Stable Zero-Valent Nickel Nanoparticles in Glycerol: Synthesis and Applications in Selective Hydrogenations

Article abstract of DOI:10.1002/adsc.201800786

Small (mean diameter, ca. 1.2 nm) and well-dispersed zero-valent nickel nanoparticles (NiNPs) stabilized by cinchona-based alkaloids and TPPTS (tris(3-sulfophenyl)phosphine trisodium salt), were synthesized from the organometallic precursor [Ni(cod)₂] in neat glycerol under hydrogen pressure. NiNPs were fully characterized ((HR)-TEM, EDX, XPS, XRD, IR, magnetization), both at solid state and directly from the corresponding colloidal solutions in glycerol due to its negligible vapour pressure. NiNPs dispersed in glycerol were applied in hydrogenation reactions, in particular in semihydrogenation of alkynes to give (Z)-alkenes under satisfactory conditions (3 bar H₂, 1 mol% Ni, 100 °C), showing remarkable activity and selectivity. The catalytic phase was recycled at least ten times without loss of activity, affording in each case metal-free organic products. Other functional groups such as nitro, nitrile and formyl groups were efficiently hydrogenated to the corresponding anilines, benzylamines and benzylalcohols respectively (77–95% yields). (Figure presented.).

Full text of DOI:10.1002/adsc.201800786

Title: Stable Zero-Valent Nickel Nanoparticles in Glycerol: Synthesis
and Applications in Selective Hydrogenations
Authors: Antonio Reina, Isabelle Favier, Christian Pradel, and
Montserrat Gómez
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10.1002/adsc.201800786
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Stable Zero-Valent Nickel Nanoparticles in Glycerol: Synthesis
and Applications in Selective Hydrogenations
Antonio Reina,a Isabelle Favier,a Christian Pradel,a and Montserrat Gómeza*
a
Laboratoire Hꢀtꢀrochimie Fondamentale et Appliquꢀe, Universitꢀ de Toulouse 3 - Paul Sabatier, UPS and CNRS UMR
5069, 118 Route de Narbonne, F-31062 Toulouse Cedex 9, France
[Phone : +33 561557738, Fax : + 33 561558204, e-mail : gomez@chimie.ups-tlse.fr]
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201######.((Please delete if not appropriate))
Abstract. Small (mean diameter, ca. 1.2 nm) and well- 1 mol% Ni, 100 ºC), showing remarkable activity and
dispersed zero-valent nickel nanoparticles (NiNPs) stabilized selectivity. The catalytic phase was recycled at least ten
by cinchona-based alkaloids and TPPTS (tris(3- times without loss of activity, affording in each case metal-
sulfophenyl)phosphine trisodium salt), were synthesized free organic products. Other functional groups such as
from the organometallic precursor [Ni(cod)2] in neat glycerol nitro, nitrile and formyl groups were efficiently
under hydrogen pressure. NiNPs were fully characterized hydrogenated to the corresponding anilines, benzylamines
((HR)-TEM, EDX, XPS, XRD, IR, magnetization), both at and benzylalcohols (77-95% yields).
solid state and directly from the corresponding colloidal
solutions in glycerol due to its negligible vapour pressure.
NiNPs dispersed in glycerol were applied in hydrogenation
reactions, in particular in semihydrogenation of alkynes to
give (Z)-alkenes under satisfactory conditions (3 bar H2,
Keywords: Nickel; Nanoparticles; Glycerol;
Hydrogenation; Semihydrogenation of alkynes; Catalyst
recycling
In the last decade, the skillful synthesis of well-
defined NiNPs has attracted the attention of several
research groups, being the main challenge the
exclusive formation of Ni(0) nano-materials due to
the easy oxidation of zero-valent nickel species.
