Soft, Wet-Chemical Synthesis of Metastable Superparamagnetic Hexagonal Close-Packed Nickel Nanoparticles in Different Ionic Liquids

Article abstract of DOI:10.1002/chem.201605251

The microwave-induced decomposition of bis{N,N′-diisopropylacetamidinate}nickel(II) [Ni{MeC(NiPr)₂}₂] or bis(1,5-cyclooctadiene)nickel(0) [Ni(COD)₂] in imidazolium-, pyridinium-, or thiophenium-based ionic liquids (ILs) with different anions (tetrafluoroborate, [BF₄]^?, hexafluorophosphate, [PF₆]^?, and bis(trifluoromethylsulfonyl)imide, [NTf₂]^?) yields small, uniform nickel nanoparticles (Ni NPs), which are stable in the absence of capping ligands (surfactants) for more than eight weeks. The soft, wet-chemical synthesis yields the metastable Ni hexagonal close-packed (hcp) and not the stable Ni face-centered cubic (fcc) phase. The size of the nickel nanoparticles increases with the molecular volume of the used anions from about 5 nm for [BF₄]^? to ≈10 nm for [NTf₂]^? (with 1-alkyl-3-methyl-imidazolium cations). The n-butyl-pyridinium, [BPy]⁺, cation ILs reproducibly yield very small nickel nanoparticles of 2(±1) nm average diameter. The Ni NPs were characterized by high-resolution transmission electron microscopy (HR-TEM) and powder X-ray diffraction. An X-ray photoelectron spectroscopic (XPS) analysis shows an increase of the binding energy (E_B) of the electron from the Ni 2p_3/2 orbital of the very small 2(±1) nm diameter Ni particles by about 0.3 eV to E_B=853.2 eV compared with bulk Ni⁰, which is traced to the small cluster size. The Ni nanoparticles show superparamagnetic behavior from 150 K up to room temperature. The saturation magnetization of a Ni (2±1 nm) sample from [BPy][NTf₂] is 2.08 A m² kg^?1 and of a Ni (10±4 nm) sample from [LMIm][NTf₂] it is 0.99 A m² kg^?1, ([LMIm]=1-lauryl-3-methyl- imidazolium). The Ni NPs were active catalysts in IL dispersions for 1-hexene or benzene hydrogenation. Over 90 % conversion was reached under 5 bar H₂ in 1 h at 100 °C for 1-hexene and a turnover frequency (TOF) up to 1330 mol_hexane (mol_Ni)^?1 h^?1 or in 60 h at 100 °C for benzene hydrogenation and TOF=23 mol_cyclohexane (mol_Ni)^?1 h^?1.

Full text of DOI:10.1002/chem.201605251

A Journal of
Accepted Article
Title: Soft wet-chemical synthesis of metastable superparamagnetic
nickel-hcp nanoparticles in different ionic liquids
Authors: Christina Rutz, Kai Schütte, Juri Barthel, Alexey Bushmelev,
Annette Schmidt, Katharina Dilchert, Roland Fischer, and
Christoph Janiak
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Soft wet-chemical synthesis of metastable superparamagnetic nickel-hcp
nanoparticles in different ionic liquids
Susann Wegner,[a] Christina Rutz,[a] Kai Schütte,[a] Juri Barthel,[b] Alexey Bushmelev,[c] Annette Schmidt,[c]
Katharina Dilchert,[d] Roland A. Fischer[d] and Christoph Janiak[a]*
Abstract: The microwave-induced decomposition of bis{N,N'-diisopropylacetamidinate}nickel(II)
[Ni{MeC(NiPr)2}2] or bis(1,5-cyclooctadiene)nickel(0) [Ni(COD)2] in imidazolium-, pyridinium- or
thiophenium-based ionic liquids with different anions (tetrafluoroborate, [BF4]–, hexafluorophosphate,
[PF6]– and bis(trifluoromethylsulfonyl)imide, [NTf2]–) yields small, uniform nickel nanoparticles (Ni-NPs)
which are stable in the absence of capping ligands (surfactants) for more than eight weeks. The soft wet-
chemical synthesis yield the metastable Ni(hcp) and not the stable Ni(fcc) phase. The size of the nickel
nanoparticles increases with the molecular volume of the used anions from about ~5 nm for [BF4]– to ~10
nm for [NTf2]– (with 1-alkyl-3-methyl-imidazoliumcations). The n-butyl-pyridinium, [BPy]+ cation ILs
reproducibly yield very small nickel nanoparticles of 2 (± 1) nm average diameter. Ni-NPs were
characterized by high-resolution transmission electron microscopy (HR-TEM) and powder X-ray
diffractometry. An X-ray photoelectron spectroscopic (XPS) analysis shows an increase of the binding
energy (EB) of the electron from the Ni 2p3/2 orbital of the very small 2 (± 1) nm diameter Ni particles by
about 0.3 eV to EB = 853.2 eV compared to bulk Ni(0) which is traced to the small cluster size. The Ni
nanoparticles show superparamagnetic behavior from 150 K up to room temperature. The saturation
magnetization of a Ni (2±1 nm) sample from [BPy][NTf2] is 2.08 A·m2·kg–1 and of a Ni (10±4 nm) sample
from [LMIm][NTf2] it is 0.99 A·m2·kg–1, ([LMIm] = 1-lauryl-3-methyl-imidazolium. The Ni-NPs were
active catalysts in the IL dispersion for 1-hexene or benzene hydrogenation. Over 90% conversion was
reached under 5 bar H2 in 1 h at 100 °C for 1-hexene and TOF up to 1330 mol hexane x (mol Ni)–1 x h–1 or
60 h at 100 °C for benzene hydrogenation and TOF = 23 mol cyclohexane x (mol Ni)–1 x h–1.
