Novel nickel nanoparticles stabilized by imidazolium-amidinate ligands for selective hydrogenation of alkynes

Article abstract of DOI:10.1039/c9cy02172h

The main challenge in the hydrogenation of alkynes into (E)- or (Z)-alkenes is to control the selective formation of the alkene, avoiding the over-reduction to the corresponding alkane. In addition, the preparation of recoverable and reusable catalysts is of high interest. In this work, we report novel nickel nanoparticles (Ni NPs) stabilized by three different imidazolium-amidinate ligands (ICy·^(Ar)NCN; L1: Ar = p-tol, L2: Ar = p-anisyl and L3: Ar = p-ClC₆H₄). The as-prepared Ni NPs were fully characterized by (HR)-TEM, XRD, WASX, XPS and VSM. The nanocatalysts are active in the hydrogenation of various substrates. They present a remarkable selectivity in the hydrogenation of alkynes towards (Z)-alkenes, particularly in the hydrogenation of 3-hexyne into (Z)-3-hexene under mild reaction conditions (room temperature, 3% mol Ni and 1 bar H₂). The catalytic behaviour of Ni NPs was influenced by the electron donor/acceptor groups (-Me, -OMe, -Cl) in the N-aryl substituents of the amidinate moiety of the ligands. Due to the magnetic character of the Ni NPs, recycling experiments were successfully performed after decantation in the presence of an external magnet, which allowed us to recover and reuse these catalysts at least 3 times preserving both activity and chemoselectivity.

Full text of DOI:10.1039/c9cy02172h

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Novel nickel nanoparticles stabilized by
imidazolium-amidinate ligands for selective
Cite this: DOI: 10.1039/c9cy02172h
hydrogenation of alkynes†
ab
a
Angela M. López-Vinasco, a Luis M. Martínez-Prieto,
Juan M. Asensio,
*
Pierre Lecante,c Bruno Chaudret, a Juan Cámpora d and
a
*
Piet W. N. M. van Leeuwen
The main challenge in the hydrogenation of alkynes into (E)- or (Z)-alkenes is to control the selective for-
mation of the alkene, avoiding the over-reduction to the corresponding alkane. In addition, the preparation
of recoverable and reusable catalysts is of high interest. In this work, we report novel nickel nanoparticles
(Ni NPs) stabilized by three different imidazolium-amidinate ligands (ICy·(Ar)NCN; L1: Ar = p-tol, L2: Ar =
p-anisyl and L3: Ar = p-ClC6H4). The as-prepared Ni NPs were fully characterized by (HR)-TEM, XRD,
WASX, XPS and VSM. The nanocatalysts are active in the hydrogenation of various substrates. They present
a remarkable selectivity in the hydrogenation of alkynes towards (Z)-alkenes, particularly in the hydrogena-
tion of 3-hexyne into (Z)-3-hexene under mild reaction conditions (room temperature, 3% mol Ni and 1
bar H2). The catalytic behaviour of Ni NPs was influenced by the electron donor/acceptor groups (–Me,
–OMe, –Cl) in the N-aryl substituents of the amidinate moiety of the ligands. Due to the magnetic character
of the Ni NPs, recycling experiments were successfully performed after decantation in the presence of an
external magnet, which allowed us to recover and reuse these catalysts at least 3 times preserving both ac-
tivity and chemoselectivity.
