Facile synthesis of NHC-stabilized Ni nanoparticles and their catalytic application in the: Z -selective hydrogenation of alkynes

Article abstract of DOI:10.1039/c7cc01779k

Well defined Ni nanoparticles (NiNPs) stabilized with N-heterocyclic carbenes (NHCs) have been synthesized through a new methodology involving the decarboxylation of a zwitterionic CO₂ adduct. Their catalytic performance was tested in the partial hydrogenation of alkynes into (Z)-alkenes under very mild reaction conditions (50 °C and 5 bar H₂ pressure), providing excellent activities and selectivities.

Full text of DOI:10.1039/c7cc01779k

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
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DOI: 10.1039/C7CC01779K
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Facile Synthesis of NHC-Stabilized Ni Nanoparticles and their
Catalytic Application in the Z-selective Hydrogenation of Alkynes
Received 00th January 20xx,
Accepted 00th January 20xx
a‡*
a‡
a
a,b
M
iriam Díaz de los Bernardos,
Sara Pérez-Rodríguez , Aitor Gual , Carmen Claver and
a,b*
Cyril Godard
DOI: 10.1039/x0xx00000x
www.rsc.org/
Well defined Ni nanoparticles (NiNPs) stabilized by N-heterocyclic carbene or for its in-situ generation. Crabtree et al. reported
carbenes (NHCs) have been synthesized through new the utilization of 1,3-dialkylimidazolium-2-carboxylate ( Im-
methodology involving the decarboxylation of a zwitterionic CO CO ) as an efficient carbene precursor for the synthesis of
a
R
2
2
2
1
5
adduct. Their catalytic performance was tested in the partial NHC-based organometallic complexes.
hydrogenation of alkynes into (Z)-alkenes under very mild reaction
conditions (50 °C and 5 bar H
activities and selectivities.
2
pressure) providing excellent
Metal nanoparticles (MNPs) display unique physical and
chemical properties, which confers them high potential for
applications in diverse areas such as medicine and catalysis, among
1
, 2
others. To enhance their performance in catalysis, the selective
formation of well-defined nano-objects is required. Furthermore,
they should be stabilized by species that will restrict the approach
of the substrate and as such induce selectivity. Nowadays, N-
heterocyclic carbenes (NHCs) are recognized as one of the most
versatile families of compounds in modern chemistry and are
employed as ligands in metal transition catalysed transformations
3
Scheme 1 Synthetic approaches to NHC-stabilized metal nanoparticles.
in both academy and the chemical industry. Although these ligands
4
-8
9
10
11, 12
These imidazolium carboxylates are water-stable, synthetically
readily available, and efficiently transfer the corresponding NHC
were used for the preparation of Au, Pd, Ir and Ru
nanoparticles, to the best of our knowledge, there is only one
1
6, 17
1
3
ligand by decarboxylation in non-polar solvent.
It was therefore
report on the use of NHCs for the stabilization of Ni NPs. To date,
NHC-stabilized MNPs have been obtained by three main
methodologies: i) by direct reduction of NHC-organometallic
thought that the use of such NHC-precursor could be applied in the
synthesis of MNPs and as such, neither the filtration of inorganic
salts nor the isolation of carbenes would be required.
6
complexes, ii) by procedures using the isolated free carbene (either
1
4
The hydrogenation of alkynes to selectively produce (E)- or (Z)-
alkenes is a useful synthetic tool for the production of valuable
by ligand exchange or by the organometallic approach developed
11
by Chaudret ), and iii) using non-isolable carbenes via in situ
1
8
1
2
compounds. Since classical heterogeneous catalysts are usually
generation of the free carbenes (Scheme 1).
19-22
based on noble metals such as Pd, Pt or Au,
new approaches
In all cases, a basic pre-treatment of an imidazolium salt
precursor is required, either for the isolation of the free
employing abundant and low-cost metals is therefore desirable.
Recently, Prechtl and co-workers reported the use of NiNPs
stabilised by ionic liquids for the selective hydrogenation of alkynes
2
0
to the corresponding (Z)-alkenes.
