Selective partial hydrogenation of alkynes to (Z)-alkenes with ionic liquid-doped nickel nanocatalysts at near ambient conditions

Article abstract of DOI:10.1039/c6cc00499g

A selective hydrogenation method for forming (Z)-alkenes from alkynes has been developed using a catalyst system of cheap Ni-NPs in a nitrile functionalised imidazolium based ionic liquid (IL) operating under very mild reaction conditions of 30-50 °C and 1-4 bar H₂ pressure.

Full text of DOI:10.1039/c6cc00499g

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Selective partial hydrogenation of alkynes to
(Z)-alkenes with ionic liquid-doped nickel
nanocatalysts at near ambient conditions†
Cite this:DOI: 10.1039/c6cc00499g
Received 18th January 2016,
Accepted 24th March 2016
Hannelore Konnerth and Martin H. G. Prechtl*
DOI: 10.1039/c6cc00499g
www.rsc.org/chemcomm
A selective hydrogenation method for forming (Z)-alkenes from alkynes reducing agents to synthesis nanoparticles and they act as
has been developed using a catalyst system of cheap Ni-NPs in a nitrile stabilising agents for these NPs.25,26 Special properties including
functionalised imidazolium based ionic liquid (IL) operating under very low vapour pressure, and high thermal and chemical stability make
mild reaction conditions of 30–50 8C and 1–4 bar H2 pressure.
ILs suitable for the use in liquid–liquid biphasic systems, enabling
facile separation of the catalyst from the substrate phase. Moreover,
The hydrogenation of alkynes to produce selectively (E)- or (Z)-alkenes the selectivity of a metal-catalysed reaction can be controlled by
is desirable as this reaction is a useful synthetic tool for the steric or electronic properties of either the cationic or anionic
production of many bioactive natural products or pheromones.1–4 component of the ionic liquid, which interacts with the nanoparticle
The synthesis of stilbene derivatives is especially interesting as these surface or the substrate. This multi-functionality promotes the use
structural motifs are present in many relevant natural products of ionic liquids which combines reducing agent, stabilising agent,
currently under investigation in pharmacological studies.5–7 Many solvent and ligand as selectivity controller in one single compound
of these alkyne transformations to (Z)-alkenes make use of the in the catalyst phase. As described in our previous works, the ionic
famous Lindlar catalyst, consisting of Pd–CaCO3 partially poisoned liquid plays a crucial role for the chemoselectivity in the hydrogena-
with Pb(OAc)2 and quinoline.8 Various heterogeneous catalyst tion of alkynes to (Z)-alkenes.12,27 Nitrile-functionalised imidazolium
systems are also known which make use of noble metals like Pd, based ionic liquids are especially suitable to tune the selectivity
Pt or Au.9–14 An approach towards more abundant and low-cost towards partial hydrogenation. Pd–NPs embedded in IL showed a
metals, thus expanding the applicability of this synthetic strategy, tunable selectivity for the production of either the corresponding
would be beneficial. Efforts have been made to investigate homo- (Z)-alkenes or alkanes in the hydrogenation of both terminal and
geneous nickel catalysts which make use of hydrogen transfer agents internal alkynes under very mild reaction conditions.12 Moreover, a
like H3PO2 or HCO2H. These systems are either limited to internal system with Fe–NPs in the same ionic liquid also showed the
alkynes to produce (E)-alkenes or require high reaction condi- conversion of a wide substrate scope of internal alkynes to the
tions.15,16 In 1981 a nickel on graphite catalyst was introduced which corresponding (Z)-alkenes; harsh reaction conditions were required
operates under mild reaction conditions; sensitivity towards oxida- however (80 1C, 60 bar H2). For the catalyst preparation, a strong
tion prohibits reusability.17 Moreover, only a few nanoscale catalyst reducing agent is necessary to form the active iron species in an
systems were reported using non-noble metals such as the binary organic solvent followed by extraction of the Fe–NPs into the IL; the
GaNi18 or ternary CuFeNi19 system. Ni(0)-NPs, in combination with procedure has to be carried out under strict absence of moisture
an alcohol as hydrogen source, have also been described, although and air to prevent oxidation of the sensitive catalyst.27
stoichiometric amounts of an alkali metal and catalyst are needed.20
A nickel phosphide catalyst has also been reported, however the taking advantage of the benefits of abundant metals, we were
activity is low applied to diphenyl acetylene derivatives.21
interested in developing a catalyst system containing a cheap and
In order to recreate the advantages of noble metal catalysts whilst
In the field of catalysis, the use of metal catalysts in ionic abundant metal such as nickel, which is not as sensitive to oxidation
liquids is of tremendous importance.22–24 Besides, the role as as iron. We thus developed a Ni-NPs catalyst system, which operates
‘‘greener’’ solvents, ILs also show remarkable properties as under very mild reaction conditions, to hydrogenate internal and
even terminal phenylalkynes to the corresponding (Z)-alkenes with
high selectivity using molecular hydrogen (Scheme 1).
