Single step synthesis of metallic nanoparticles using dihydroxyl functionalized ionic liquids as reductive agent

Article abstract of DOI:10.1039/c3ra43909g

In the last few years, ionic liquids (ILs) have revealed their broad versatility for application towards the synthesis of shape and size controlled nanoscale metal catalysts, respectively metal(0) nanoparticles (M-NPs) in the absence of additive ligand. ILs play the role of solvent and ligand. In our work, we depict the synthesis of M-NPs (M = Cu, Ag, Ni, Ru) using dihydroxyl functionalized imidazolium ILs 1-(2,3-dihydroxypropyl)-2,3-dimethylimidazolium bis(trifluoromethanesulfonimide) [C₁C₁(EG)Im][NTf ₂] as solvent, stabilizer and reductive agent simultaneously. In this IL the formation of stable dispersion of NPs from silver and copper salts in the absence of any reducing agent has been observed. The comparison of the ¹³C-NMR, and IR data of pure IL and IL-NP (Cu and Ag) suspensions proved the oxidation of the hydroxyl group to a carbonyl group, confirming that the dihydroxyl function of the IL acted as the reducing agent. This oxido-reduction depends on the nature of the metal. No reaction occurred with Ni^II and Ru⁰ precursors, and with Ru^II and Ni⁰ salts the formation of M-NPs is due to the activation of the acidic protons on the imidazolium ring. The Ru-NPs were then further evaluated for selective partial hydrogenation, here we observed good selectivities for α,β-unsaturated compounds such as cyclohexenone and cinnamic aldehyde, which can be converted exclusively into the unsaturated alcohols or saturated alcohols. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra43909g

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Full Paper
Single step synthesis of metallic nanoparticles using dihydroxyl
functionalized ionic liquids as reductive agent
Walid Darwich,a Christian Gedig,b Hassan Srour,a Catherine C. Santini,a* Martin H. G. Prechtlb*
Received (in XXX, XXX) 25th July 2013, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
In the last years, ionic liquids (ILs) have revealed their broad versatility for application towards the
synthesis of shape and size controlled nanoscale metal catalysts, respectively metal(0) nanoparticles (Mꢀ
NPs) in absence of additive ligand. ILs play the role of solvent and ligand. In our work, we depict the
synthesis of MꢀNPs (M = Cu, Ag, Ni, Ru) using dihydroxyl functionalized imidazolium ILs 1ꢀ(2,3ꢀ
10 dihydroxypropyl)ꢀ2,3ꢀdimethylimidazolium bis(trifluoromethanesulfonimide) [C1C1(EG)Im][NTf2] as
simultaneously solvent, stabilizer and reductive agent. In this IL the formation of stable dispersion of NPs
from silver and copper salts in the absence of any reducing agent has been observed. The comparison of
the 13CꢀNMR, and IR data of pure IL and ILꢀNP (Cu and Ag) suspensions proved the oxidation of the
hydroxyl group to a carbonyl group, confirming that the dihydroxyl function of the IL acted as the
15 reducing agent. This oxidoꢀreduction depends on the nature of metal. No reaction occurred with NiII and
Ru0 precursors, and with RuII and Ni0 salts the formation of MꢀNPs is due to the activation of the acidic
protons on the imidazolium ring. The RuꢀNPs were then further evaluated for selective partial
hydrogenation, here we observed good selectivities for α,βꢀunsaturated compounds such cyclohexenone
and cinnamic aldehyde, which can be converted exclusively into the unsaturated alcohols or saturated
20 alcohols.