Received 28th October 2019,
Accepted 3rd December 2019
DOI: 10.1039/c9cy02172h
rsc.li/catalysis
substituted alkynes or the partial isomerization of (Z)-alkenes
to (E)-alkenes for the disubstituted ones.1c,3c Therefore, the
Introduction
The selective hydrogenation of alkynes into (E)- or (Z)-alkenes
is an important synthesis method in fine chemistry and the
polymerization industry.1 The classical catalysts for this trans-
formation are based on passivated, supported Pd catalysts.2
However, they often involve the use of toxic lead salts as a cat-
alyst inhibitor. The most widely used one industrially is
Lindlar's catalyst (Pd/CaCO3 poisoned with PbIJOAc)2).3 This
catalyst has shown good selectivity in the semi-hydrogenation
of simple internal alkynes, but it exhibits some drawbacks
such as poor selectivity in the semi-hydrogenation of mono-
development of environmentally friendly, chemoselective and
recyclable catalysts based on earth-abundant and low-cost
metals such as nickel is highly desirable. In this respect, me-
tallic nanoparticles (MNPs) are good alternatives,4 because
they present the advantages of heterogeneous and homoge-
neous catalysts such as good activity and selectivity, as well as
the possibility to separate and reuse the catalyst at the end of
the reaction.5 Many recent NP catalysts for semi-
hydrogenation reactions are based on the noble metal Pd.6
Nevertheless, well-defined nickel nanoparticles (Ni NPs) have
been also successfully applied to the selective hydrogenation
of alkynes into alkenes.7 In addition, Ni NPs have shown bet-
ter catalytic activity than conventional heterogeneous catalysts
because they have higher active surfaces areas. Ni NPs also
present magnetic properties, which could be exploited to re-
cover them from the reaction medium after precipitation in
the presence of an external magnet. Indeed, magnetic recov-
ery has emerged as a robust, efficient and fast catalyst separa-
tion technique.8 Thus, the intrinsic magnetic properties of Ni
NPs, which facilitate their recycling, together with their poten-
tial as catalysts in hydrogenation reactions, makes them good
candidates for catalytic semi-hydrogenation reactions in an
environmentally friendly way.
a LPCNO; Laboratoire de Physique et Chimie des Nano-Objets, UMR5215 INSA-
CNRS-UPS, Institut National des Sciences Appliquées-Toulouse, 135, Avenue de
Rangueil, F-31077 Toulouse, France. E-mail: vanleeuw@insa-toulouse.fr
b ITQ, Instituto de Tecnología Química, CSIC-Universitat Politécnica de València,
Avda. Los Naranjos S/N, 46022, Valencia, Spain.
E-mail: luismiguel.martinez@csic.es
c CEMES (Centre d'Elaboration de Matériaux et d'Etudes Structurales), CNRS, 29
Rue J. Marvig, F-31055 Toulouse, France
d IIQ, Instituto de Investigaciones Químicas, CSIC-Universidad de Sevilla, C/
Américo Vespucio, 49, 41092 Sevilla, Spain
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cy02172h
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It has been already shown in the literature that the cata- Results and discussion
lytic properties of MNPs can be tuned by the use of polymeric
stabilizers, surface active agents, solid supports, but notably
by ligands, which in turn also provide stability and solubility
to the MNPs.9 Imidazolium-amidinate ligands demonstrated
to be a good stabilizer for Ru NPs10 and Pt NPs,11 enabling
the synthesis of very small nanoparticles but until now they
have never been used on nickel NPs. Due to the zwitterionic
structure of the imidazolium-amidinate ligands, the nitrogen
atoms exhibit a large electron-donor capability and coordi-
nate strongly to transition metals without adding electric
charges to the system. Furthermore, a large influence of the
substituents of the ligands on catalytic activity of the Pt NPs
in the hydrogenation of activated ketones was found.11 Here,
we describe novel Ni NPs stabilized by three different
imidazolium-amidinate ligands, ICy–(Ar)NCN (L1: Ar = p-tol,
L2: Ar = p-anisyl and L3: Ar = p-ClC6H4). The three systems
were fully characterized by multiple techniques ((HR)TEM,
XRD, WAXS, AAS, XPS and VSM). In this contribution, we ex-
plore the three catalytic systems containing modified ligands
in various hydrogenation reactions, paying special attention
to the semi-hydrogenation of 3-hexyne, and we investigate
the recyclability of such systems by utilizing their magnetic
properties.
Synthesis and characterization of Ni@L nanoparticles
Ni NPs were prepared from the Ni precursor bisIJ1,5-
cyclooctadiene)nickelIJ0), [NiIJCOD)2], which was decomposed
in THF at 70 °C under 3 bar H2 in the presence of 0.2 equiva-
lents of the corresponding ligand. Three different
imidazolium-amidinate compounds were employed as stabi-
lizing ligands, L1 = p-tol, L2 = p-anisyl and L3 = p-ClC6H4,
leading to Ni@L1, Ni@L2 and Ni@L3 nanoparticles, respec-
tively (Fig. 1, top). TEM micrographs of Ni@L1 and Ni@L2 re-
vealed the formation of spherical Ni NPs of similar sizes (ca.