Some of these methodologies describe the synthesis
of Ni-based alloys[9,11,13,14] and also nickel
nanoparticles supported on different kinds of
solids,[8,12,15-18] even under continuous millifluidic
conditions.[19] The lack of control in the oxidation
state of the metal in the preformed nano-catalysts is a
recurrent reported issue, highlighted by different
authors: Tilley et al. in the synthesis of nickel
nanocubes by reduction of Ni(acac)2 under
hydrogen;[20] Hyeon’s group in the synthesis of
NiNPs prepared under thermal decomposition;[21] and
Zarbin et al. in the synthesis of NiNPs by the polyol
methodology.[22]
Introduction
The selective hydrogenation of carbon-carbon triple
bonds is a very valuable tool in organic synthesis
because alkenes are useful building blocks, both at
laboratory and industrial scale.[1] In particular, this
process represents a crucial step in polymerization
reactions in order to eliminate alkynes and dienes
from alkene raw materials.[2] The two main
challenges rely on controlling the stereochemistry
and avoiding over reduction towards the
corresponding alkane. For the synthesis of (Z)-
alkenes from internal alkynes and also for the
synthesis of alkenes from terminal alkynes, several
methodologies have been applied, such as i) Lindlar
catalyst,[3] including lead-free metal-based systems;[4]
ii) complex reducing agents of the type NaH-NaOR-
[5]
MXn and iii) dispersed nickel metal on graphite.[6]
However, over reduction and isomerization reactions
happen and consequently the chemoselectivity is
difficult to be controlled. In recent years, nickel
catalysts and in particular supported nickel
nanoparticles including nickel-based alloys, have
been studied, due to their ability to selectively
hydrogenate alkynes into alkenes.[7-13]
Chaudret’s group reported an efficient method for the
synthesis of nickel(0) nanorods exhibiting magnetic
properties, obtained from [Ni(cod)2] under hydrogen
atmosphere.[23] Based on this procedure, Philippot
and co-workers have recently described the
preparation of Ni(0)NPs supported on SiO2 for the
catalytic hydrogenation of alkenes.[24]
1
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10.1002/adsc.201800786
Advanced Synthesis & Catalysis
In contrast to the use of NiNPs as solid catalysts, few
works have been published in liquid catalytic phase,
the main part of them using ionic liquids. Dupont et
al. synthesized Ni(0)NPs in imidazolium-based ionic
liquids which showed a thin layer of nickel oxide,
being active in the reduction of alkenes.[25] Janiak’s
group reported the synthesis of small crystalline
Ni(0)NPs exhibiting a hcp structure, in imidazolium-,
pyridinium- and thiophenium-based ionic liquids
under micro-wave conditions.[26] Shen’s group
described the formation of Ni and Ni/NiO core-shell
nanoparticles in water using glycerol as co-solvent.[27]
In this context, we were interested in the efficient
synthesis of zero-valent nickel nanoparticles in neat
glycerol based on the methodology previously
developed in our group.[28,29] The ability of glycerol
to trap metal catalysts most likely due to its
supramolecular arrangement and the stability
exhibited by both Pd[28] and Cu[29] based nanoparticles,
encouraged us to study more challenging Ni(0)
nanoparticles, with the aim of overcoming the surface
oxidation and consequently obtaining suitable and
reusable catalysts, leading to nickel-free products.
Scheme 1. Synthesis of Ni(0) nanoparticles NiL (L
denotes the stabilizer) in glycerol from decomposition of
[Ni(cod)2].
The as-prepared colloidal NiA-NiC solutions were
analyzed by low resolution TEM. The obtained
micrographs mainly showed the formation of small,
spherical and quite well-dispersed NiNPs (1.1 - 1.3 ±
0.3 nm), together with few bigger particles (ca. 2 – 5
nm; Fig. 1).
Results and Discussion
(a) NiA
(b) NiB
(c) NiC
NiA
NiB
NiC
Synthesis
and characterization of
Ni(0)
nanoparticles. Zero-valent nickel nanoparticles
(NiNPs) were synthesized by decomposition of the
organometallic precursor [Ni(cod)2] in glycerol under
hydrogen atmosphere (3 bar) at 100 °C, in the
presence of different types of stabilizers (Ni/stabilizer
= 1/1): quinidine (A), cinchonidine (B) and TPPTS
(C), based on the methodology previously reported
for palladium nano-systems (Scheme 1).[28] For the
three NiNPs, black colloidal solutions were obtained.
Lower pressure (1 bar) or lower temperature (80 °C)
led to the formation of agglomerates (see Fig. S1 in
the Supporting Information). Moreover, lower
content on stabilizer (Ni/stabilizer = 1/0.5 and 1/0.3
for both TPPTS and quinidine) or in the absence of
any stabilizer (it means, only in the presence of
glycerol) did not led to stable colloidal solutions,
observing the instantaneous precipitation of bulk
metal under hydrogen pressure.
dmean = 1.3 ± 0.3 nm
(1196 particles)
dmean = 1.3 ± 0.3 nm
(1316 particles)
dmean = 1.1 ± 0.3 nm
(1004 particles)
Figure 1. TEM images of NiA (a), NiB (b) and NiC (c) in
glycerol. Mean diameters excluding the bigger particles.