Here, we report a new and straightforward methodology to
prepare small and well defined NHC-stabilized nickel nanoparticles,
both colloidal and supported in carbon nanotubes (CNTs), using a
simple “one-pot” procedure based on the decarboxylation of 1,3-
2
).
3
dialkylimidazolium-2-carboxylate (R
2
Im-CO
2
This procedure
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avoids basic pre-treatment. The catalytic performance of these the nickel surface. However, for Ni-1 NPs a mean diameter of ca.
latter species was evaluated in the selective hydrogenation of 2.9 0.7 nm was measured and a high degree of agglomeration was
internal alkynes into Z-alkenes under mild conditions. observed.
To evaluate Me Im-CO as ligand precursor for the stabilization TGA analysis of the samples revealed that the NHC content for
Ni-NPs, the organometallic precursor Ni(COD) these NPs ranges from ca. 15 wt.% (Ni-0.2 NPs) to 44 wt.% (Ni-1.0
COD=cyclooctadiene) was reduced in THF under 3 bar of hydrogen NPs) (Fig. S1, SI). Ni-0.5 NPs presented a ligand content of 27 wt.%.
±
2
2
of
2
(
2
4
2 2
at 60°C in the presence of Me Im-CO (Scheme 2). The amount of Similar NHC contents were previously reported for Ru and
2
5
NHC-based ligand used during the synthesis was varied using 0.1, Pt NPs. In addition, no weight loss associated to the presence of
0
.2, 0.5 and 1 equivalent per Ni.
imidazolium carboxylate (at ca. 200 °C) was observed, confirming
that this species is not acting as stabilizer. XRD patterns of a
selected sample (Ni-0.5 NPs, Fig. S3, SI) revealed the presence of
crystalline Ni with fcc packing. XPS measurements (Fig. S4, SI)
showed a high percentage of NiO on the surface of Ni nanoparticles
(67.6 at.%), which was attributed to nanoparticle oxidation during
the sample preparation for XPS analysis. Indeed, nickel
nanoparticles are very reactive and burn spontaneously in the
presence of air producing white clouds of the oxide which is
explained by their very small size and clean surface state. Upon Ar
sputtering, the Ni content substantially increased (from 32.4 to 95.5
at.%) confirming the presence of metal core nanoparticles. This new
route was first validated by comparing two batches of NiNPs
R'
R
N
N
N
R
N
R'
N
O
N
O
K
1
(-CO
2
)
3 bar H
2
R'
R
Ni
+
R'
N
N
R
K
2
(+CO
2
)
THF
60ºC
R'
R
N
N
-
CO
2
N
N
R
R'
Ni(COD)
2
R= Me: Me
2
Im-CO
2
Me
2
Im
Ni-0.1 to 1 NPs
(0.1, 0.2, 0.5 or 1 eq.)
Scheme 2 Synthetic methodology used for the preparation of NHC-
stabilized Ni-NPs using Me Im-CO as stabilizer precursor.
2
2
In the presence of 0.1 equivalent of ligand, nickel particles were
not efficiently stabilized. When the amount of ligand was increased,
the NPs Ni-0.2, Ni-0.5and Ni-1 were formed and could be isolated.
The TEM images of these particles are shown in Fig. 1.
stabilized with 1,3-dimethyimidazol-2-ylidene (Me Im), prepared by
2
Ni-0.2 NPs
i) the above mentioned methodology and ii) the reported in situ
0
0
0
0
0
.20
.15
.10
.05
.00
12
2
.4 ± 0.9 nm
generation of non-isolable NHC, which is first employed here for
the preparation of Ni nanoparticles. Following the last protocol,
2
Me Im·HCl (0.5 molar equiv/Ni) was deprotonated via the addition
of a slight excess of KOtBu in THF, and the resulting suspension
(Me Im) was filtered through celite prior to its transfer into a
0
1
2
3
4
5
6
7
8
9
10
2
Mean Diameter (nm)
solution of Ni(COD)2 in THF. The reaction mixture was then
pressurized with 3 bar of H
NPs with a mean size of 2.0
the TEM data obtained by the new methodology clearly revealed
similar dispersion of the particles on the grid and size (2.0 0.4 nm).