45 generated in ILs mainly from the following routes a) simple
reduction of M(n+) salts, b) decomposition of the organometallic
complexes in the formal zero oxidation state, c) bombardment of
bulk metal precursors with deposition onto the ILs and d) phase
transfer of preꢀformed NPs in water or organic solvents to the IL
50 (Scheme 1).23
Introduction
The progress in the comprehension of metallic nanoparticles
(NPs) is a central aspect due to their great potential in the
development of new and innovative materials for applications in
25 areas such as catalysis or microelectronics.1ꢀ11 Their small sizes,
generally reported between 1 and 100 nm, lead to unique
physicoꢀchemical properties between the bulk and molecular
states, which vary greatly with small changes in NP size. For
example, the catalytic properties of NPs are largely determined
30 by the energy of the surface atoms, in turn controlled by the
number of neighbouring atoms, dictated by their size, as well as
the presence and nature of ligands or supports.12ꢀ16 Particles less
than 10 nm in diameter are particularly interesting in catalysis,
due to their high surface to volume ratio, and are also greatly
18
35 influenced by the aforementioned sizeꢀeffect.17,
To prevent
aggregation, NPs must be stabilized by the use of capping agents
such as (waterꢀsoluble) polymers, quaternary ammonium salts,
surfactants, ligands or polyoxoanions, providing electronic and/or
steric protection.19, 20 In this context, ionic liquids (ILs), defined
40 as low temperature molten salts, have emerged as one of the most
important and the most investigated media for the synthesis of
metal NPs.21ꢀ24 Moreover, an advantage of using ILs is that
stabilizing additives such as ligands, polymers and supports are
not required.23 The soluble transitionꢀmetal NPs are in situ
Scheme 1: General methods used to prepare metal NPs in ILs.
In general, appropriate metal complexes or metal salts are
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dispersed / dissolved in the ILs and subsequently reduced to the
corresponding MꢀNPs, in the ubiquitous presences such as
works we reported that functional groups in the moieties of ionic
liquid sideꢀchain show significant influence towards the
27
molecular dihydrogen gas,25 ethylene glycol,26 ascorbic acid,
55 selectivity while using RuꢀNPs as hydrogenation catalyst for
hydrazine,28 and disulfide.29 In some cases, the IL has been
claimed to be the reducing agent.30ꢀ32 However, in certain cases,
an aqueous solution of ILs was used. For instance, the formation
arene versus nitrile entities.
There, we showed that nitriles
38ꢀ39
5
completely suppress the usual occurring arene hydrogenation.
of
Au0
microprisms
in
1ꢀbutylꢀ3ꢀmethylimidazolium
hexafluorophosphate in the presence of water has been explained
by an oxidoꢀreduction reaction consuming the IL (Scheme 2) and
Results and Discussion
60 In order to prove the specific reactivity of [C1C1(EG)Im][NTf2],
the thermal stability of all chosen metal precursors (Table 1) in
this IL under argon have also been studied in [C1C4Im][NTf2].
Each precursor has been successively heated in IL under argon
until a colour change was observed (Scheme 4).
10 yielding [Clꢀ] anions in the media.
Scheme 2: The formation of Au0 microprisms in 1ꢀbutylꢀ3ꢀ
methylimidazolium hexafluorophosphate in the presence of water.30
15 In water, using halide salts of mono and diꢀhydroxyl ILs as
surfactants, a suspension of narrow size Ag0ꢀNPs (means size 2
or 4 nm following the IL) stabilized by the hydroxyl groups has
been obtained using AgNO3 as metal source (Scheme 3).31 The
oxidation of the OHꢀgroup to the corresponding aldehyde during
20 the reduction of AgI into Ag0 has been evidenced by IR
spectroscopy.
65
Scheme 4: General overview for the decomposition/reduction of metal
precursors in [C1C1(EG)Im][NTf2] at the applied reaction temperatures
and durations (refer Table 1)
Experiments with AgNO3, CuMes, anhydrous CuSO4 and
70 Ru(COD)(COT) in [C1C4Im][NTf2] showed no reaction. In
contrast Ru(Meꢀallyl)2(COD) and Ni(COD)2 already decompose
at low temperature.32 In [C1C1(EG)Im][NTf2] no reactions were
observed with anhydrous CuSO4, Ru(COD)(COT) and Ni(NTf2)2
after stirring overnight at 130 °C, neither colour changes of the
75 solutions were visible, nor NPs could be located in TEM images
(Table 1, Entries 1, 3, 5). In [C1C1(EG)Im][NTf2], Ru(Meꢀ
allyl)2(COD), Ni(COD)2 and AgNO3 led to NPs at 90 °C in 1, 2
and 4 h, respectively (Table 1, Entries 2, 4, 6). The formation of
CuꢀNPs in [C1C1(EG)Im][NTf2] from mesityl copper (CuMes)
80 was observed after 12 h at 130 °C (Table 1, Entry 7). All results
with the corresponding mean diameters are listed in Table 1.
