An N-heterocyclic carbene ligand promotes highly selective alkyne semihydrogenation with copper nanoparticles supported on passivated silica

Article abstract of DOI:10.1039/c8sc01924j

We report a surface organometallic route that generates copper nanoparticles (NPs) on a silica support while simultaneously passivating the silica surface with trimethylsiloxy groups. The material is active for the catalytic semihydrogenation of phenylalkyl-, dialkyl- and diaryl-alkynes and displays high chemo- and stereoselectivity at full alkyne conversion to corresponding (Z)-olefins in the presence of an N-heterocyclic carbene (NHC) ligand. Solid-state NMR spectroscopy using the NHC ligand ¹³C-labeled at the carbenic carbon reveals a genuine coordination of the carbene to Cu NPs. The presence of distinct Cu surface environments and the coordination of the NHC to specific Cu sites likely account for the increased selectivity.

Full text of DOI:10.1039/c8sc01924j

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An N-Heterocyclic Carbene Ligand Promotes Highly Selective
Alkyne Semihydrogenation with Copper Nanoparticles Supported
on Passivated Silica
Received 00th January 20xx,
Accepted 00th January 20xx
a
a,b
a
a
a
DOI: 10.1039/x0xx00000x
Nicolas Kaeffer, Hsueh-Ju Liu, Hung-Kun Lo, Alexey Fedorov * and Christophe Copéret *
www.rsc.org/
We report a surface organometallic route that generates copper nanoparticles (NPs) on a silica support while
simultaneously passivating the silica surface with trimethylsiloxy groups. The material is active for the catalytic
semihydrogenation of phenylalkyl-, dialkyl- and diaryl-alkynes and displays high chemo- and stereoselectivity at full alkyne
conversion to corresponding (Z)-olefins in the presence of a N-heterocyclic carbene (NHC) ligand. Solid-state NMR
1
3
spectroscopy using the NHC ligand C-labeled at the carbenic carbon reveals a genuine coordination of the carbene to Cu
NPs. The presence of distinct Cu surface environments and the coordination of the NHC to specific Cu sites likely accounts
for the increased selectivity.
batch semihydrogenation of alkynes in the presence of ligands
2
9, 30
Introduction
(e.g. PCy )
3
with performances comparable to the Lindlar
3
1
catalyst. However, with certain substrates these earth-
abundant Cu catalysts still display limited selectivity at full
alkyne conversion due to the secondary hydrogenation of
formed alkenes.
N-Heterocyclic carbenes (NHCs) belong to a class of prominent
1
ligands, especially in homogeneous catalysis, primarily due
-3
4
to their strong
σ-donor properties that yield organometallic
2
, 5
complexes with very stable metal-carbon bonds.
The NHC
2
0, 22-24, 32-36
We reasoned that ligand functionalization of NPs
ligands have been utilized to improve performance of
numerous molecular catalysts in a broad range of applications
would improve chemoselectivity towards alkenes through
competitive adsorption or poisoning of unselective sites by
6
7
such as olefin metathesis, cross-coupling or hydrogenation
2
9
8
reactions, to mention just a few. NHCs also bind to surfaces
-10
strong
σ-donor ligands such as NHCs. However, free NHCs
are strong bases (imidazolium/-ylidene couples typically have
1
1-14
of metallic nanoparticles (NPs) and wafers,
which was
and recently exploited in
such as electrocatalytic reduction of
Another example is the use of Pd NPs decorated with
NHC ligands for the Buchwald-Hartwig amination of aryl
3
7, 38
1
5, 16
pKa values in 20-29 range)
and as such deprotonate
initially demonstrated for Au NPs
1
7-21
surface silanols of silica. In order to favor the selective
coordination of an NHC ligand on the Cu sites, we use Surface
catalytic applications,
2
2, 23
2
CO .
3
9
Organometallic Chemistry (SOMC) with a tailored molecular
precursor, [Cu (HMDS) ], to synthesize Cu nanoparticles on
passivated silica (that is silica where surface silanols are
2
4
4
4
chlorides.
Selective semihydrogenation of alkynes to alkenes is an
4
0, 41
2
5
replaced by trimethylsilyloxy groups, SiO2-TMS).