In addition, the IR spectra recorded from the two batches of Ni-
Me Im NPs were similar. In the sample of NPs prepared via the
2
, leading to a colloidal solution of nickel
Ni-0.5NPs
0
0
0
0
0
0
0
0
0
0
.45
.40
.35
.30
.25
.20
.15
.10
.05
.00
±
0.5 nm (Fig. S5, SI). Comparison with
2
.0 ± 0.4 nm
±
2
0
1
2
3
4
5
6
7
8
9
10
decarboxylation method, the characteristic features of the
Mean Diameter (nm)
compound Me Im-CO were not detected, confirming that this
2
2
Ni-1NPs
species does not participate to the NP stabilization (Fig. S6, SI).
Furthermore, the TGA analysis recorded from the two batches of
0
0
0
0
0
0
.25
.20
.15
.10
.05
.00
2
.9 ± 0.7 nm
Ni/Me Im NPs showed similar profiles (Fig. S7, SI). Based on these
2
results, it was concluded that both synthetic procedures yield very
similar Ni nanoparticles, and thus evidenced that the new
methodology developed in this work allows a rapid access to NHC-
stabilized nanoparticles without the need of a basic pre-treatment.
With these results in hand, supported Ni nanocatalysts were
synthesized following the same methodology in the presence of
multiwalled carbon nanotubes (MWCNTs). The amount of NHC-
based ligand used during the synthesis was varied from 0.2, 0.5 to 1
equivalent per Ni. TEM analysis of the resulting hybrid material is
shown in Fig. 2. Interestingly, the TEM micrographs of Ni-0.2
NPs@CNTs and Ni-0.5 NPs@CNTs revealed the formation of very
similar nanoparticles with no significant differences in size, shape
0
1
2
3
4
5
6
7
8
9
10
Mean Diameter (nm)
Fig. 1 TEM micrographs and size distributions of Ni-0.2,Ni-0.5 and Ni-1
nanoparticles.
1
1
As previously observed with NHCs, smaller NPs were formed
upon increasing the amount of the ligand from 0.2 to 0.5 equivalent
per Ni. Indeed, in sample Ni-0.2, the NPs exhibited a mean diameter
of ca. 2.4± 0.9 nm, whilst for Ni-0.5, the mean size of ca. 2.0± 0.4
nm. Under these conditions, particles were well separated and
neither agglomeration nor coalescence was detected on the carbon
grid, indicating that the NHC-based ligand is strongly adsorbed on
and distribution since both exhibited a diameter of ca. 3.6± 0.8 nm.
2
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Ni-0.2 NPs@CNTs
in Table 1. First, the catalytic performance of Ni-0.2NPs@CNTs, Ni-
0.5NPs@CNTs and Ni-1NPs@CNTs were compared using
diphenylacetylene as model substrate using 3 mol% of Ni at 50°C
0
0
0
0
0
0
0
.30
.25
.20
.15
.10
.05
.00
3
.5 ± 0.7 nm
and under 5 bar of H
2
. After 16h, full conversion was achieved for
0
.2NPs@CNTs and Ni-0.5NPs@CNTs while a lower value was
measured using Ni-1NPs@CNTs (Table 1, Entries 1-3). This indicated
that the large amount of NHC used during the synthesis of the latter
nanocatalyst could hinder the substrate coordination at their
surface. In all cases, no over-hydrogenation product was detected
while distinct degree of stereoselectivity was measured: using
0
1
2
3
4
5
6
7
8
9
10
Mean Diameter (nm)
Ni-0.5 NPs@CNTs
0.25
0.20
0.15
0.10
0.05
0.00
3
.6 ± 0.8 nm
0
.2NPs@CNTs, excellent selectivity (94%) to the Z-alkene was
observed while using Ni-0.5NPs@CNTs as the catalytic system,
diphenylacetylene was selectively converted to (Z)-stilbene in
quantitative yield (Table 1, entry 2). In contrast, using Ni-
1NPs@CNTs, a drop in selectivity was detected.