With the aim of determining the structure of NiA-
NiC in glycerol, HR-TEM analyses on those bigger
nanoparticles only exhibited the presence of
crystalline fcc Ni(0) particles, in contrast to hcp Ni(0)
nanoparticles prepared in ionic liquids under thermal
decomposition conditions (Fig. 2).[26] EDX analyses
proved the presence of the corresponding stabilizers
(Fig. 2). It is important to draw attention to the
stability of Ni(0)NPs in glycerol. Therefore, the
colloidal solution corresponding to NiA was exposed
Ni(II) salts as metal precursors, such as Ni(OAc)2 and
Ni(NO3)2, led to green or yellow dissolutions
depending on the stabilizer, suggesting the presence
of Ni(II) species.
2
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10.1002/adsc.201800786
Advanced Synthesis & Catalysis
to air and moisture for ten days; after this time, HR-
TEM analysis revealed that NiNPs remained
dispersed without any sign of agglomeration,
showing the same crystalline structure (Fig. S2 in the
Supporting Information); this test is coherent with the
low solubility of molecular oxygen in glycerol.[30]
Moreover, the synthesis of NiA was scaled-up ten
times (50 mL of NiA colloidal solution), preserving
size and morphology of nanoparticles (Fig. S3 in the
Supporting Information), and catalytic reactivity (see
below).
(absence of any absorption band at 425-475 cm-1
corresponding to Ni-O stretching[31]); however,
NiNPs stabilized by cinchona-based alkaloids present
absorption bands in the 550-400 cm-1 range,
hindering the observation of bands coming from
nickel oxides (Fig. S5d in the Supporting
Information). HR-TEM images of NiA(s)-NiC(s) at
solid state did not exhibit any significant difference in
relation to the analyses of NiA-NiC dispersed in
glycerol, also showing Ni(0) crystalline phases (Fig.
S6 in the Supporting Information).
Elemental and ICP-AES analyses gave similar
metal/ligand ratio: Ni/stabilizer = 1/0.06 for both
cinchona ligands (A and B) and 1/0.05 for the
phosphine (C), which led to the idealized formula of
Ni147A10, Ni147B15 and Ni147C6, taking into account
the compact fcc structure of these nanoparticles. With
the aim of quantifying the content of hydrides present
in the nanoparticles (molecular hydrogen was used as
reducing agent in the synthesis of NiNPs), we carried
out a hydrogen transfer reaction between the
nanoparticles stabilized by quinidine, NiA(s), and
phenylacetylene. The alkyne consumption to give
NiA
3.5
NiA
3.0
C
C
2.5
2.0
1.5
Ni, Cu
Ni
N
1.0
Ni
N
Ni
O
O
Cu
0.5
0.0
NiNi
(0,2,2)
(0,0,2)
0
1
2
3
4
5
6
7
8
keV
(1,1,1)
NiB
Ni, Cu
Ni
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1
styrene was monitored by H NMR up to no further
C
Ni, Cu
C*
NiB
evolution was observed, resulting in a Ni/H ratio of
1/1.4 (Fig. S7 in the Supporting Information). This
high hydride content can probably be due to the
presence of interstitial hydrides in the nanocluster
structure. Unfortunately, XPS analyses showed very
low intense signals for the Ni 2p binding region,
probably associated to a low electronic density on the
surface related to the small size of particles together
with the common surface defaults (Fig. S8 in the
Supporting Information).