The
important process used both in industry and academia. The
corresponding catalysts are typically based on Pd, among
resulting material (Cu/SiO2-TMS) is a highly active alkyne
hydrogenation catalyst that becomes selective towards
semihydrogenation by addition of a prototypical NHC ligand,
2
6
which the Lindlar catalyst is most frequently used, even
though it relies on scarce and expensive Pd metal and requires
toxic Pb additive to achieve high selectivity. Copper-based
materials are known to catalyze semihydrogenation of alkynes
IMes (IMes
4
=
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-
2
ylidene), through the binding of IMes to mostly Cu surface
sites as revealed by solid-state NMR spectroscopy.
2
7, 28
under flow conditions.
In that context, our group has
recently reported that copper nanoparticles supported on a
partially dehydroxylated silica (Cu/SiO2-700) efficiently catalyze
Results and discussion
Preparation of Cu/SiO2-TMS
The SOMC approach exploits the reactivity of [Cu (HMDS) ]
4
4
4
that was prepared on gram scale from [Cu (COD) Cl ] and
3
2
2
2
4
4
NaHMDS (39% yield, see ESI for details). This Cu precursor
reacts with SiO2-700 (2.1 equiv. Cu per surface Si-OH site) in
≡
THF at 60 °C (Scheme 1) to yield a material containing by
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4
0, 41
elemental analysis 2.12, 1.81, 0.29 and 0.26 wt% loadings in silanols
([CuHMDS]/SiO2-TMS, Scheme 1). Treatment of
Cu, C, H and N, respectively.
Isolated
silanols
[CuHMDS]/SiO2-TMS under a flow of H
change from white to dark red and affords Cu/SiO2-TMS
TEM images of show the presence of small, well dispersed Cu
NPs with a narrow size distribution of 2.0 ± 0.6 nm (Figure
1e,f). Elemental analysis of indicates a Cu loading of 2.31
wt% and H chemisorption gives ca. 0.07 mmol Cusurface
Cu surface sites (Figure S2). This loading is lower compared to
that obtained using [Cu (Mes) ] precursor (5.5 wt%, 0.31 mmol
g ), in agreement with the competing passivation of
silanol sites by Me Si groups that consequently limits the
density of Cu sites upon grafting.
The IR spectrum of displays a strong C–H stretching band at
2
at 300 °C results in a color
(
1). HR-
H
H
[(HMDS)
Cu
]
TMS
m
n
O
O
O
O
1
[
Cu4(HMDS)4]
Si
Si
Si
Si
SiO2–700
SiO2–700
THF, 60 oC,
overnight
[
CuHMDS]/SiO2–TMS
1
–
g
1
2
⋅
in
IMes(*)
N
(*)
N
Mes
Mes
IMes(*)
IMes(*)
H2 flow,
5
5
IMes(*)
o
–1 29
3
00
C
Cu
TMS
Cu
TMS
Cusurface
⋅
O
O
1
2 h,
Toluene, R.T.,
overnight
Si
Si
3
–
NH3
SiO2–700
SiO2–700
Mes = mesityl
Cu/SiO2–TMS (1)
IMes-Cu/SiO2–TMS (2)
IMes*-Cu/SiO2–TMS (2*)
1
Scheme 1. Synthetic route to 1 and 2.
–1
–1
2
967 cm , along with a weak band at 3747 cm that is
associated with the presence of a small amount of residual
Si-OH sites (ca. 10% with respect to initial silanols, Figure 1d).
NMR analysis (Figure S1) shows that features a single set of
IR spectroscopy (Figure 1) reveals that isolated silanols are
fully consumed when [Cu (HMDS) ] is contacted with SiO2-700
disappearance of the sharp band at 3747 cm with
concomitant appearance of C–H stretching bands at 2902 and
≡
4
4
1
–
1
(
1
13
29
peaks at 0.5 ppm ( H), 0 ppm ( C) and 15 ppm ( Si), pointing
to a single chemical environment for SiMe groups. These
observations are consistent with the formation of Cu NPs from
Si–O–[Cu (HMDS) ] sites upon H treatment, regenerating
surface silanols that are mostly directly passivated into Si-O-
SiMe sites by released HN(SiMe
–1
1
13
29
3
2
954 cm associated with SiMe
state NMR spectra of the grafted material indicate the
presence of SiMe groups in two different chemical
3
groups). H, C and Si solid-
≡
n
m
2
3
≡
1
13
29
environments ( H: 0.8 and 0.5 ppm, C: 7 and 0 ppm and Si:
3
3 2
) .
