org. Chem., 2010,
6904–6907.
P. Selvam, S. U. Sonavane, K. Mohapatra and R. V Jayaram,
Tetrahedron Lett., 2004, 45, 3071–3075.
M. Kidwai, V. Bansal, A. Saxena and S. Mozumdar,
Tetrahedron Lett., 2006, 47, 4161–4165.
5
9
0
A. Saha and B. Ranu, J. Org. Chem., 2008, 73, 6867–6870.
T. Galeandro-Diamant, M. L. Zanota, R. Sayah, L. Veyre, C.
Nikitine, C. De Bellefon, S. Marrot, V. Meille and C.
Thieuleux, Chem. Commun., 2015, 51, 16194–16196.
M. A. Rivero-Crespo, A. Leyva-Pérez and A. Corma, Chem. -
A Eur. J., 2017, 23, 1702–1708.
6
61
62
63
B. P. S. Chauhan and A. Sarkar, Dalt. Trans., 2017, 46,
8709–8715.
M. Horn, L. H. Schappele, G. Lang-Wittkowski, H. Mayr and
A. R. Ofial, Chem. - A Eur. J., 2013, 19, 249–263.
8
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Catalysis Science & Technology
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DOI: 10.1039/C8CY01516C
Depending on the ligand, ligand-induced leaching of copper nanoparticles may produce catalytically
active species for the reduction of benzaldehyde.