N
Ni
N
O
O
Cu
O
Ni
Ni
(0,0,2)
0
2
4
6
8
(1,1,1)
keV
NiC
Ni, Cu
Ni
12
10
8
NiC
Ni, Cu
C*
C
6
Magnetic measurements for Ni(0)NPs stabilized by
quinidine were carried out, both in glycerol solution
and solid state (Fig. 3). Actually, both materials
exhibited narrow hysteresis at 2 K (coercive fields of
64 Oe and 89 Oe for NiA dispersed in glycerol and
NiA(s), respectively), together with moderate
remanence magnetizations (0.66 and 2.15 emu/g for
NiA dispersed in glycerol and NiA(s), respectively),
showing a weak ferromagnetic behavior. The
saturation magnetization of NiA(s) (31 emu/g) is
slightly lower than that reported for bulk nickel (54
emu/g)[32]). Data corresponding to NiA(s) is in
agreement with that reported for cubic nickel
nanocrystals.[33] The lower values of coercive fields
and remanence magnetization of nickel nanoparticles
in glycerol (NiA) than those observed for
nanoparticles in solid state (NiA(s)) can be related to
the higher amount of capping agents around the metal
nanoparticles (i.e. both glycerol and quinidine),
leading to a magnetic material constituted by a
crystalline core surrounded by a “disordered”
surface.[34]
(1,1,1)
4
OO
Ni
2
S
S
Cu
(0,2,2)
Ca
P
Ca
P
Ca
Ni
0
0
2
4
6
8
(0,0,2)
keV
Figure 2. HR-TEM micrographs (left) and EDX analyses
(right) of NiNPs in glycerol NiA, NiB and NiC in glycerol;
inserts show the crystallographic planes (corresponding to
fcc Ni(0) structure) observed by Fast Fourier Transform on
selected nanoparticles.
NiNPs, NiA(s)-NiC(s), were isolated at solid state
from the corresponding colloidal glycerol solutions
by centrifugation. The obtained black powders were
stocked in the glove-box to avoid their oxidation.
Powder X-Ray Diffraction (PXRD) merely evidenced
the fcc structure of nickel(0); small NiNPs (ca. 1 nm)
could not be observed by PXRD and only large
particles were detected (Fig. S4 in the Supporting
Information). Their infrared spectra showed the
presence of the corresponding ligand (Fig. S5 in the
Supporting Information). In addition, the IR spectrum
of NiC(s) excluded the presence of nickel oxide
3
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10.1002/adsc.201800786
Advanced Synthesis & Catalysis
1.25
1
NiA
catalyst), but highly chemo-selective towards the
corresponding alkene (57% yield). Unfortunately, 1-
NiA(s)
0.75
0.5
(4-chlorophenyl)-2-phenylacetylene
was
nearly
inactive at low hydrogen pressure (ca. 10%
conversion using NiA as catalyst under 3 bar H2);
under 20 bar H2 pressure, a mixture of alkene/alkane
of 75/25 was achieved (80% conversion).
0.25
0
-20000
-15000
-10000
-5000
0
5000
10000
15000
20000
0.5
NiA
NiA(s)
-0.25
-0.5
-0.75
-1
0.3
0.1
-700
-500
-300
-100
-0.1
100
300
500
700
Table 1. Semihydrogenation of internal alkynes
catalyzed by NiNPs in glycerol.a)
-0.3
-0.5
Magnetic field (Oe)
-1.25
Magnetic field (Oe)
Figure 3. Magnetization curves of nickel nanoparticles
stabilized by quinidine dispersed in glycerol (black line)
and isolated NiNPs at solid state (red line) at 2 K.
NiNPs catalyzed semihydrogenation reactions. The
work developed by Yus and co-workers using Ni(0)
nanoparticles obtained in situ by reduction of
nickel(II) salts in THF, led to a highly active catalytic
system for the reduction of dienes and both internal
and terminal alkynes, obtaining the corresponding
alkenes in high yields under room conditions;
however, this catalytic system could not be efficiently
recycled.[7] Supported catalysts including Cu- and Ni-
Al hydrotalcites,[9] nickel phosphide nanoparticles,[10]
NiNPs decorated on polyaniline/graphite oxide
composite[12] and Rh-Ni nanoflowers immobilized in
a metal organic framework,[13] have been used for the
selective hydrogenation of alkynes obtaining
moderate to good yields, but generally under harsh
conditions, showing unsuccessful catalyst recycling.
Magnetic nano-Ni-Fe2O4 catalyst[35] and magnetic
amine-nickel complex coating Fe3O4 nanoparticles
allowed the recycling due to the easy separation of
the catalyst.[8] NiGa bimetallic nanoparticles in
imidazolium-based ionic liquid were active for the
reduction of alkyl terminal alkynes and aryl internal
alkynes, and the liquid support allows the recycling
of the catalytic phase for at least 4 runs.[11]
Entry Substrate
NiA
NiB
NiC
Conv.
Conv.
Conv.