Department of Chemistry, Institute of Inorganic Chemistry, University of Cologne,
In a first step using diphenylacetylene as model substrate,
Greinstraße 6, 50939 Cologne, Germany. E-mail: martin.prechtl@uni-koeln.de;
an IL screening and optimisation of the reaction conditions
Web: http://catalysis.uni-koeln.de; Fax: +49 221 4701788
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cc00499g towards a high selectivity for the (Z)-alkene was investigated.
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compared to alkyne, a better accessibility for close proximity to the
surface. The solubility of the alkene in the catalyst phase could also
be enhanced, this might lead to an increase of hydrogenation activity
resulting in production of the alkane. In the alkyl sidechain
functionalised ILs (Table 1, entries 4–9), longer reaction times of
16 h are needed for quantitative conversion of the alkyne, although
in the dihydroxyl functionalised IL only 10% conversion was
achieved after 16.5 h. Moreover, in all functionalised ILs the
selectivity for the formation of the alkene is very high. These
two results indicate a different coordination mode of the ILs on
Scheme 1 Semihydrogenation of alkynes using Ni-NPs in nitrile function-
alised IL.
The NPs were synthesised using different ionic liquids, through the Ni-NPs. As reported previously for nitrile-ILs, a coordination on
modification to known literature protocols, for the formation of the nanoparticle surface is suggested which involves the nitrile-
Ni-NPs in imidazolium based ILs.25,28–30 The reactivity of all group.12,26,27 A higher coordination tendency further impedes
samples was seen to be high using mild reaction conditions of displacement with the substrate, and thus a longer reaction
30 1C within 6–16 h reaction times (Table 1). In all reactions, an time is necessary. Steric factors might also shield the surface of
excellent stereoselectivity for the formation of the (Z)-isomer the nanoparticle or electronic modifications by the stabilizer,
was apparent, in several cases however, hydrogenation to the which might change the absorption/desorption mode of the
corresponding alkane occurred (Table 1; entries 1–3). The more electron-rich alkyne-moiety.31,32 In the case of the diol-
quantitative Ni-NP catalysed hydrogenation of diphenylacetylene is functionalised IL drawing of conclusions is difficult due to the
already apparent after only 6 h when using the non-functionalised low conversion. A decomposition route of [Ni(COD)2] in
ILs [BMIM]NTf2 and [BMMIM]NTf2. Low selectivities for the produc- [C1C1(EG)IM]NTf2-IL is proposed via the formation of transitory
tion of the alkene relative to overreduction to the alkane are seen non-classical NHC-ligands, confirmed by the generation of
however (Table 1, entries 1 and 2). With the IL [C10MMIM]NTf2 1,3-COD, rather than a decomposition by simple ligand exchange
(Table 1, entry 3) even a higher amount of alkane was produced of the OH-groups on the surface of the Ni-NPs.28 Thus, the diol-
with 83% yield. Compared to the imidazolium based ionic functionality might simply create a very hydrophilic area around
liquids bearing a butyl side chain, which result in Ni-NPs size the nanoparticle surface in the hydrogenation reaction of the
of 7.8–8.2 nm, the NPs synthesised in the IL with the decyl side alkyne. In such an environment it would be difficult for the
chain are much smaller with around 4.4 nm in diameter. Aside lipophilic substrates to get in close proximity to the nanoparticle
from the different size and shape of the formed NPs in different surface. In the IL [CNC3MIM]NTf2 a decreased conversion was
ILs, it is possible that the surrounding of the NPs by the IL itself observed, which is in accordance with the previously reported
influences the metal–surface interaction with the stabilizer and Fe–NPs-catalysed reaction in the same ionic liquid.27 Here, a
thus the outcome of this hydrogenation reaction. The longer reduced activity, compared to the IL [CNC3MMIM]NTf2, was also
hydrophobic alkylchain might increase the non-polar region observed for the hydrogenation of diphenylacetylene. In this case
around the nanoparticle surface and allows the less polar alkene, the generation of NHC-complexes might be involved, although
the selectivity for the alkene is still very high with about 93%
(Table 1, entry 9). In all investigated systems with different ILs
the formed NPs have a similar shape and a size of about 7–8 nm,
Table 1 Optimisation reactions with different ionic liquids
except the NPs which were synthesised in [C10MMIM]NTf2 and
[C1C1(EG)IM]NTf2, which have a size of about 4 nm. Comparing
only the systems with a similar size and shape, changes in
activity and selectivity are obvious, which appear to be especially
influenced by the respective ionic liquid.