2.8 nm), independently of the electron donor/acceptor char-
acter of the N-aryl substituent in the amidinate ligands
(Fig. 1a and b). However, the electron-poor system for Ni@L3
was composed by slightly bigger (3.4 (2.0) nm) and consider-
ably more polydisperse NPs (Fig. 1c and S1 of ESI†). High-
resolution TEM (HRTEM) images of Ni@L1 showed the pres-
ence of crystalline fcc-Ni NPs (face centred cubic), which is
characteristic of metallic nickel (Fig. S2 of ESI†). Also, Fourier
analysis applied to this image showed reflections of the
(11−1), (0−11) and (0−11) atomic planes.
X-ray powder diffraction (XRD) of Ni@L1, Ni@L2 and
Ni@L3 also revealed that the nanoparticles are well
Fig. 1 Top: Synthesis of the nickel nanoparticles stabilized by ICy·(Ar)NCN (Ni@L1, Ni@L2 and Ni@L3). Bottom: TEM images and size distribution
histograms of (a) Ni@L1, (b) Ni@L2 and (c) Ni@L3.
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crystallized according to the fcc metal structure. In addition,
very small peaks at 2θ values of ca. 42°, a characteristic of
NiO, were observed in all cases (Fig. 2). As the Ni NPs were
handled under argon and the XRD analyses were performed
under inert atmosphere, it must be assumed that the oxide is
formed during the synthesis of the NPs in THF. Wide-angle
X-ray scattering (WAXS) analyses of the Ni@L NPs (Fig. S3 of
ESI†) revealed mostly metallic nanoparticles with a compact
particle (Table S1 of ESI†). However, in the case of Ni@L3,
the Ni(s)/L ratio has a value of 2.2. As each ligand coordinates
to two Ni atoms due to steric reasons, it is not possible to ac-
commodate all ligands onto the Ni NPs surface. Therefore, as
was previously observed for Ru and Pt NPs ligated with the
same amidinate ligands, the excess of L3 may be organized
in a second sphere of non-coordinated ICy·(p-ClC H )NCN, prob-
ably bonded by π–π stacking and ionic interactions of the
zwitterions.
6
4
structure (fcc) and
a coherence length of ca. 3 nm,
confirming the crystalline structure and size observed by
XRD and HRTEM. Again, some signs of oxidation for the
three nickel systems were observed. Atomic absorption
spectroscopy (AAS) of the Ni@L NPs gave metal contents of
ca. 75.9, 71.2 and 45.5 wt% for Ni@L1, Ni@L2 and Ni@L3 re-
spectively. For Ni@L1 and Ni@L2, the Ni/L ratio is large
enough to allow the coordination of all ICy·NCN molecules,
since the estimated number of Ni surface atoms [Ni(s)] is up
to 7–9 times larger than the calculated number of ligands per
The chemical state and coordination mode of the
imidazolium-amidinate ligands (L1, L2 and L3) to the Ni sur-
face were further investigated by X-ray photoelectron spectro-
scopy (XPS). The N 1s signals of the ligands present two bind-
ing energies (BEs) at ∼401.5 and ∼397.5 eV (Fig. 3a, blue).
The first peak can be assigned to the nitrogen atoms of the
imidazolium fragment, which bear a partial positive charge.
The second one at lower BE corresponds to the N atoms of
the amidinate fragment with a partial negative charge. The
coordination of ligand L1 through the amidinate moiety is
reflected in the loss of electron density on these N atoms, as
their BE increases from 397.5 to 399.9 eV. The overlapping
of this signal with the one corresponding to the
imidazolium N atoms, gave a new broad peak centred at
399.9 eV (Fig. 3a, red). The latter could be deconvoluted in
three contributions at 401.3, 399.8 and 398.5 eV, which cor-
respond to the δ−, neutral and δ+ N atoms, respectively
(Fig. 3b). The N 1s signal of Ni@L2 presents a peak similar
to that in Ni@L1, but here the contribution corresponding
to Nδ− (398.7 eV) increases at the expense of the Nδ+ contri-
bution (401.3 eV), due to the presence of an electron donor
group in the ligand (–OMe). On the other hand, in the N 1s
signal of Ni@L3, which contains the electron-withdrawing
chloro substituent, the δ+ is the most important contribu-
tion (401.4 eV). This can be explained by the presence of
the second sphere of non-coordinated ligands, as proposed
above.