(yield)
(yield)
(yield)
(%)b)
(%)b)
(%)b)
1c)
>99 (98)d) >99 (97) 98 (94)
>99 (90) >99 (98) 93 (90)
1
2
3
2
3
4
5
88 (84)
98 (96)
92 (89)
85 (80)
94 (91)
93 (89)
78 (74)
89 (87)
89 (84)
4
5e),f)
a) Results from duplicated experiments. No reactivity in the
absence of metal and/or stabilizer. Reaction conditions: 1
mmol of substrate (1-4) and 1 mL of the catalytic glycerol
solution of NiA-NiC (10-2 molL-1, 0.01 mmol of total Ni;
b)
determined by ICP).
Determined by GC and GC-MS
c)
using decane as internal standard. Cyclooctane used as
d)
internal standard. Using large scale synthesis of NiA (50
mL scale), 97% conversion and 94% yield of 1a was
obtained. e) Substrate 5 corresponds to phenylacetylene (see
Table 2). f) Under 1 bar of H2.
The stereochemistry of the unsymmetrically
substituted alkene 3a was established by a NOESY-
NMR experiment, evidencing the exclusive formation
of the (Z)-stereoisomer (Fig. S9 in the Supporting
Information); this fact points to a syn hydrogen
addition, following a Horiuti-Polanyi mechanism.[36]
No important differences were observed among the
three catalysts employed, showing a slightly better
With the aim of studying the selectivity on the
hydrogenation of alkynes, we chose the
hydrogenation of 5-decyne (1) to compare the
reactivity of NiA-NiC (entry 1, Table 1). Under 3 bar
of H2 at 100 °C using 1 mol% of NiNPs in glycerol,
full conversion was achieved after 2 h of reaction,
exclusively obtaining 5-decene (1a). Using NiA
prepared at large scale (50 mL scale; for all the other
catalytic results, smaller synthesis scales were used, 5
or 10 mL scale), the same reactivity was observed
than using smaller NiNPs synthesis scale (entry 1,
Table 1). Other internal alkynes (2-4) led to the
expected alkenes in high yields (2a-4a; entries 2-4,
Table 1). The conversion for the hydrogenation of 3-
phenylpropargyl alcohol was low (62%, using NiA as
performance for those containing
a
cinchona
stabilizer (NiA, NiB) in relation to that stabilized by
the phosphine TPPTS (NiC); this trend was also
noticed for the phenylacetylene hydrogenation to give
styrene (entry 5, Table 1). The over-reduction to
afford the corresponding saturated product was only
observed under harsher conditions; actually, full
conversion of diphenylacetylene took place under 20
bar H2 at 100 °C using 5 mol% NiA, leading to a
4
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10.1002/adsc.201800786
Advanced Synthesis & Catalysis
mixture of stilbene/1,2-diphenylethane (90/10 ratio
respectively) after 24 h of reaction. When these
catalysts (NiA-NiC, see Scheme S1 in the Supporting
Information) were applied in the hydrogenation of 4-
phenyl-3-buten-2-one, no reaction was observed, in
contrast to the recent published NiNPs stabilized by
carboxylic acids and supported on silica, where the
corresponding butanone was obtained under smooth
conditions.[24] The high selectivity observed by NiA-
NiC towards the semihydrogenation process,
evidences the effect of the capping agents (A-C),
hindering the coordination of alkenes to the surface,
in agreement with the higher adsorption enthalpies
for internal alkynes than for internal alkenes.[37]
bond interaction, favoring the over-reduction of the
alkyne.
Table 2. Hydrogenation of terminal alkynes
catalyzed by NiA in glycerol.a)
Hydrogenation of the 1,6-enyne 6a gave exclusively
the corresponding diene under high pressure (20 bar
H2), without observing the reduction of either of the
two alkene groups (Scheme 2).
Entry Substrate
NiA
(mol%)
Conv.
(%)b)
Selectivity
(a/b)b),c)
1d)
1
1
1
1
5
5
5
5
1
1
92
69
71
98
95
94
97
94
100
90
100/0 (89)
100/0 (66)
100/0 (68)
85/15
5
7
8
9
5
7
8
9
2d)
3d)
Scheme 2. Hydrogenation of the 1,6-enyne 6 catalyzed by
NiA.