nickel nanoparticles show a cytotoxic effect against leukaemia
cancer cells.[7]
Introduction
Nickel nanoparticles can be widely used as catalysts, for example
in reductive hydrogenation[ 1 ] or Suzuki coupling reactions.[ 2 , 3 ]
Also, the application as sensor electrode[4] or anode material in fuel
cells[ 5 ] or as a graphene-nickel nanoparticle hybrid for the
enrichment of proteins and peptides is reported.[6] Functionalized
Soft wet-chemical synthesis in organic solvents from metal-
organic complexes is an important route to nanometals and allows
preparing metals and alloys in the nanometer scale regime.[8-11]
Metal amidinates (Scheme 1) were investigated and widely used as
precursors for metal nanoparticles,[12,13] thin metal films in low
pressure chemical vapor deposition (CVD) or atomic layer
deposition (ALD).[14,15]
Zero-valent organometallic compounds like [Ru(COD)(COT)]
and [Ni(COD)2] (COD = 1,5-cyclooctadiene, COT = 1,3,5-
cyclooctatriene) were already employed as metal nanoparticles
precursors in imidazolium-based ionic liquids (ILs).[16-20] Hydrogen
was used to hydrogenate the ligands COD and COT to cyclooctane
which was subsequently lost from the Ru(0) or Ni(0) metal atom
and removed under reduced pressure from the reaction mixture.
[a]
Susann Wegner, Dr. Christina Rutz, Dr. Kai Schütte, Prof. Dr.
Christoph Janiak
Institut für Anorganische Chemie und Strukturchemie, Heine-
Universität Düsseldorf, 40204 Düsseldorf, Germany. Fax: + 49-211-
81-12287, Tel: +49-211-81-12286.
E-mail: janiak@uni-duesseldorf.de
[b]
Dr. Juri Barthel
Gemeinschaftslabor für Elektronenmikroskopie RWTH-Aachen,
Ernst Ruska-Centrum für Mikroskopie und Spektroskopie mit
Elektronen D-52425 Jülich, Germany.
E-mail: Ju.Barthel@fz-juelich.de
[c]
[d]
Dr. Alexey Bushmelev, Prof. Dr. Annette Schmidt
Physical Chemistry Dept. AK Schmidt, University of Cologne,
Luxemburger Str. 116, D-50939 Cologne, Germany
E-mail: annette.schmidt@uni-koeln.de
Katharina Dilchert, Prof. Dr. Roland A. Fischer
Lehrstuhl für Anorganische und Metallorganische Chemie, TU
München, Lichtenbergstr. 4, 85748 Garching. Germany
E-mail: roland.fischer@tum.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.xxxxx.
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Ni-NPs in imidazolium-based ILs
Bis{N,N'-diisopropylacetamidinate}nickel(II), [Ni(AMD)2] or
bis(1,5-cyclooctadiene)nickel(0), [Ni(COD)2] were dissolved or
suspended under nitrogen atmosphere in dried and deoxygenated
ionic liquid. Complete decomposition by microwave heating of
both precursors in IL was achieved after only 10 minutes using a
low power of 100 W to give an approximate temperature of 220 °C
in the reaction mixture (Scheme 1).