The Ni 2p3/2 spectra of Ni@L1 and Ni@L2 showed a broad
signal centred at ca. 855.4 eV accompanied by a satellite peak
(Sat) at 861.4 eV, also known as shake-up satellite.12 The peak
at 855.4 eV could be deconvoluted in three contributions at
∼852.6, ∼854.6 and ∼856.1 eV that correspond to Ni(0),
nickel oxide (NiO) and nickel oxyhydroxide [Ni(O)OH], respec-
tively (Fig. 3c, top and centre).13 As the main contributions
are the ones corresponding to NiO and nickel oxyhydroxide
species,12c we can assume that practically all the surface
atoms of Ni@L1 and Ni@L2 were oxidized during the XPS
analyses as the samples were exposed to air. Interestingly, the
surface of Ni@L3 was only partially oxidized during the XPS
analysis, presenting a clear peak at 852.6 eV that belongs to
Ni(0) (Fig. 3c, bottom). We ascribe these differences to the
higher surface coverage of Ni@L3 and the lower electron den-
sity of the Ni surface atoms of these Ni NPs because of the
comparatively lower electron-donor capacity of L3.
The as-prepared Ni NPs were further characterized by vi-
brating sample magnetometry (VSM). Magnetization versus
Fig. 2 XRD diffractograms of (a) Ni@L1 (b) Ni@L2 and (c) Ni@L3. The
peaks labelled in red correspond to fcc Ni(0) and the peaks labelled in
blue correspond to NiO.
applied field curves evidenced
a
superparamagnetic
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Fig. 3 XPS spectra of Ni@L1, Ni@L2 and Ni@L3 (experimental spectra plotted in red line). Left inset, N 1s signals; (a) comparison with the spectra
of the free ligands L1, L2 and L3 (blue lines); (b) same spectra, showing their deconvolution in three components: δ+ (yellow), δ0 (green), and δ−
(orange). Right inset, (c) The corresponding Ni 2p3/2 signals, showing the deconvolution of the main signal in three components (yellow, green and
orange), assigned to NiIJIII) oxyhydroxide species [Ni(O)OH], NiIJII) oxide (NiO) and elemental Ni [Ni(0)].
behaviour at room temperature and an absence of saturation,
with magnetization (MS) values at 3 T of 28, 23 and 35 A2 m2
kg−1 for Ni@L1, Ni@L2 and Ni@L3 (Fig. 4a), respectively. At
low temperature (5 K), after cooling down in the presence of
μ0H of 3 T (field-cooling), the three samples were close to sat-
uration at 3 T with magnetization values of 44, 38 and 44 A2
m2 kg−1 for Ni@L1, Ni@L2 and Ni@L3, respectively (Fig. 4a).
In all three cases, the MS values were below that of bulk Ni
(55 A m2 kg−1).14 This can result from the surface/magnetic-
core ratio which increases for small Ni NPs, increasing the
amount of nonmagnetic layers at their surface.15 A further
electronic role of the ligand acting as a π-acceptor cannot be
discarded.16 Furthermore, the coercive field (HC) values of 10,
5 and 20 mT were observed for Ni@L1, Ni@L2 and Ni@L3,
respectively (Fig. 4b). The small discrepancies observed be-
tween the three samples can result from the polydispersity of
the NPs, since larger HC values are found in the most polydis-
perse system, Ni@L3, which contains the larger NPs. The ab-
sence in the hysteresis loops of any exchange bias, character-
istic of the coupling between ferromagnetic and
antiferromagnetic layers,17 demonstrates the absence of sig-
nificant oxidation of the particles and suggests that the pres-
ence of NiO that was observed in the XRD and WAXS analyses
should be almost negligible.