4d)
5d)
15/85
6e)
0/100 (90)
0/100 (95)
0/100 (91)
40/60
7e)
Terminal alkynes (5, 7-9) selectively gave the
corresponding styrene derivative under low hydrogen
pressure (1 bar H2, 1 mol% NiA, 24 h; entries 1-3,
Table 2), except for 9 where a mixture of styrene and
ethylbenzene derivatives were formed (9a/9b = 85/15,
entry 4, Table 2). However, working at higher
catalyst load (5 mol%) and longer times (up to 48 h),
full hydrogenation was favored towards the
corresponding alkyl-benzene derivative (entries 5-8,
Table 2), in contrast to that observed for internal
alkynes 1-4. The preferred over-reduction for
terminal alkynes can be presumed by the faster
hydrogenation of both terminal alkynes and alkenes
on the metal surfaces than that observed for internal
ones, mainly due to coordination constraints at the
nanoparticle surface.[37] For non-aromatic substrates,
such as alkynes 10 and 11, the full reduction of the C-
C triple bond was favored even using low catalyst
load (entries 9 and 10, Table 2).
8d)
9d)
10
10d)
25/75
11
a) Results from duplicated experiments. No reactivity in the
absence of metal and/or stabilizer. Reaction conditions: 1
mmol (for 1 mol%) or 0.2 mmol (for 5 mol%) of substrate
(5-7) and 1 mL of the catalytic glycerol solution of NiA
b)
(10-2 molL-1, 0.01 mmol of total Ni; determined by ICP).
Determined by GC and GC-MS using decane as internal
standard. c) Yields given in brackets. d) For 24 h. e) For 48 h.
The catalytic glycerol phase was recycled at least ten
times, preserving its catalytic behavior, as shown for
the semihydrogenation of 5-decyne (Fig. 4). No
presence of nickel in the extracted organic products
was evidenced by ICP-AES analyses after the 1st, 4th
and 10th runs; neither significant differences by TEM
were observed for the catalytic phase after the 1st and
8th runs (Fig. S10 in the Supporting Information).
Therefore, the metal trapping effect of glycerol
avoids the leaching of nickel from the catalytic
solution and hence metal-free compounds can be
obtained, valuable aspect taking into account the
nickel toxicity.[39]
It is noteworthy that even in the presence of a
hydroxyl or amino group (alkynes 7, 10 and 11), the
full hydrogenation of the C-C triple bond was favored
(entries 6, 9 and 10, Table 2), in contrast to the
behavior observed using iron nanoparticles, where the
formation of alkenes was preferred, due to the
coordination of the OH group at the surface.[38] In our
case, glycerol probably prevents the coordination of
OH and NH2 groups to the metal surface by hydrogen
5
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10.1002/adsc.201800786
Advanced Synthesis & Catalysis
that may have a greater impact on sterically hindered
substrates, such as 1,3-dinitrobenzene (14).
For benzaldehyde derivatives (15-17), higher
hydrogen pressure (20 bar) was required to achieve
good yields in benzyl alcohols (up to 92%, entries 4-6,
Table 3) with both catalysts NiA and NiB. No
formation of acetal coming from the reaction between
aldehyde and glycerol was detected, in contrast to
that observed using NiC (entries 4 and 6, Table 3).
For this latter catalyst, only when the hydrogenation
is faster, such as for 4-trifloromethylbenzadehyde
(16; entry 5, Table 3), the formation of the
corresponding acetal was precluded. NiA also
hydrogenated hexanal, giving 1-hexanol in moderate
yield (35%). Unfortunately, NiA catalyst was not
active in the hydrogenation of acetophenone. The
synthesis of benzylamines by hydrogenation of
Figure 4. NiA-catalyzed semihydrogentaion of 5-decyne
(1), showing the recycling of the catalytic phase up to ten
runs. For catalytic conditions, see entry 1 of Table 1.
nitriles
was
possible
when
4-
We were also interested in the reduction of other
types of functions, such as nitro-, nitrile- and formyl-
substituted aromatics. In the literature, NiNPs have
been applied in the reduction of nitro-substituted
aromatic compounds, including NiPd bimetallic
trifluoromethylbenzonitrile (19) was used as substrate
(entry 8, Table 3). In the case of 4-methylbenzonitrile,
no reaction was observed using any of the three
catalysts; for benzonitrile (18; entry 7, Table 3), only
a
mixture
of
dibenzylamine
and
N-
nanoparticles.[40]
Monometallic
Ni(0)-based
benzylidenebenzylamine was obtained (see Fig. S11
in the Supporting Information), common by-products
in the hydrogenation of nitriles.[45]
nanoparticles supported on different types of supports
led to the formation of anilines under harsh
conditions,[41-43] giving over-reduction of the aromatic
rings[42] and by-products coming from the reaction of
the formed anilines and stabilizers of the
nanocatalysts.[43] However, few NiNPs colloidal
systems have been used,[8,44] probably due to the easy
oxidation of the catalyst, losing its catalytic activity.