2
3 2
and 15 ppm, Figure S1) corresponding to N(SiMe ) and
4
0
3
OSiMe , respectively.
Activity in alkyne hydrogenation
Next, we recorded consumption profiles during
hydrogenation of a prototypical substrate 1-phenyl-1-propyne
S1) catalyzed by (20 bar H , 60 °C, 1.8 mol% Cutotal, 0.4 mol%
H
2
(
1
2
Cusurface), both with or without additional ligands in the
reaction media (Figure 2). Two subsequent linear trends are
2
observed in the H consumption profile for the unmodified
catalyst 1 (Figure 2c, black trace). The first linear slope of ca.
–1 –1
–
2
–1 –1
–5
4
2 2
.3 10 molH ⋅L ⋅h (1.2 10 molH ⋅L ⋅s ) at early stages is
associated with hydrogenation of S1 to the (Z)-S12H. Following
the full hydrogenation to S12H, the overhydrogenation of S12H
to S14H occurs with a faster rate (second, steeper linear slope)
–
1
–1 –1
–5
–1 –1
2 2
of ca. 1.8 10 molH ⋅L ⋅h (5.0 10 molH ⋅L ⋅s ). GC analysis
of the reaction mixture after 16 h confirms quantitative
conversion of S1 to S14H (Figure 2c, black, and Table 1, entry 1).
The two-step behavior of the process likely originates from
inhibition of the overhydrogenation by the starting alkyne until
the latter gets depleted and only then formation of S14H
proceeds. The fast rate of overhydrogenation at high
conversion is thus responsible for the quick deterioration of
chemoselectivity in the desired cis-olefinic product.
Average size
.0 ± 0.6 nm
e)
f)
2
Introducing 2 mol% of IMes (ca. 1:6 Cusurface/IMes ratio) to the
catalytic mixture does not significantly change the initial rate
of hydrogenation of 1-phenyl-1-propyne, as follows from the
–
2
–1
–
very similar slope in the H
1
2
uptake profile (5.0 10 molH
2
⋅L ⋅h
–5
–1 –1
,
1.4 10 molH ⋅L ⋅s ; Figure 2c, red trace). These unaltered
2
rates suggest no poisoning of catalytic sites by IMes binding.
However, the overhydrogenation to S14H is fully inhibited in
the presence of IMes and the chemoselectivity to (Z)-S12H
Figure 1. FT-IR spectra of a) [Cu
Cu/SiO2-TMS (1) and e) HR-TEM image and f) particle size distribution of 1.
4
4
(HMDS) ], b) SiO2-700, c) [CuHMDS]/SiO2-TMS, d)
These observations are consistent with the formation of remains at 97 % even after 16 h of reaction (Figure 2c, red, and
isolated multinuclear Cu along with Si-OSiMe surface sites Table 1, entry 2). The high (Z)-S12H chemoselectivity is also
by grafting of [Cu (HMDS) ] on SiO2-700 via protonolysis, retained at a lower IMes/S1 ratio of 1:100 (Table 1, entry 3 and
releasing HN(TMS) that further passivates remaining free Figure S3). Furthermore, GC analysis using an internal standard
≡
3
4
4
2
2
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reveals only minor amounts of unidentified side products (1%, Control experiments reveal that while (Z)-S12H is readily
Table S1, entry 2), possibly oligomers, confirming the very high hydrogenated by , introducing IMes (2 mol% with respect to
overall selectivity to (Z)-S12H with added IMes. (Z)-S12H) completely shuts down the conversion of the alkene
Table S2). This result clearly demonstrates that the NHC ligand
1
(
Semihydrogenation Overhydrogenation
inhibits hydrogenation of (Z)-S12H to S14H, thereby avoiding
a)
R
overhydrogenation. For comparison, when IMes is replaced by
R’
+
R’
R
3
tricyclohexylphosphine (2 mol% PCy ), overhydrogenation is
R’
1
(+L)
not completely hindered, thus compromising the selectivity, in
particular at high conversion (Figure 2c,d, blue trace, and
Tables 1 and S2).