Catalytic evaluation
In order to learn about the catalytic properties of the novel
Ni NPs, Ni@L1 was tested in the hydrogenation of various
substrates containing different functional groups including
alkenes, alkynes and carbonyl groups. The catalytic results
show that the nickel systems are capable of hydrogenating
double and triple bonds, but the carbonyl groups were hardly
Fig. 4 Magnetization against applied field curves for the Ni@L NPs at
(a) room temperature (300 K) and (b) 5 K.
affected. Table 1 presents the scope of substrates in
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Table 1 Hydrogenation reaction catalysed by Ni@L1a
Entry
Substrate
Product
Conv.b (%) select. (%)
>92%, 1a : 1b : 1c = 93 : 4 : 3
1c
1a
1
1b
1c
2c
47%, 2a : 2b : 2c = 92 : 8 : 0
2a
2b
2
2c
3
4
>99%, 3a : 3b = 94 : 6
66%, 4a : 4b = 97 : 3
3
3a
3b
4
4a
4b
5
>99%, 5a : 5b = 92 : 8
5
5a
5b
6
7
17%
10%
6a
6
7
7a
8
2%
8
8a
a
Reaction conditions: substrate (0.5 mmol), Ni@L1 (0.015 mmol Ni assuming % of Ni from AAS. 3 mol% Ni loading), toluene (0.75 mL), H2
(40 bar), overnight and r.t. b Conversions and selectivities were determined by GC/MS (average of two runs). c H2 (1 bar), 5 h and r.t.
hydrogenation reactions. The best results were obtained in
the hydrogenation of 3-hexyne (1) and diphenylacetylene (2)
at 1 bar H2 and room temperature for 5 hours. The selective
hydrogenation of 1 and 2 gave a high selectivity towards the
(Z)-alkene, with some traces of (E)-alkene and the correspond-
ing alkane (Table 1, entries 1 and 2). The conversion values
for 1 and 2 were of 92% (selec. 93 : 4 : 3) and 47% (selec. 92 :
8 : 0), respectively, showing that the steric effects between
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Table 2 Selective hydrogenation reaction of 3-hexyne catalysed by Ni@L1, Ni@L2 and Ni@L3a
Selectivityb (%)
Entry
Ni@L
Time
Conv.b (%)
1a
1b
1c
1
2
3
4
5
6
7
8
Ni@L1
Ni@L2
Ni@L3
Ni@L1
Ni@L2
Ni@L3
Ni@L1
Ni@L2
Ni@L3
Ni@L1
Ni@L2
Ni@L3
2
2
2
5
5
5
6
6
6
8
8
8
32
45
27
92
97
73
98
98
92
88
83
93
91
83
82
86
89
78
83
90
7
8
9
4
1
3
8
3
4
5
7
6
4
11
8
4
5
12
11
8
7
11
9
9
89
10
11
12
>99
>99
>99
6
a
Reaction conditions: 3-hexyne (0.5 mmol), Ni@L (0.015 mmol Ni assuming % of Ni from AAS. 3 mol% Ni loading), toluene (0.75 mL), H2 (1
bar), and r.t. b Conversions and selectivities were determined by H NMR (average of two runs).
1
substrate and nanoparticles influence the catalytic activity of
Ni@L1. Substrates 3–5 that contain double bonds and car-
bonyl groups are hydrogenated with a high preference for the
double bond (selec. 92%) and they show a low conversion of
the carbonyl group (Table 1, entries 3–5). Substrate 6 (furfu-
ral) reached a 17% conversion to the corresponding alcohol
(2-furfuryl alcohol) with no hydrogenation of the conjugated
aromatic ring (Table 1, entry 6). The conversion of 7 was of
10% containing an electron-withdrawing CF3 group, whereas
acetophenone (8) conversion was negligible.
Encouraged by the result obtained with 3-hexyne (Table 1,
entry 1), we decided to test the as-prepared Ni NPs in the se-
lective hydrogenation of alkynes into alkenes. Only a few ex-
amples have been reported up to date that employ colloidal
Ni NPs for the selective semi-hydrogenation of alkynes using
molecular hydrogen.7 In order to investigate the influence of
the electron donor/acceptor group of each ligand (R = –Me,
–OMe, –Cl) on their activity and selectivity of the Ni NPs
herein prepared, we chose the semi-hydrogenation of
3-hexyne as a model reaction. Table 2 summarizes the results
obtained in the semi-hydrogenation reaction of 3-hexyne
catalysed by Ni@L1, Ni@L2 and Ni@L3. Toluene was chosen
as a reaction solvent because of the good dispersibility of the
NPs in it. Interestingly, although the NPs were slightly oxi-
dized, they still showed good and comparable activities at
long reaction times (8 h, entries 10–12). At short reaction
times (2 h, entries 1–3), we observed an effect of the ligand
on the conversion; the stronger the electron donor on the
N-aryl group, the more active is the Ni@L catalyst. After 2 h
of reaction, a conversion of 45% was obtained with the cata-
lyst that bears the –OMe group (Ni@L2), whereas the conver-
sion for Ni@L1 and Ni@L3 were 32 and 27%, respectively.