Best results were achieved with [CNC3MMIM]NTf2 at 4 bar
H2 at 30 1C as quantitative conversion was achieved after 16 h
with high selectivity for (Z)-stilbene (Table 1, entry 7). Even a
smaller amount of ionic liquid could be applied without loss of
activity and selectivity; this makes its use in large scale even
more attractive due to reduced costs (Table 1, entries 6 and 7).
Interestingly, neither the synthesis of the Ni-NPs nor the
hydrogenation conditions of diphenylacetylene (Table 1, entry 7)
showed modification of the ionic liquid, which was confirmed by
1H, 13C or 19F-NMR spectroscopy (see ESI†). Only small amounts of
Yield [%]
Ni-NPs
size [nm]
Entry IL
t [h] Conv. [%]
2
3
4
1a
2a
3a
4b
5a
6a
7
[BMIM]NTf2
[BMMIM]NTf2
[C10MMIM]NTf2
8.2 Æ 1.3
7.8 Æ 1.4
4.4 Æ 0.7
6
6
6
99
100
97
34
59
11
7
36
88
90
57
58
3
1
2
0
2
1
2
2
2
62
38
83
2
[C1C1(EG)IM]NTf2 3.5 Æ 0.528 22
9
[CNC3MMIM]NTf2 7.0 Æ 1.1
[CNC3MMIM]NTf2 7.0 Æ 1.1
[CNC3MMIM]NTf2 7.8 Æ 1.1
[CNC3MMIM]NTf2 7.8 Æ 1.1
6
16
16
22
16
44
97
100
63
65
4
8
7
4
8c
9
[CNC3MIM]NTf2
8.4 Æ 1.6
4
The volume of cyclohexane was 1 ml in all reactions. a For the synthesis cyclooctadiene were visible in the spectra after the decomposition
of the NPs 0.3 g IL was used, the other reactions were conducted with
of the [Ni(COD)2] precursor. This is in vast contrast to iron and
b
0.15 g IL. The synthesis of Ni-NPs was conducted accordingly to
ruthenium NPs in such ILs,26,27 and underlines the suitability
of this IL with Ni-NPs under the applied reaction conditions.
literature method.28 c The hydrogenation reaction was conducted with
a constant pressure of 1 bar H2.
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Table 2 Substrate screening with Ni-NPs in [CNC3MMIM]NTf2
Test reactions without Ni-catalyst or without applied hydrogen
pressure under an atmosphere of argon yielded in no conversion
(see ESI†); this confirms the crucial role of nickel as the active
catalyst. Furthermore, we tested the reaction in [BMMIM]NTf2
with acetonitrile as additive. The nitrile group might coordinate as
a ligand on the NP surface, similar to the coordination mode
suggested for the nitrile functionalised IL. Indeed, in the hydro-
genation reaction of diphenylacetylene, quantitative conversion
resulted in 90% (Z)-stilbene, which confirms the results of the IL
screening that the functionalised side chain of the IL is respon-
sible for the high chemoselectivity in this hydrogenation reaction.
We concentrated our investigation to evaluate the optimized
catalyst system in the hydrogenation of diphenylacetylene
to (Z)-stilbene, using [Ni(COD)2] as precursor and the IL
[CNC3MMIM]NTf2 (Table 1, entry 7). TEM analysis of the
Ni-NPs embedded in IL shows well dispersed spherical shaped
nanoparticles with a mean diameter of 7.8 Æ 1.1 nm (Fig. 1).