(
Z)-S2H
S
4H
R
2
0 bar H2,
+
S
toluene, 60 oC
R’
Other products
side reaction)
R
To illustrate that IMes generally improves selectivity for
semihydrogenation, we submitted several additional
+
(
(
E)-S2H
4
5
phenylalkyl-, dialkyl- and diarylalkynes
hydrogenation over with or without added IMes. We
optimized conditions (1:50 IMes/substrate ratio, 20 bar, 60 °C,
4 h, 9 mol% Cutotal, 2 mol% Cusurface) to ensure full conversion
(S2-S7, Figure 2b) to
b)
X
1
X
2
of the starting alkyne, that was successfully achieved for all
substrates except for S7 (vide infra, Table 2). At these
extended reaction time and significantly higher catalyst
loading, S1 retains good selectivity to S12H (88%) in the
presence of IMes that is however completely lost due to S14H
X = CH3
n-C H
S1
S2
S3
X =
H
OCH
Cl
S4
S5
S6
S7
4
9
3
Br
(
>99%) when IMes is not added (Table 2, entries 1-2). Being
c)
d)
structurally related to S1, 1-phenyl-1-hexyne (S2) likewise
displays high selectivity for semihydrogenation to S24H (91 %)
only in the presence of IMes, whereas complete
overhydrogenation to the corresponding alkane is observed
using pristine 1; both reactions proceed without detected side-
products (Table 2, entries 3-4). The selectivity to the (Z)-olefin
is very high for the aliphatic alkyne S3 (99% to S32H, >99:1 Z/E
ratio) when IMes is added to the reaction mixture, in contrast
to what is observed in the absence of the ligand, where lower
Z selectivity (Z/E = 91:9) and substantial by-product formation
occurs (17%), presumably due to oligomerization (Table 2,
entries 5-6).
Table 2. Conversions and overall selectivities for hydrogenation (20 bar, 60 °C, 24
h) with catalyst 1 (9 mol% Cutotal) in the presence or absence of IMes using a
a
molar ratio IMes/substrate = 1:50 (see ESI for details). Mass balance indicates
the formation of undetected side products, likely oligomers.
Figure 2. Hydrogenation of alkynes with catalyst
b) substrate scope, c) H consumption profiles for hydrogenation of 1-phenyl-1-
propyne (S1, 400 mol) at 20 bar, 60 °C in toluene over unmodified (black, 1.8
mol% Cutotal) or in the presence of IMes (red) or PCy (blue) using a molar ratio
ligand/S1 = 1:50, and d) selectivity to (Z)-S12H among hydrogenation products
S12H and S14H (selectivity[(Z)-S12H S12H+4H]) at final time (16 h).
1. a) General reaction scheme,
Entry Substrate Ligand Conv.
%)
Sel. S2H
(%)
<1
S
2H Z/E
Sel. others (S4H
)
2
(
ratio
–
(%)
ꢀ
1
1
2
3
4
5
6
7
8
9
S1
S1
S2
S2
S3
S3
S4
S4
S5
S5
S6
S6
S7
S7
–
IMes
–
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
79
>99 (>99)
<1 (–)
3
88
90:10
–
/
<1
>99 (>99)
9 (9)
IMes
–
91
91:9
91:9
>99:1
–
Table 1. Selectivities among hydrogenation products (S12H and S14H) for hydrogenation
of S1 (20 bar, 60 °C, 16 h) with catalyst 1 at 1.8 mol% Cutotal loading. All reactions lead
to complete conversion (>99%) of S1.