The same trend could be observed for reaction times of 5
and 6 hours (entries 4–9), which also showed a lower activity
for the NPs that contain the electron withdrawing Cl-
substituent (Ni@L3). A similar catalytic behaviour was previ-
ously observed with Pt NPs ligated by the same ligands in the
hydrogenation of ketones.11 After 8 h, the conversion was
complete for all three catalytic systems.
Fig. 5 shows the evolution of the products in the semi-
hydrogenation of 3-hexyne catalysed by Ni@L1 with respect
to time. The formation of the (Z)-alkene runs parallel with
the consumption of the alkyne until a maximum is reached
after 5 hours for Ni@L1 and Ni@L2, and 8 h for Ni@L3
(Fig. 5 and S4†). When comparing the 3-hexyne consumption
slopes during the initial stages of the reactions (between 0–5
hours), it can be seen that Ni@L2 is the most active catalyst
(slope: 19.25 mol s−1 Fig. S4†), followed by Ni@L1 (slope:
Fig. 5 Time course of the product yield in the semi-hydrogenation of
3-hexyne using Ni@L1 as the catalyst. Reaction conditions: 0.5 mmol
of 3-hexyne, 3 mmol% catalyst, 0.75 mL toluene, and 1 bar H2.
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Table 3 Semi-hydrogenation of terminal alkynes catalysed by Ni@L1a
Selectivityb (%)
a
Entry
Substrate
Conv.b (%)
b
1
2
3
4
5
9
63
86
85
60
96
89
89
91
90
39
11
11
9
10
61
10
11
12
13
a
Reaction conditions: substrate (0.5 mmol), Ni@L (0.015 mmol Ni assuming % of Ni from AAS. 3 mol% Ni loading), toluene (0.75 mL), H2 (1
bar), 8 h and r.t. b Conversions and selectivities were determined by GC/MS (average of two runs).
18.42 mol s−1) and Ni@L3 (slope: 14.86 mol s−1 Fig. S4†). Two
combined phenomena, an effect of the ligand and polydis-
persity, may explain these differences. In terms of selectivity,
when comparing the three systems at the end of the reaction,
Ni@L3 is slightly more selective towards the formation of (Z)-
3-hexene, which might be due to the lower activity. Though
somewhat less active, Ni@L3 exhibits a remarkable chemo-
and stereocontrol, achieving a 90% selectivity in the semi-
hydrogenation to (Z)-3-hexene at full conversion of the alkyne.
The catalytic activity of the Ni@L systems herein prepared
is higher than those of the Ni-based systems previously
reported in the literature.7a,b,d For example, Godard and co-
workers prepared Ni NPs stabilized by NHC ligands that were
supported on multi-walled carbon nanotubes, which were
used in the hydrogenation of internal alkynes under 5 bar H2
and at 50 °C.7b The catalytic systems prepared in the present
work are three times faster under milder reaction conditions
(1 bar H2 and room temperature), likely due to the non-
supported nature of the catalysts. Similarly, recoverable
magnetic Fe3O4@Ni nanoparticles have been employed in the
selective hydrogenation of terminal alkynes to alkenes under
7 bar H2 and at room temperature, but higher catalyst load-
ings (7%) and longer reaction times (48 h) were required in
that case to reach an 80% conversion.7g
The hydrogenation of terminal alkynes catalysed by
Ni@L1 under mild conditions (1 bar H2 and r.t.) gave a good
selectivity to the corresponding alkenes showing mixtures of
a/b ca. 90/10 (Table 3), except for 13 which showed a mixture
of 39/61. In addition, there was no full conversion of the sub-
strates after 8 h and only for 13 the conversion reached 96%.