Scheme 3: Preparation of Ag NPs using the hydroxyl group linked to the
imidazolium cation as reducing agents.31
25 Pointing out that the particle size control can also be influenced
by dissolved halides, especially in aqueous solutions of ILs. For
instance, [Clꢀ] anions and water may coordinate onto the metal
surface, acting as extraꢀstabilizers, but also influencing the
nanoparticle properties (optical, magnetic and catalytic).33
30 Contrary, in our previous studies using neat anhydrous alkyl
imidazolium IL like [C1CnImNTf2] (n = 4 and 6) as reaction
media under argon, the formation of Ru0ꢀNPs and Ni0ꢀNPs by
reduction of bis(2ꢀmethylallyl)(1,5ꢀcyclooctadiene) ruthenium
[RuII(Meꢀallyl)2(COD)], respectively autoꢀdecomposition of
35 bis(η4ꢀ1,5ꢀcyclooctadiene)nickel
[Ni0(COD)2] have
been
observed.32 These methods afforded smallꢀsized and monoꢀ
dispersed RuꢀNPs (1ꢀ3 nm; mean diameter: 2.0±0.3 nm) and for
NiꢀNPs a broader size distribution (2ꢀ12 nm; mean diameter:
7.0±2.0 nm and higher agglomeration.
40 Generally, the NP formation in solution depends on three key
factors (i) the solvent medium, (ii) the reducing agent, and (iii)
the stabilizer. ILs are largely depicted as solvent and stabilizer for
NPs and are scarcely used as a reducing agent. In this work, we
report, that an anhydrous dihydroxyl functionalized imidazolium
45 IL, namely 1ꢀ(2,3ꢀdihydroxypropyl)ꢀ2,3ꢀdimethylimidazolium
bis(trifluoromethanesulfonimide ([C1C1(EG)Im][NTf2]) based on
Table 1: Experimental conditions and the mean sizes of the MꢀNPs
formed in [C1C1(EG)Im][NTf2] under argon
Entry
Metal Source [M]a
CuSO4
AgNO3
Ru(COD)(COT)
Ni(COD)2
Ni(NTf2)2
Ru(COD)(Meꢀallyl)2
CuMes
t[h]/T[°C]
12 h/130 °C
4 h/90 °C
12 h/130 °C
2 h/90 °C
12 h/130 °C
1 h/90 °C
12 h/130 °C
NP size (nm)
n.r*
4.7± 0.8nm
n.r*
3.5 ± 0.5nm
n.r*
6.9 ± 1.3nm
5.7 ± 0.8nm
1
2
3
4
5
6
7
85 a[C] 0.01 M of metal precursor in 1.5 ml of [C1C1(EG)Im][NTf2], n.r*=
no reaction
.
a
building block derived from abundant biomass namely
glycerine, conjuncts these three roles: solvent, stabilizer, surface
modifier and reductant in the in situ synthesis of NPs.
50 The role as metal nanoparticle surface modifier we have
investigated in particular regarding the partial hydrogenation of
organic substrates with ruthenium nanoparticles. In previous
90
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the13C NMR spectra of the AgꢀNP suspension (Figure 3).
Silver Nanoparticle Synthesis:
Silver nitrate is reduced in aqueous monoꢀ and dihydroxyl
functionalized imidazolium ionic liquid within in 2 h at 60°C, the
generated AgꢀNPs show sizes of 4ꢀ8 nm and 2ꢀ4 nm,
respectively.31 The presence of dihydroxyl functional group
induces probably a higher size control. In our experimental
Copper Nanoparticle Synthesis:
decomposition of mesitylꢀcopper
[C1C1(EG)Im][NTf2] at 130 °C within 12 h yielded well
dispersed CuꢀNPs with a mean size around 5.7 nm (Figure 4).
40 The
(CuMes)
in
5
conditions, (Table 1, Entry 2:
2 h at 90 °C in neat
While, CuMes in [C1C4Im][NTf2] under 0.9 MPa of H2 resulted
[C1C1(EG)Im][NTf2]) the AgꢀNP were formed with a mean size
about 4.7 ± 0.8nm (Figure 1) for 90% of the NP population and
in CuꢀNPs with a size distribution centred around 6.4 nm at
45 30 °C, respectively 4.5 nm at 100 °C reaction temperatures.4
10 at ca. 10 nm for 10 %. The structure of individual particles was
investigated using HRTEM, Figure 1. The interplanar distance
measured from this micrograph is 0.2295nm (table 2),
corresponding to the (101) interplanar distance of hexagonal
metallic Ag.