Benzonitriles containing formyl or nitro groups, such
as 4-nitrobenzonitrile (20) and 4-formylbenzonitrile
(21) were selectively hydrogenated towards the
corresponding aniline (20a) and benzyl alcohol (21a)
using NiA as catalyst, preserving the nitrile moiety
(Scheme 3). Nitro (20) and formyl (21) groups are
faster reduced than nitrile function. The resulting
aniline 20a and benzyl alcohol 21a disfavors the
reduction of CN group because of the electron-donor
character of NH2 and CH2OH groups, in agreement
with the lack of reactivity observed for 4-
methylbenzonitrile (see Scheme S2 in the Supporting
Information), and in agreement with the behavior
observed using bimetallic CuNi nanoparticles in the
hydrogenation of nitriles containing nitro groups, as
recently reported by Sun and co-workers.[46]
With the aim of evaluating our NiNPs dispersed in
glycerol, we first studied their catalytic behaviour in
the hydrogenation of nitro-derivatives. In comparison
with the hydrogenation of alkynes (see above), higher
catalyst load was required (5 mol% Ni) for the
reduction of these functional groups. Analogously to
the trend observed for alkyne hydrogenations, NiNPs
stabilized by TPPTS were much less efficient than
those containing cinchona ligands as stabilizers, NiA
and NiB (see Table 3), probably due to the stronger
interaction of the phosphine to the Ni surface,
hampering the coordination of the substrate to the
metal. Nitrobenzene (12) and 4-nitrophenol (13) were
efficiently hydrogenated by NiNPs stabilized by
cinchona ligands (NiA and NiB), giving high yield of
the corresponding aniline derivative, 12a and 13a,
under low H2 pressure (3 bar; entries 1-2, Table 3).
Curiously, 1,3-dinitrobenzene (14) was hydrogenated
by NiA obtaining 1,3-diaminobenzene (14a) in 77%
yield (entry 3, Table 3), however, NiB was
substantially less active (only 30% yield under the
same conditions). This behavior can be attributed to
two related effects: stabilizer A is probably more
labile than B due to the presence of the methoxide
group on the quinoline ring, hence favoring the
coordination of substrate to be hydrogenated, effect
6
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10.1002/adsc.201800786
Advanced Synthesis & Catalysis
Scheme 3. NiA catalyzed hydrogenation of benzonitriles
20 and 21 (figures indicate conversions (yields) determined
by GC using decane as internal standard)
smooth conditions by a single step methodology,
using three different stabilizers (TPPTS and two
cinchona-based
alkaloids,
quinidine
and
cinchonidine), and fully characterized both in
glycerol solution and solid state. The as-prepared
nanocatalysts were highly selective for the
hydrogenation of alkyl and aryl alkynes without
reducing neither conjugated C=C bonds nor aromatic
rings. In addition, nitro aromatic compounds,
benzonitrile derivatives and formyl containing
compounds were reduced under relatively mild
conditions. Studies concerning the synthesis of
bimetallic nanocatalysts in glycerol are currently
under investigation, with the aim of being applied in
the synthesis of fine chemicals.
Table 3. NiNPs in glycerol catalyzed hydrogenation
of aromatic substrates containing nitro, formyl and
nitrile functional groups.a)
Experimental Section
Synthesis of NiNPs in glycerol, NiA-NiC. 0.05 mmol
(13.6 mg) of [Ni(cod)2] and 0.05 mmol of ligand (14.7 mg
for quinidne (A); 16 mg for cinchonidine (B); 28.4 mg for
TPPTS (C)) were dissolved in 5 mL of glycerol and stirred
under argon in a Fisher-Porter bottle at room temperature
until complete dissolution. The system was then
pressurized under 3 bar of dihydrogen and stirred at 100 °C
for 18 h. A black colloidal solution was then obtained.