Comparing these results with the hydrogenation of internal
alkynes 1 and 2, a 92% conversion was observed for 1
(Table 1, entry 1) and 47% for 2 (Table 1, entry 2), which indi-
cates that steric factors affect both selectivity and conversion.
In general, the hydrogenation of the substrates 2 and 9–12
(containing aromatic rings) is slow compared to 1 and 13
(the aliphatic ones), but the hydrogenation of aryl terminal
alkynes 9–12 is faster than diarylalkyne 2. This can also be at-
tributed to the steric factors caused by hindered coordination
Fig. 6 Recycling experiments using Ni@L1 as the catalyst. Reaction
conditions: 0.5 mmol of 3-hexyne, 3 mmol% catalyst, 0.75 mL toluene,
1 bar H2, and 5 h. Conversions and selectivities were determined by 1H
NMR (average of two runs).
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to the nanoparticle surface. To evaluate the robustness of the
Ni NPs prepared in this work, recycling experiments were
performed with Ni@L1 (Fig. 6). Taking advantage of the mag-
netic character of Ni@L, the nanoparticles were effortlessly
recovered from the reaction medium after precipitation in
the presence of an external magnet (adhesive force: 4 kg).
Fig. 6 shows the recycling results during five catalytic runs (1
bar H2; 5 h). Both the conversion and selectivity towards (Z)-
3-hexene were practically maintained during the first 3 runs,
but after the 4th run the activity of Ni@L1 considerably de-
creased. This effect can be attributed to an agglomeration of
the nanoparticles after 3 catalytic cycles, as evidenced by
TEM (Fig. S5 and S6 of ESI†), but oxidation of the NPs during
the recycling cycles cannot be discarded.
THF solution of 30 mL containing 0.2 equiv. (0.11 mmol) of
the appropriate ligand was added (L1, L2 or L3). The Fisher–
Porter was pressurized with 3 bar H2, and when the solution
reached room temperature it was stirred at 70 °C during 48
h, leading to a black suspension. After that period, the H2
pressure was released and the suspension was concentrated
to 2–3 mL under reduced pressure. Next, 50 mL of pentane
were added resulting in a black precipitate, which was
washed with pentane (25 mL × 2) and dried overnight under
dynamic vacuum. NP diameters determined by TEM, Ni@L1:
2.8 (0.4) nm, Ni@L2: 2.8 (0.5) nm, Ni@L3: 3.4 (2.0) nm. Ni
content determined by AAS, Ni@L1: 75.9%, Ni@L2: 71.1%,
Ni@L3: 45.5%.
Catalytic hydrogenation reactions
Experimental
A HEL 24-multireactor (Vvial = 1.5 mL) was used to perform
the catalytic experiments. In a typical experiment, Ni@L
(0.015 mmol of Ni) in 0.75 mL of toluene (from a standard
solution previously prepared) was added to a vial with 0.5
mmol of substrate. The reactor was then sealed under argon,
purged with H2 three times, and finally pressurized with 1
bar of hydrogen. Then, the reaction mixture was stirred using
a magnetic stir bar at room temperature during the time in-
dicated. After that, the samples from each reaction were
analysed by 1H NMR and/or GC-MS. More specifically, after
the catalytic reactions, Ni NPs were easily separated from the
organic solution with the help of an external magnet. For the
NMR analyses, the samples were evaporated, and redissolved
in CDCl3. For the GC-MS analysis, the samples were directly
injected in the equipment. For the recycling experiments, the
NPs were decanted with the help of a magnet, the superna-
tant was removed, and the NPs were washed three times with
toluene (2 mL). Then, the substrate and the solvent were in-
troduced into the HEL 24-multireactor and the reaction was
loaded with H2.
All manipulations were carried out using standard Schlenk
tubes, Fisher–Porter bottle techniques or in a glove-box under
argon atmosphere. The organic solvents were purified before
their use by a solvent purification system (MBraun). The com-
mercial products were obtained from commercial providers
and used as received. [NiIJCOD)2] and 3-hexyne were bought
from Sigma Aldrich. Zwitterionic imidazolium-amidinate li-
gands ICy·(Ar)NCN (L1, L2 and L3) were synthesized according
to the literature.11 The AAS analyses of Ni@L1, Ni@L2 and
Ni@L3 were performed by Kolbe (Mülheim, Germany), using
a Perkin Elmer Analyst 200 atomic absorption spectrometer.