Figure 2 FTꢀIR spectra of suspension of (a) AgꢀNPs in
[C1C1(EG)Im][NTf2] after reaction (b) pure [C1C1(EG)Im][NTf2]
15
Figure 1: TEM and HRTEM images of AgꢀNPs in [C1C1(EG)Im][NTf2]
and their particles size distribution
Table 2 : Theorical‡ (JCPD 04ꢀ016ꢀ1388) and experimental interplanar
20
distances (nm), and angles° for AgNPs
spot
1
2
hkl
d(exp) nm d(theo) nm
Angle (exp) Angle (theo)
101
011
ꢀ110
0.2295
0.2466
0.2529
0.224270
0.224270
0.224270
0
0
50.29
119.67
52.44
116.22
3
FTꢀIR spectroscopy was used to investigate the interactions
between different species and changes in chemical composition
of the mixtures. Figure 2 shows the FTꢀIR spectra of pure
50 Figure 3: 13C NMR spectra a)pure [C1C1(EG)Im][NTf2]; b) suspension of
AgꢀNPs in [C1C1(EG)Im][NTf2]
25 [C1C1(EG)Im][NTf2] (b), and of the reaction media after the
formation of AgꢀNPs (a). In the IR spectra of the neat
[C1C1(EG)Im][NTf2], a broad peak of (3220ꢀ3720) cmꢀ1 centred
at 3504 cmꢀ1 and peaks at 1530 cm ꢀ1 and 1590 cm ꢀ1are related to
the hydroxyl function. In the IR spectra of the AgꢀNP
30 suspensions, all these bands are less wide and changed from
(3220ꢀ3720 cmꢀ1) to (3500ꢀ3600 cmꢀ1) and are centred at 3525
cmꢀ1 and a new peak is observed at 1645 cm ꢀ1 assigned to the
vibration of conjugated carbonyl groups resulting from the
oxidation of the OHꢀgroup, during the reduction of metal ions
35 (Figure 2 and Scheme 5). The formation of this carbonyl function
is also confirmed by the peak at 205 ppm assigned to a δC=O in
Note that no reaction occurs with CuSO4 in [C1C1(EG)Im][NTf2]
whatever the experimental conditions are. Contrarily, CuꢀNPs are
formed using this salt in C1C4ImBF4 in the presence of hydrazine
55 as a reducing agent and ethylene glycol playing the role as
ligand.28 The comparison of the FTꢀIR spectra of the CuꢀNP
suspension in [C1C1(EG)Im][NTf2] (Figure S1a) and the pure
[C1C1(EG)Im][NTf2] (Figure S1b) shows the presence of
stretching peaks at 1645 cmꢀ1 corresponding to a carbonyl group
60 vibration, similar to the AgꢀNPs synthesis (see above).
With AgNO3 and CuMes an oxidation of one of the hydroxyl to
carbonyl function was observed affording AgꢀNPs and CuꢀNPs,
following the oxidoꢀreduction mechanism depicted in Scheme 5.
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The interplanar distance measured from this micrograph is
0.2012 nm (table 3), which coincides with the (1 1 1) interplanar
distance of cubic metallic Cu.
with a methyl group at the C2ꢀposition. When Ni(COD)2 was
dissolved in [C1C1(EG)Im][NTf2] under argon, no reaction was
25 observed at room temperature. At 90 °C after 2 h, a homogeneous
suspension of NiꢀNPs was obtained with
a narrow size
M-NPs
distribution centred around 3.5 ± 0.5nm (Figure 5).
+
N
OH
O
N
[M+]
O
N
OH
-
N
NTf2
Ar, ꢀT
OH
-
N
NTf2
[C1C1(EG)Im][NTf2]
N
O
M = Ag or Cu
-
NTf2
5
Scheme 5: Proposed oxidoꢀreduction mechanism
30 Figure 5 : TEM image of NiꢀNPs in [C1C1(EG)Im][NTf2] and particles
size distribution
The analysis of the resulting solution by gas chromatography
showed as major product cyclooctaꢀ1,5ꢀdiene (1,5ꢀCOD),
35 accompanied by its isomer cyclooctaꢀ1,3ꢀdiene (1,3ꢀCOD) and
cyclooctene and traces of cyclooctane (Figure S2). The presence
of large amount of 1,5ꢀCOD could be explained by a ligand
exchange with a diol functionality. Following Route A (Scheme
6), the formation of NiIIꢀH is necessary to explain the
40 isomerization of 1,5ꢀCOD into 1,3ꢀCOD. But, NiII salt are not
reduced to NiꢀNPs, in this media, vide supra, thus Route A
(Scheme 6) could not explain the experimental results.