In previous works, we reported the formation of Ni(0)-NPs
(o10 nm) and sponge-like agglomerates in [BMIM]NTf2.25 This
underlines again the beneficial effect of the nitrile moiety for a
more defined synthesis. Other works reported the preparation of
Ni-NP systems in non-functionalised imidazolium based NTf2
ILs under hydrogen pressure with similar Ni(0)-NPs.29,30 It is well
accepted that the IL acts as protective layer on the surface of the
nanoparticles to inhibit further oxidation,23–25,29,31 this thin
layer of IL around the NPs is also visible in the TEM images.
A partially oxidation of the surface can never be excluded in
nanoparticles owing to the exposure to air of the robust catalyst
system.29,30 In addition, it is known that NiO can be reduced to
Ni(0) under hydrogenative conditions, therefore our results
show that this perspective does not play such a crucial role in
contrast to highly air sensitive Fe–NPs.27
Entry Alkyne
1a
pH2 [bar] t [h] Conv. [%] Yield [%] S Z/E
4
1
3
100
79
73
—
—
2
16 100
3b,c
4
4
3
100
22
81
12
—
4d
17
95/5
5
1
1
1
16 100
72
16 100
54
47
53
—
—
—
6
7
7e
8c,f
1
4
4
17
16
16
92
7
82
6
100h
100h
—
9e,f
10b,c,d
1
0
88/
12
11g
4
4
6
6
100
73
26
33
12b,c
99/1
We further examined the application of the optimized catalyst
system to a broader substrate scope of terminal phenyl alkynes,
aliphatic internal and terminal alkynes as well as functionalised
alkynes (Table 2). Interestingly, with precise control of the reaction
conditions, including time and hydrogen pressure, even terminal
alkynes bearing a phenyl-moiety such as phenylacetylene and the
methoxy-substituted 1-ethynyl-4-methoxybenzene can be hydro-
genated to the corresponding olefins with high selectivity of 79–81%
(Table 2, entries 1 and 3). Interestingly, H2 pressure variation showed
that even pressures below 5 bar H2 are sufficient and beneficial
a
The volume of the co-solvent cyclohexane was 0.5 ml. Further experi-
ments with the variation of the H2 pressure are shown in the ESI.
b
c
Reactions were conducted in a scale of 0.38 mmol substrate. An
d
external H2 reservoir was connected to the reactor. A reaction temp. of
50 1C was used. The reaction was conducted with a reduced catalyst
loading with 1.8 mmol substrate. A reaction temperature of 40 1C was
e
f
g
used. A volume of 1 ml of the co-solvent cyclohexane was used.
h
According to NMR analysis.
for the selectivity of the partial hydrogenation (Table 2, entries 1 system superior to known heterogeneous literature procedures
and 2 and Fig. 1, ESI†). This makes this biphasic Ni-catalyst with non-noble metals for the hydrogenation of terminal alkynes
to the alkenes with molecular hydrogen. The phenylic and
aliphatic internal alkynes bearing a trimethylsilyl group reacted
poorly under similar reaction conditions (Table 2, entries 4 and 9).
Even with a higher reaction temperature of 50 1C, 22% conversion
of 1-phenyl-2-trimethylsilylacetylene yielded only 12% of the
alkene (Table 2, entry 4). The reaction might be hindered by the
sterically bulky trialkylsilanes. However, aliphatic terminal
alkynes like 1-octyne resulted in a mixture of alkene and alkane
(Table 2, entries 5–7). Whilst the aliphatic internal alkyne
4-octyne required a higher reaction temperature of 40 1C for a
high conversion of 92%, this alkyne was selectively converted to
the corresponding (Z)-alkene with 82% yield (Table 2, entry 8).
Fig. 1 Particle size distribution and TEM image of Ni-NPs dispersed in
[CNC3MMIM]NTf2 (38 mmol metal in 0.15 g IL, 70 1C; 20 h).
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2014, the Alexander-von-Humboldt Foundation (BRIGFOS
Connect program) and H. Konnerth acknowledges the Ju¨rgen
Manchot Stiftung for a research scholarship. S. Roitsch
is acknowledged for TEM analysis. And we are grateful to
H. G. Schmalz for access to GC analysis and for some alkynes.
Notes and references
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