a
83
17 (<1)
a
IMes
–
99
1 (<1)
<1
>99 (>99)
<1 (–)
>99 (>99)
<1 (–)
72 (72)
4 (4)
Entry Ligand L/S1 ratio Sel. S12H (Z/E) (%) Sel. S14H (%)
IMes
–
>99
<1
97:3
–
1
2
3
4
5
–
–
<1
>99
3
IMes
IMes
IMes
1:50
1:100
1:500
1:50
97 (99:1)
95 (98:2)
12 (58:42)
66 (89:11)
1
0
1
2
3
4
IMes
–
>99
28
97:3
48:52
98:2
99:1
99:1
5
1
1
1
1
88
34
IMes
–
96
PCy
3
>99
>99
<1
IMes
66
<1
Hot filtration experiments showed no further conversion of S1
after removal of catalysts from the reaction mixture with or
without added IMes, thereby demonstrating that no leaching
The semihydrogenation of diphenylacetylenes S4 and S5 in the
presence of IMes provides respective (Z)-olefins in very high
yields (>96%); in contrast, complete overhydrogenation to the
2
9
of copper occurs, consistent with our previous observations.
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corresponding diphenylethanes occurs when IMes is not added Furthermore, X-Ray absorption near edge structure (XANES)
>99% selectivity, Table 2, entries 7-10). The chloro-derivative spectroscopy of and demonstrates a slight change in the
S6 yields mostly S64H (72% selectivity) over the unmodified Cu-K white line from 8996.5 to 8996.7 eV that also supports
catalyst while S62H is obtained with 28% selectivity. binding of IMes on Cu NPs (Figure S6).
Interestingly, S62H formed with pristine is roughly an equal NMR spectroscopy is particularly instrumental in elucidating
mixture of - and - isomers (Table 2, entry 11). Addition of the coordination of ligands, including NHCs, on transition-
IMes ligand alleviates low chemo- and stereoselectivities, metal NPs.
yielding -stilbene S62H in 93% yield (Table 2, entry 12). The * (Figure 3b) reveals a major, broad peak centered at 178
bromo-derivative S7 does not reach full conversion both with ppm and a shoulder at 189 ppm, which are also present in the
(
1
2
1
1
Z
E
1
8, 19, 22, 24, 46-48
13
The C direct excitation spectrum of
Z
2
1
3
or without added IMes (66 and 79% conversion, respectively)
and features high selectivity to ( )-S62H in both cases (>99%, these peaks disappear in the C CP spectrum of
entries 13-14). This high chemoselectivity to (
C cross-polarization (CP) spectrum (Figure 3c). In contrast,
1
3
Z
2
(Figure 3e)
13
Z
)-S72H likely and can thus be attributed to C-labeled carbons in 2*. In
originates from the remaining alkyne S7, consistent with the addition, the intensity of the peak at 178 ppm relative to that
inhibition of the overhydrogenation by the starting alkyne, as of TMS groups of the support (-2 ppm) increases in the direct
2
9
was also reported previsouly. We note that both halogen- excitation spectrum compared to the CP one (Figure S7b,c),
containing substrates S6 and S7 react slower over catalyst pointing to carbon atoms not directly bonded to protons. With
1,
13
possibly due to the interaction of the Cu surface with the these observations, we assign the peak at 178 ppm to ( C-
halogen substituent. The latter effect would also be consistent labeled) carbenic carbons coordinated to Cu NPs. This
with the high fraction of a chloro-stilbene (
pristine catalyst , probably through a secondary isomerization peak compared to the carbenic carbon in free IMes* (219 ppm,
of ( )-S62H on a Cu surface that had possibly been modified by Figure 3a) and by the similarity of the observed shifts to
an interaction with the Cl-substituent (Table 2, entry 11). values reported for NHC bound to Cu sites in [Cu(IMes)(L)]
E)-S62H formed with assignment is further supported by the upfield shift of this
1
4
9
Z
5
0
Overall, these results demonstrate that the enhancement in molecular complexes. In the same ppm range, the shoulder
selectivity provided by IMes can be extended to phenylalkyl, at 189 ppm is also attributed to Cu-bound carbenic carbons,
dialkyl and diaryl internal alkynes. These findings further likely in a different environment.
underline the impressive effect of IMes in inhibiting the The relative intensity of this peak is similar in the direct
undesired overhydrogenation of internal olefins, and also excitation and CP spectra (Figure S7b,c), suggesting the
hindering side reactions such as oligomerization and olefin presence of protons in the vicinity of the carbene ligand,
o
29
isomerization, especially at elevated temperature (60 C).