Consequently the decomposition can only be explained by CꢀH
insertion at the less acidic C4ꢀH, C5ꢀH positions of the
45 imidazolium ring and generation of transitory nonꢀclassical NHC
ligands; Route B in Scheme 6.32, 34
Figure 4 : TEM and HRTEM images of CuꢀNPs in [C1C1(EG)Im][NTf2]
and their particles size distribution
10
Table 3 : Theoretical‡ (JCPD 04ꢀ001ꢀ3342) and experimental interplanar
distances (nm) and angles of CuNPs
spot
1
2
hkl
d(exp) nm d(theo) nm
Angle (exp) Angle(theo)
1 1 1 0.2012
2 0 0 0.1753
2 2 0 0.1344
0.2087240
0.1807600
0.1278170
0
0
90.11
42.90
90
45
3
15
Ruthenium Nanoparticle Synthesis:
Nickel Nanoparticle Synthesis:
While Ru(COD)(COT) under H2 since 0 °C readily affords Ruꢀ
50 NPs in any ILs; contrarily, in [C1C1(EG)Im][NTf2] under argon,
as well in [C1C4Im][NTf2] no RuꢀNPs are observed.
An autoꢀdecomposition of the organometallic complex Ni(COD)2
is observed affording nickel nanoparticles NiNPs in neat
[C1C4Im][NTf2] at room temperature under argon. This occurs
In [C1C4Im][NTf2], Ru(Meꢀallyl)2(COD) is readily decomposed
at room temperature after prolonged reaction time (Scheme S1).32
In [C1C1(EG)Im][NTf2], RuꢀNPs were formed at 90°C after 1h,
20 via a CꢀH insertion at the acidic C2ꢀposition of the imidazolium
ring and the formation of Nꢀheterocyclic carbenes.32 To overcome
this side reaction, we have used [C1C1(EG)Im][NTf2] substituted
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leading to a broader size distribution (6.9 ± 1.3 nm; Figure 6;
Figure S3). These NP could result from a decomposition as
already reported,32 through an oxidoꢀreductive reaction as
observed with Cu and Ag precursors or by the attack on the two
acidic protons C4ꢀH, C5ꢀH as observed with Ni(COD)2. No
difference was observed between the FTꢀIR and 13C NMR spectra
of the media before and after reaction (Figure S4).
5
Route A
N
OH
N
OH
+
N
N
Ni
OH
O
Ni (1,5-COD)
-
NTf2
-
H
NTf2
1,5ꢀCOD
-
NTf2
N
OH
N
OH
N
N
NiNPs
+
O
O
Ni (1,5-COD)
H
-
NTf2
Route B
OH
OH
-
NTf2
+
OH
N
2 (1,5-COD)
OH
N
-
Me
1,3-COD
1,5-COD
NTf2
N
+
N
Ni
H
[Ni]
Me
H
10
25 Figure 6 : TEM image of AgꢀNPs in [C1C1(EG)Im][NTf2] and particles
size distribution
Scheme 6 : Possible mechanism to explain the formation of NiꢀNPs in
[C1C1(EG)Im][NTf2].32
The gas chromatogram of the reaction media was similar to the
one obtained with Ni(COD)2 (Figure S5) due to ligand loss.
Catalytic Hydrogenation with Ruthenium Nanoparticles:
For further evaluation of the properties of the ruthenium
30 nanoparticles we tested their catalytic activity for hydrogenation
of different unsaturated substrates with arene, carbonyl and
alkene functionalities (Table 4). Moreover, recycling experiments
were performed for two aromatic substrates (Figure 7).