0
1
2
3
4
5
6
7
8
9
10
Mean Diameter (nm)
Ni-1 NPs@CNTs
Table 1 Catalytic semi-hydrogenation of internal alkynes using
NiMe ImNPs@CNTs
2
a
0
.20
.15
.10
.05
.00
2
.9 ± 1.1 nm
0
0
0
0
b
Conv.
Yield B)
b
b
0
1
2
3
4
5
6
7
8
9
10
E. NiNPs@CNT
R
1
R
2
c
B/C
%D
(
Mean Diameter (nm)
1
2
3
4
5
6
7
Ni-0.2@CNT
Ni-0.5@CNT
Ni-1@CNT
Ni-0.5@CNT
Ni-0.5@CNT
Ni-0.5@CNT
Ni-0.5@CNT
Ph
Ph
Ph
(CH
Ph
Ph
Ph
(CH
(CH
Ph
100
100(97)
78
100(91)
81(90)
100(70)
100(80)
94/6
97/3
78/22
99/1
98/2
94/6
96/4
0
0
0
0
10
12
13
Fig. 2 TEM micrographs and size distributions of supported Ni-0.2,Ni-0.5
and Ni-1 NPs@CNTs.
2
)
2
CH
3
2
)
)
4
2
CH
CH
3
It is remarkable that an increase of the mean particle size and
size distribution was observed in comparison with the colloidal
systems. When the amount of the NHC-based stabilizer was
increased to 1 equivalent per Ni (Ni-1NPs@CNTs), a wider
distribution was observed with no significant difference in size with
respect to the colloidal system. In general, the small Ni
CH
CH
CH
3
2
3
2
CH
OH
3
2
Ph
a
2
General conditions: 1 mmol of substrate, 3 mol% of Ni, 10 ml of THF, 50°C, 5 bar H , t =
b
c
1
6h. Determined by GC-MS spectrometry and NMR analysis. Isolated yeld.
In view of these results, the reaction using Ni-0.5NPs@CNTs as
nanoparticles were mostly deposited in the inner cavity of CNTs, as catalytic system was repeated and monitored over time (Fig. 4).
confirmed by STEM (Fig. 3). The supported nanoparticles were Total conversion of the substrate was observed after a reaction
characterized by XRD analysis, evidencing the presence of Ni time of 7 hours with total selectivity for the corresponding (Z)-
crystalline systems with fcc packing (Fig. S11, SI). XPS alkene. Interestingly, no over-hydrogenation of the substrate into
measurements (Fig. S12, SI) again revealed a high surface NiO the corresponding alkane was observed even after 24 hours of
content (82.9 at.%) due to the strong oxygen sensitivity of the reaction.
nanoparticles even after immobilization onto CNTs. However, after
Conversion
Diphenylacetylene
(Z)-stilbene
(E)-stilbene
Ar sputtering the metal content increased (72.7 at.%). Finally, TGA
and ICP revealed a nickel content in Ni-Me Im@CNTs that was
1
10
00
2
1
similar to the nominal loading (10 wt.%) with values of 10.0 wt.%
and 8.8 wt.%, respectively.
9
0
80
70
60
50
40
30
20
10
0
-10
0
1
2
3
4
5
6
7
8
9
23 24 25
Fig. 3 STEM micrographs of Ni-0.5 NPs@CNTs.
Time (h)
Fig. 4 Evolution in time of diphenylacetylene semi-hydrogenation in the
presence of NiMe Im-0.5 NPs@CNTs at 50ºC, 5 bar H for 24h.
Next, the catalytic performance of the NPs was evaluated in the
2
2
semi-hydrogenation of internal alkynes. The results are summarized
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The remaining solution after catalytic reaction was analysed by
The authors are grateful to the European Union’s Horizon 2020
inductively coupled plasma (ICP) revealing a nickel content of research and innovation programme (ref. 677471), TERRA project
.008% with respect to all Ni initially introduced. Furthermore, no and the Ministerio de Economia y Competividad and the Fondo
0
evolution of the reaction was observed when the semi- Europeo de Desarrollo Regional FEDER (CTQ2016-75016-R,
hydrogenation of diphenylacetylene was tested using the remaining AEI/FEDER, UE) for funding.
solution of a catalytic test after filtration of the heterogeneous
catalyst. These results indicated that no relevant leaching had taken
Notes and references
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1
2
3
.