possibly associated with Cu sites closer to the interface with
the SiO2-TMS support. Alternative interpretations are that
distinct resonances at 178 and 189 ppm arise from IMes bound
Characterizing the ligand-particle interaction
To understand the origin of the improved chemoselectivity, we on (111) and (200) facets, which are expected to be present on
5
1
studied the nature of the IMes–Cu NP interaction at the Cu nanoparticles, or from high (terraces) and low (corners or
molecular level using solid-state NMR. Contacting with a edges) coordination sites present on Cu NPs.
1
1
3
toluene solution of IMes*, C-labeled at the carbenic carbon,
followed by washing of the resulting material with toluene and
pentane and drying under vacuum yields IMes*-Cu/SiO2-TMS
(
2*, the labeled analogue of
2, Scheme 1). Elemental analysis
of 2* gives 1.82, 3.37, 0.48 and 0.44 wt% Cu, C, H and N,
respectively, consistent with the immobilization of IMes* on
Cu/SiO2-TMS (Table S3). Elemental analysis data allows
estimating the Cusurface/IMes ratio to be ca. 1:1.4, suggesting
concomitant binding of IMes* on both Cu NPs and the support.
The IR spectrum of
ligand, confirming adsorption of IMes onto
Exposure of to 33 mbar of CO (Figure S5) gives a broad IR
band between 2000-2200 cm that can be decomposed into 3
2
contains bands reminiscent of the NHC
1
(Figure S4).
1
–1
–
1
main components centered at 2119, 2095 and 2055 cm .
Repeating the experiment with (Figure S5) produces CO
2
bands that are less intense, possibly because its adsorption is
hindered by the bulky NHC ligand, as already observed with
2
9
PCy
3
-functionnalized non-passivated Cu/SiO2-700
.
Moreover,
–
the feature at 2119 cm disappears whereas a new band
1
–1
appears at 2004 cm , resulting in an overall red shift of CO
adsorption modes. This red shift points to a stronger back-
donation to CO, consistent with a more electron-rich Cu NPs as
1
Figure 3. (a) C NMR solution-state spectrum of free IMes* (C
C NMR solid-state HPDEC spectrum of
3
6
D
6
, 75 MHz); (b)
2* (30000 scans); C NMR solid-state
13
13
expected from binding of an IMes ligand to the Cu surface. CP-MAS spectra of (c) 2* (90000 scans), (d) IMes*/SiO2-TMS (27513 scans) and (e)
4
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2
(90000 scans). Broader spectral windows are provided in Figures S7 and S8 and
H NMR spectra for IMes* and 2* are given in Figure S9.
for assistance with the NMR measurements, Dr. K. Larmier and
Dr. T. Margossian for XAS measurements and Dr F. Krumeich
1
The peak at 175 ppm on the CP spectrum of 2* (Figure 3c) is
(ScopeM) for TEM images.
1
3
attributed to the C-labeled carbenic carbon of IMes* in
interaction with the support, as this peak is also found for the
fully passivated SiO2-TMS support contacted with IMes*
Notes and references
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52
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2
3
.
.
(
Figure 3c) are also observed for non-passivated SiO2
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1
2
48, 2247-2273.
3
treated with IMes* (Figure S7f); they are assigned to C-
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6
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8
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.
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Collectively, these results prove the immobilization of IMes in
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.
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.
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6
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There are no conflicts to declare.
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Acknowledgements
We are grateful to the Scientific Equipment Program of ETH 20.
Zürich and the SNSF (R’Equip grant 206021_150709/1) for
financial support of the high throughput catalyst screening
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facility (HTE@ETH). N.K. acknowledges support from the ETHZ
Postdoctoral Fellowship Program and from the Marie Curie
Actions for People COFUND Program. H.-J.L. was partially
funded by CCEM. A.F. thanks the Holcim Stiftung for a
habilitation fellowship. The development of supported Cu
catalysts was also partially funded by the SCCER Heat and
Energy Storage. The authors thanks W.-C. Liao and C. Gordon
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