For the study of nanoscale catalysts, one important factor is the
35 surface to volume ratio of the particles. Of course one cannot
exclude partial aggregation of the nanoparticles, and even
topology changes,23 but the metal atoms located on the surface
are accessible to interact with substrates, hence are potentially
catalytically active. For the calculation of the amount of surface
40 atoms on a nanoparticle, we estimated the volume of a spherical
particle with a mean diameter of 6.9 nm. This diameter
corresponds to a particle volume of 172.0 nm3 consisting of
approx. 11862 atoms. According to the magic numbers
methodology as convenient tool for the evaluation of cluster
45 formation,35ꢀ37 the number of shells is 16 and this corresponds to
15 Consequently, as for [Ni(COD)2], the formation of MꢀNPs from
[Ru(COD)(2ꢀmethylallyl)2], was probably due to the activation of
the acidic protons on the imidazolium ring.32 However, in
[C1C1(EG)Im][NTf2], the formation RuꢀNPs differs to those
previously reported in nonꢀfunctionalized ILs such as
20 [C1C4Im][NTf2] because contrary in [C1C1(EG)Im][NTf2] no
imidazolium ring fragmentation was observed during the reaction
of the ruthenium precursor with the IL.32
~18
% surface atoms which are potentially active for
heterogeneous catalysed reactions such as arene hydrogenation.
The surface to volume ratio was used to determine the substrate
catalyst ratio and TONs for the hydrogenation reactions. The Ruꢀ
50 NPs dispersions in ILs are stable in presence of other solvents.
Therefore these ILꢀphases are interesting for their application in
multiphase catalysis.
The hydrogenation of toluene (Table 4; Entry 1ꢀ2) only occurred
at higher pressure. This stays in contrast to previous studies with
55 unfunctionalised ILs like [C1C4Im][NTf2] where hydrogenation
already occurred in high conversions at H2 pressures as low as 4
bar.32, 38 This observation can be related to two properties of this
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IL: (I) The gas solubility depends on the viscosity of the IL, as
[C1C1(EG)Im][NTf2] is highly viscous material, higher
pressures are necessary to guarantee significant amounts in the
still coordinate as Oꢀligand onto the surface via ligand exchange
(substrate vs. IL) which also brings the arene moiety close to the
surface, hence accessible for hydrogenation (Table 4, Entry 3;
a
ILꢀphase of dissolved hydrogen. (II) Functional groups linked to 40 Figure 8). Interestingly and contrary, the hydrogenation of
5
the IL may act as ligands on the metal surface, and these ligands
may hinder the coordination of the substrate, thus suppressing the
hydrogenation of arenes, but still allowing the hydrogenation of
functional groups via ligandꢀexchange on the surface.39 At higher
cinnamic aldehyde gives under similar conditions selectively 3ꢀ
phenylpropanol at high conversions (Entry 4; >99%), respectively
3ꢀphenylpropanal 9 at moderate conversions (Entry 5; 56%). The
aldehyde and olefin are hydrogenated subsequently and can be
pressure (40 bars) toluene was completely converted into 45 kinetically distinguished, but the arene remains untouched. This
10 methylcyclohexane in five runs with a total TON of >>2600, this
proves the capability for recycling without significant loss of
activity (Entry 2).
is again most likely due to steric aspects. During the coordination
onto the surface via the oxygen, the propylꢀchain acts as a spacer
and the arene moiety is not accessible for hydrogenation (Figure
9). Furthermore, we tried then to distinguish between an olefin
Table 4: Hydrogenation of different substrates with RuNPs.