The samples were prepared with an acid digestion procedure.
Ni@L were analysed by TEM after the deposition of a drop of
a colloidal dispersion in THF on a copper grid covered with a
carbon layer. The TEM pictures were taken at the UMS-
Castaing by using a JEOL JEM 1011 CX-T electron microscope
(100 kV; point resolution of 4.5 Å). The particles mean size
was obtained through a manual analysis of the magnified
micrographs by measuring at least 200 nanoparticles on a
given grid. XRD measurements were performed on
a
Conclusions
PANalytical Empyrean diffractometer [Co-Kα radiation (λ =
0.1789 nm); 45 kV; 40 mA]. The XRD samples were prepared
and sealed under argon atmosphere. Magnetic studies were
executed on a vibrating sample magnetometer (VSM, Quan-
tum Device PPMS Evercool II). X-Ray photoelectron spectro-
scopy (XPS) analyses were carried out at CIRIMAT Laboratory
(Toulouse) with the help of a Thermoelectron K-Alpha ma-
chine. The photoelectron emission spectra were obtained
using a monochromatized source of Al-Kα radiation (hν =
1486.6 eV). Analyzed area = 0.15 mm2. Pass energy = 40 eV.
The spectrometer was calibrated using the Au 4f7/2 (83.9 0.1
Novel Ni NPs were obtained by reaction of [NiIJCOD)2] with
zwitterionic imidazolium-amidinate ligands. Specifically,
three amidinate ligands with different electronic properties
were employed as stabilizers (L1, L2 and L3). The Ni NPs
ligated with the weakest electron donor ligand (L3) presum-
ably have a densely covered surface that protects them to-
wards oxidation, as has been observed by XPS, probably
thanks to the electron-withdrawing character of the ligand.
The three different catalytic systems herein prepared were ac-
tive in the hydrogenation of various substrates and especially
in the semi-hydrogenation of 3-hexyne to (Z)-3-hexene under
very mild reaction conditions. An influence on the activity
and selectivity was observed depending on the ligand used as
a stabilizer. Ni@L2 stabilized with a strongest donor ligand
(R = –OMe) was the most active catalyst. The slowest catalyst
was the system containing an electron-withdrawing group
Ni@L3 (R = –Cl). The higher surface coverage can induce a
higher selectivity in this system, as Ni@L3 showed the best
eV) and Cu2p3/2 (932.8
0.1 eV) photoelectron lines. XPS
spectra were recorded in direct mode N(Ec). The XPS spectra
were analysed with CasaXPS processing software.
Synthesis of the Ni NPs
[NiIJCOD)2] (150 mg, 0.55 mmol) and 30 mL of dried and de-
oxygenated THF, were placed in a 180 mL Fisher–Porter
bottle. The resulting solution was cooled at −40 °C and a
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selectivity towards the (Z)-3-hexene formation at full conver-
sions. It should be highlighted that ligands do not only play
a role in the stabilization of the NPs, but also in the capacity
to modulate the activity and selectivity of the NPs in catalysis.
The magnetic character of Ni@L permits the recovery of the
Ni NPs from the reaction medium after the catalysis by an ex-
ternal magnet. The NPs were recyclable up to 3 times
maintaining good activity values. This fact, together with the
good selectivity of these catalytic systems without the need of
toxic salts as poison, makes them potentially useful catalysts
for semi-hydrogenation reactions.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank CNRS, UPS-Toulouse, INSA, “IDEX/Chaires
d'attractivité l'Université Fédérale Toulouse Midi-Pyrénées”,
“Instituto de Tecnología Química” (ITQ; UPV-CSIC), “Juan de
la Cierva” programme (IJCI-2016-27966), “Primero Proyectos
de Investigación” (PAID-06-18), “Instituto de Investigaciones
Químicas” (IIQ; CSIC-US), “Ministerio de Ciencia, Innovación
y Universidades” (MCIU/AEI), FEDER funds of the European
Union (PGC2018-095768-B-I00) and ERC Advanced Grant
(MONACAT 2015-694159) for financial support. We also thank
L. Datas for the TEM facilities (UMS Castaing) and S. Cayez
for the HRTEM measurements.
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