Entry
Substrate
(R)
NiA
NiB
NiC
Isolation of NiNPs from the glycerol solution. The as-
prepared NiNPs in glycerol were transferred to a
centrifugation tube under argon. Centrifugation was carried
out at 5,000 rpm for 15 min and the solution was then
separated by decantation. This process was repeated 5
times until complete removal of glycerol. The remaining
black powder was then dried under vacuum at 80 °C
overnight. Elementary analysis (nickel load determined by
ICP-AES): NiA: Ni 73.1%, C 22.5%, H 1.7%, N 2.7%;
NiB: Ni 67.1%, C 27.2%, H 2.3%, N 3.4%; NiC: Ni 74.9%,
C 16.9%, H 0.8%, S 7.4%. Expected data for proposed
nanoclusters: Ni147A10: Ni 72.7%, C 20.2%, H 2.0% ; N
2.4%,; Ni147B15: Ni 66.15%, C 26.2%, H 2.55% ; N 3.2%;
Ni147C6 : Ni 71.7%, C 10.8%, H 0.6% ; S 4.8%;
Conv.
Conv.
Conv.
(yield)
(yield)
(yield)
(%)b)
(%)b)
(%)b)
1
2
3
4
5
6
7
12 (H)
13 (4-OH)
14 (3-NO2) 80 (77)
15 (H)
16 (4-CF3)
17 (4-OMe) 90 (87)
90 (89)
98 (95)
96 (91)
91 (88)
35 (30)
31 (27)
30 (26)
<5
95 (92)
95 (90)
93 (89) 77 (30)c)
94 (90)
75 (70)
83 (78) 71 (25)d)
18 (H)
95
(70/30)e) (20/80)f)
95 (92) 89 (82)
95
<5
<5
8
19 (4-CF3)
a) Results from duplicated experiments. No reactivity in the
absence of metal and/or stabilizer. Reaction conditions: 0.2
mmol of substrate (9-16) and 1 mL of the catalytic glycerol
Hydride titration of NiA. Isolated NiA nanoparticles (0.1
mmol, 6 mg) were dispersed in 0.6 mL of THF-d8 in a
Young NMR tube and then phenylacetylene (0.1 mmol, 10
mg) was added, in the presence of an external standard (0.1
mmol, 12 mg of cyclooctane). The formation of styrene
was monitored by 1H NMR.
solution of NiA-NiC (10-2 molL-1, 0.01 mmol of total Ni;
b)
determined by ICP).
Determined by GC and GC-MS
c)
using decane as internal standard. 40% formation of the
five-membered acetal coming from the reaction of
d)
aldehyde 12 and glycerol. 40% formation of the five-
membered acetal coming from the reaction of aldehyde 14
e)
and glycerol. N-benzylidenebenzylamine/dibenzylamine
ratio = 70/30. N-benzylidenebenzylamine/dibenzylamine
ratio = 20/80.
General procedure for NiNPs-catalyzed hydrogenation
in glycerol. In a Fisher-Porter bottle (from 1 to 3 bar) or an
autoclave (from 4 to 20 bar), the appropriate substrate (1
mmol for 1 mol% of catalyst or 0.2 mmol for 5 mol%) was
added to 1 mL of preformed nickel nanoparticles in
glycerol under argon (total amount of nickel: 0.01 mmol).
The reaction mixture was treated under vacuum and then
pressurized with H2 at the appropriate pressure, heated up
to the desired temperature, stirred for the established time,
f)
Conclusions
For the first time, highly stable zero-valent NiNPs
were efficiently synthesized and trapped in neat
glycerol. The nanomaterials were obtained under
7
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10.1002/adsc.201800786
Advanced Synthesis & Catalysis
and then cooled down to room temperature. Organic
[8] V. Polshettiwar, B. Baruwati, R. S. Varma, Green
Chem. 2009, 11, 127-131.
products
were
extracted
from
glycerol
with
dichloromethane (5 x 3 mL) and the solvent was
evaporated under vacuum. All the products were identified
by GC-MS analysis, 1H and 13C NMR spectroscopy,
comparing with the reported data.
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General procedure for recycling of the catalytic phase.
After extraction of the organic products, the catalytic phase
was treated under vacuum at 50 ºC for 2 h and the
appropriated reagents were then added under argon to the
catalytic phase. The reaction mixture was pressurized with
H2 at the appropriated pressure, heated up to the desired
temperature, stirred for the established time, and then
cooled down to room temperature. Organic products were
extracted from glycerol with dichloromethane (5 x 3 mL)
and dried under reduced pressure. This process was
repeated for each of the different runs.
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Advanced Synthesis & Catalysis
FULL PAPER
Highly Stable Zero-Valent Nickel Nanoparticles in
Glycerol: Synthesis and Applications in Selective
Hydrogenations
Adv. Synth. Catal. Year, Volume, Page – Page
Antonio Reina,a Christian Pradel,a and Montserrat
Gómeza*
10
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