50 and ketone in
a hydrogenation. The hydrogenation of
Entry
Substratea
t
p
conv. TON
product
selectivityb
ꢀ
cyclohexene gave also quantitative conversion within one hour
(Table 4; Entry 6). Following, we used cyclohexenꢀ2ꢀone 4 which
is also selectively converted into cyclohexanol 10 in high
conversions (Table 2; Entry 7; >99%) and at shorter reaction
55 times cyclohexanone 11 is formed exclusively. Again both
double bonds can be hydrogenated, similar to Entry 4, and a
kinetic distinction is possible with high selectivity for the
partially and fully hydrogenated products. This shows that the
tested catalytic system hydrogenate olefins and aldehydes/ketones
60 with kinetic selectivity. Arene hydrogenation occurs effectively
in absence of functional groups (Entry 1ꢀ2), but it is slightly
hindered already in presence of a neighbouring functional group
(Entry 3), and it can be fully blocked while incorporating a
functional group via a spacer (Entry 4) leading to a suppression
65 of the arene coordination, hence, the arene moiety is not
[h] (bar)a [%]b
1
2
3
4
5
6
7
8
toluene 1
toluene 1
acetophenone 2
cinnamic aldehyde 5 24 15 >99 438
cinnamic aldehyde 5
cyclohexene 3
24
4
0
0
24 40 >99 2640e
24 15 91d 1847e
ꢀ
50:50d (6:7)
100 (8)
100 (9)
ꢀ
4
15
56 245
20 15 >99 550
cyclohexenꢀ2ꢀone 4 24 15 >99 572
cyclohexenꢀ2ꢀone 4 15 >99 572
100 (10)
100 (11)
3
a
b
1 mL (7.9ꢀ10.3 mmol); reaction temp. = 75 °C; determined by 1Hꢀ
15 NMR. in all five runs. dfirst run. etotal TON of 5 (toluene) or 4
c
(acetophenone) runs. 1,1ꢀphenylethanol 6; 1,1ꢀcyclohexylethanol 7; 3ꢀ
phenylpropanol 8, 3ꢀphenylpropanal 9, cyclohexanol 10, cyclohexanone
11
20 For proving the chemoꢀselectivity of the catalyst phase, hence to
distinguish between a suppressed hydrogenation due to gas
solubility/ILꢀviscosity or ligandꢀlike poisoning of the surface (see
above), we used acetophenone 2 as model substrate (Table 4;
Entry 3). Here, we obtained a mixture of 1ꢀphenylethanol 6 and
25 1ꢀcyclohexylethanol 7 in a ~1:1 ratio after 24 h in high
conversions (91 %; Entry 3). This ratio undergoes a variation in
four subsequent runs during the recycling of the catalyst phase
(Figure 7).
accessible for hydrogenation.
Entry 3
IL
IL
HO OH
H
HO OH
H2
H
H
OH
Ru-surface
Entry 4
IL
IL
HO OH
Ru-surface
H
HO OH
H2
H
H
OH
Figure 8: Model for the coordination of intermediates/products during the
hydrogenation in presence of [C1C1(EG)Im][NTf2].
70 Experimental
All operations were performed in the strict absence of oxygen and
water under a purified argon atmosphere using glovebox
(Jacomex or MBraun) or vacuumꢀline techniques if not otherwise
indicated. Materials: Mesitylcopper CuꢀMes (Nanomeps, 97%),
75 Bis(η3ꢀ2ꢀmethylallyl)(η4ꢀ1,5ꢀcyclooctadiene)ruthenium Ru(Meꢀ
allyl)2(COD) (Strem, 97%), bis(η4ꢀ1,5ꢀcyclooctadiene) nickel
30 Figure 7: Hydrogenation of acetophenone 2 in recycling runs, yielding 1ꢀ
phenylethanol 8 (dark grey) and 1ꢀcyclohexylethanol 9 (light grey)
However, one could say that there is no significant chemoꢀ
selectivity for the hydrogenation of the arene moiety and the
ketone. This implies that in this certain case there is no blocking
35 of the catalyst surface by the diol of [C1C1(EG)Im][NTf2] as
surface ligand.39 Indeed, the intermediate 1ꢀphenylethanol 8 can
Ni(COD)2
(Strem)
,
bis(trifluoromethylsulphonyl)nickel
Ni(NTf2)2 (Solvionic), Silver nitrate (AgNO3) (ACS reagent,
≥99.0% ) and Copper sulfate (CuSO4) (SigmaꢀAldrich) were used
80 without further purification. (η4ꢀ1,5ꢀcyclooctadiene)(η6ꢀ1,3,5ꢀ
6
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RSC Advances
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DOI: 10.1039/C3RA43909G
cyclooctatriene) ruthenium(0) Ru(COD)(COT), 1ꢀbutylꢀ3ꢀ
ring. For all these in situ NP syntheses the [C1C1(EG)Im][NTf2]
methylimidazolium
[C1C4Im][NTf2]
dimethylimidazolium
[C1C1(EG)Im][NTf2] have been prepared as described in
literature.40ꢀ42
bis(trifluoromethane
and
ꢀsulfonimide),
1ꢀ(2,3ꢀdihydroxypropyl)ꢀ2,3ꢀ 60 catalytic
bis(trifluoromethanesulfonimide),
is simultaneously solvent, stabilizer and reductive agent. The
hydrogenation
with
Ru
nanoparticles
in
[C1C1(EG)Im][NTf2] showed that it is possible to hydrogenate
effectively arenes only in absence of functional groups. And, the
arene hydrogenation is completely blocked by incorporation of a
functional group via a spacer. In this case only alkenes and
65 aldehydes are accessible for hydrogenation via a ligand exchange
reaction. Pointing out, that hydrogenation of alkenes is possible
while aldehydes and ketones remain untouched
5
General procedure for the nanoparticle synthesis:
In 1.5ml of [C1C4Im][NTf2] or [C1C1(EG)Im][NTf2] with a
10 concentration of 0.01 M metallic salt or organometallic precursor
were mixed, then the reaction media was kept under stirring
overnight. At this step if no NP formation occurred, the
temperature was maintained, until a colour change appeared,
successively 2 h at 90 °C, then 4 h at 110 °C, and finally 130 °C
15 for 12 h under argon atmosphere. No reaction was claimed, when
after overnight at 130 °C, no colour change and no NPs in TEM
images were observed. The reaction time and the results of these
experiments are depicted in Table 1.