.
.
G. Schmid, Nanoparticles. From theory to applications.,
Weinheim, 2004.
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Next, we examined the application of the optimized catalyst
system for a broader scope of internal alkynes (Table 1, Entries 4-7).
When 4-octyne was tested as substrate (Entry 4), the corresponding
3
778.
(Z)-alkene was again obtained in quantitative yield. However, when
M. N. Hopkinson, C. Richter, M. Schedler and F. Glorius, Nature,
2014, 510, 485-496.
2-octyne was used (Entry 5), 81% conversion was measured and a
mixture of alkene and alkane was observed. However, the Z/E ratio 4. D. Canseco-Gonzalez, A. Petronilho, H. Mueller-Bunz, K.
Ohmatsu, T. Ooi and M. Albrecht, J. Am. Chem. Soc., 2013, 135,
remained at ca. 98%. In the case of 1-phenyl-1-butyne substrate, a
1
3193-13203.
greater amount of alkane was obtained (12%) at 100% conversion
and the selectivity Z/E was slightly lower (ca. 92%) (Entry 6). In the
hydrogenation of 3-phenyl-2-propyn-1-ol (Entry 7), the selective
5
6
.
.
E. C. Hurst, K. Wilson, I. J. S. Fairlamb and V. Chechik, New J.
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hydrogenated product was detected with full conversion.
Johnson, J. Am. Chem. Soc., 2013, 135, 7418-7421.
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.
.
G. Wang, A. Ruhling, S. Amirjalayer, M. Knor, J. B. Ernst, C.
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2
To evaluate the robustness of these catalysts, the NiMe Im-0.5
NPs@CNTs was recycled several times in the hydrogenation of
diphenylacetylene as model substrate (Fig. 5). No relevant loss of
activity nor selectivity was observed over 3 runs although a slight
decrease in activity was observed in the last run.
9
1
1
0. L. S. Ott, M. L. Cline, M. Deetlefs, K. R. Seddon and R. G. Finke, J.
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Philippot and B. Chaudret, Angew. Chem., Int. Ed., 2011, 50,
Conversion
Yield (E)-Stilbene
Yield (Z)-Stilbene
1
00
100
80
60
40
20
0
1
2080-12084.
8
6
4
2
0
0
0
0
0
1
1
2. L. M. Martinez-Prieto, A. Ferry, P. Lara, C. Richter, K. Philippot,
F. Glorius, B. Chaudret, Chem. Eur. J., 2015, 21, 17495-17502.
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Fig. 5 Recycling experiment of the hydrogenation of diphenylacetylene
over NiMe Im-0.5 NPs@CNTs at 50ºC, 5 bar H for 7h.
2
2
1
9. M. Crespo-Quesada, F. Cardenas-Lizana, A.-L. Dessimoz and L.
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9129-9132.
zwitterionic salt has been developed. This methodology was 21. A. Molnar, A. Sarkany and M. Varga, J. Mol. Catal. A: Chem.,
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immobilization of NiNPs onto carbon nanotubes by a simple
2001, 173, 185-221.
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2
2
2
1
986, 949-952.
“
one-pot” procedure without surface modification. These NPs
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were thoroughly characterised and the supported NPs
revealed efficient catalysts in the selective hydrogenation of
terminal alkynes into the corresponding (Z)-alkenes under very
mild reaction conditions. Furthermore, these heterogeneous
catalysts can be readily recovered by simple filtration and
reused 3 times without relevant decrease in activity.
2
5. E. A. Baquero, S. Tricard, J. C. Flores, E. de Jesus and B.
Chaudret, Angew. Chem. Int. Ed., 2014, 53, 13220-13224.
4
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