Acknowledgment
We acknowledge the Ministerium für Innovation, Wissenschaft
70 und Forschung NRW (MIWFꢀNRW) for financial support within
the Energy Research Program for the Scientist Returnee Award
(NRWꢀRückkehrerprogramm) for Dr M. H. G. Prechtl.
Moreover, C. Gedig is thankful for helpful discussions with
Sebastian Sahler and Michael T. Keßler. The HRTEM images
75 were carried out by Dr.Philippe Arquillière to whom we are
thankful.
20 General procedure for the hydrogenation reactions:
1 mL of an unsaturated substrate (see Table 2) was added to
ruthenium nanoparticles in [C1C1(EG)Im][NTf2] (see above; 1
mL IL) on air in a screwꢀcapped vial with a septa and needle. The
vial was placed in a stainless reactor and heated to 75 °C and
25 pressurised with hydrogen (pressure see Table 2). After the
indicated reaction time (Table 2), the autoclave was cooled to
room temperature, the pressure was carefully released and an
aliquot was analysed by 1HꢀNMR in C6D6.
For the recycling, the ILꢀlayer was extracted with pentane or
30 ether (at least three times), the catalystꢀ/ILꢀphase was dried under
reduced pressure and then the pure catalystꢀ/ILꢀphase was
charged with substrate again.
Notes and references
a Université de Lyon, Institut de Chimie de Lyon, UMR 5265 CNRS-
Université de Lyon 1-ESCPE Lyon, LC2P2, Equipe Chimie
80 Organométallique de Surface, ESCPE 43 Boulevard du 11 Novembre
1918, F-69616 Villeurbanne, France. Fax: 33(0)472431795; Tel:
33(0)472431810; catherine.santini@univ-lyon1.fr
b University of Cologne, Institute of Inorganic Chemistry - Research
Group Catalysis, Greinstr. 6, D-50939 Köln, Germany. Fax: +49 -221-
85 470-1788; Tel: +49 -221-470-1981; martin.prechtl@uni-koeln.de
(corresponding)
† Electronic Supplementary Information (ESI) available: For additional
spectral data. See DOI: 10.1039/b000000x/
‡ M. I. Novgorodova,, A. Gorshkov and A. V. Mokhov, Zap. Vses.
90 Mineral. O-va, 1979, 108, 552ꢀ563; H. Brodowsky, A. Fruma, H.
Sagunski and H. J. Schaller, Z. Metallkd. , 1982, 73, 354 for the
crystallographic data.
Characterization:
35 The UVꢀVis absorption spectra of the NP dispersions were
recorded on a Unicowfz UVꢀ2000 spectrophotometer. The Cuꢀ,
Niꢀ, Ruꢀ and AgꢀNp sizes were determined by transmission
electron microscopy (TEM) on a Philips CM120 (120 keV), and
the structure of the nanoparticles were examined using a Jeolꢀ
40 2010 transmission electron microscope (TEM) operated at a 200
KV acceleration voltage. TEM and HRTEM samples were
prepared by dropping the IL solution onto a copper TEM grid
with amphorous carbon over layers, followed by the removal of
excess amounts of solution with filter paper. The preparation is
45 stored in pillꢀbox filled with argon and taken from the glove box
only right before the analysis. 1Hꢀ and 13CꢀNMR spectra were
recorded on a 300 MHz Bruker, with C6D6, CD2Cl2 or d6ꢀDMSO
as external reference.
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This journal is © The Royal Society of Chemistry [2013]