Comparative NMR analyses of selected neat ILs before and after
10 min microwave heating to approximately 220 °C indicated no
IL decomposition (see Fig. S5.1-S5.9 in Supp. Info.). Black Ni-
NP/IL dispersions were reproducibly obtained by repeated
decompositions of [Ni(AMD)2] or [Ni(COD)2]. An 1H-NMR
analysis after the decomposition of the nickel amidinate in
[BMIm][NTf2] shows signals in addition to the IL (Fig. S1.1b in
Supp. Info.), which we assign to the formation of 1-methyl-3-
butylimidazoliumylidene from 2-H deprotonation of the 1-methyl-
3-butylimidazolium cation by amidinate to give N,N'-
diisopropylacetamidine according to eq. S1 in Supp. Info. GC/MS
Scheme 1. Synthesis of nickel nanoparticles, Ni-NPs from nickel amidinate
[Ni{MeC(NiPr)2}2], in short [Ni(AMD)2], or [Ni(COD)2] by microwave-assisted
thermal decomposition in ILs.
For nickel nanoparticle synthesis we selected and compared
different ionic liquids as reaction media for the following reasons:
In recent years ionic liquids were developed into an alternative to
traditional solvents.[21,22] They appear particularly suited for the
preparation of metal nanoparticles (M-NPs).[23-25] The ionothermal
synthesis in ionic liquids appears highly promising, because of IL-
properties such as negligible vapor pressure, high thermal stability,
high ionic conductivity, a broad liquid-state temperature range, and
the ability to dissolve a variety of materials.[23-35] Ionic liquids,
especially with imidazolium cations (Scheme 2), are reported for
the preparation and stabilization of metal nanoparticles[23] which
were prepared from metal salts with[36,37] or without reducing H2
gas [12, 38], from organometallic -complexes[16-20,39,40] and from
analysis of
a hexane extract of the IL dispersion after
decomposition gave as the only non-IL peak m/z = 97 which is
assigned to [MeC(NiPr)2 – HiPr]+.
For [Ni(COD)2] there were only very minor signals in the NMR
analyses after decomposition (Fig S1.2 in Supp. Info.). More clear
was the observation of 1,5-COD and 1,3-COD by GC/MS in a
hexane extract of the IL dispersion after decomposition at m/z =
108 together with cyclooctene at m/z = 110. The differentiation of
1,5-COD and 1,3-COD was achieved by their difference in GC
retention time upon comparison with genuine COD samples. The
formation of 1,5- and 1,3-COD was already reported by Prechtl et
al. from the spontaneous decomposition of [Ni(COD)2] in [EMIm]
and [BMIm][NTf2].[47]
metal carbonyls[ 41 , 42 , 43 ] through thermal or photochemical[ 44 ]
45 ]
decomposition or electroreduction/electrodeposition.[23,
The
electrostatic and steric properties of ionic liquids allow for the
stabilization of M-NPs without the need of additional stabilizers,
surfactants or capping ligands.[23-26,37-46]
Analyses of the Ni-NP/IL dispersions by high-resolution
transmission electron microscopy (HR-TEM) (Fig. 1-3 and Fig. S2
and S3 in Supp. Info.) indicate the formation of Ni nanoparticles
with typical diameters between 5 and 10 nm for imidazolium ILs
with [BF4]–, [PF6]– and [NTf2]– anions (Table 1). Because of their
superparamagnetic behavior the samples show agglomerated, but
still isolated particles. In direct comparison both precursors yield
Ni-NPs of very similar size. In the imidazolium ILs the at first
sight slightly larger nanoparticle size from [Ni(COD)2] than from
[Ni(AMD)2] is still well within experimental errors (see also Fig.
6). Thus, the nature of the Ni precursor does not affect the
nanoparticle size.
Scheme 2. Ionic liquids used here for nickel nanoparticle synthesis and
Table 1. Ni-NP crystal phase, diameter and distribution from different
precursors and in different ILs.a
stabilization (lauryl = dodecyl).
Ni-precursor,a IL
A = [Ni(AMD)2]
B = [Ni(COD)2]
Phase b
TEM Ø
Image in Figure
Herein we compare nickel(II)-amidinate [Ni{MeC(NiPr)2}2], in
short [Ni(AMD)2], and [Ni(COD)2] (Scheme 1) as precursors in
different ionic liquids to yield nickel nanoparticles as stable
colloids. The different ILs contained the cations 1-n-butyl-, 1-n-
octyl- and 1-n-lauryl-3-methyl-imidazolium, 1-n-butyl- and 1-n-
(σ) [nm] c
Imidazolium
cations
A, [BMIm][BF4]
B, [BMIm][BF4]
A, [BMIm][PF6]
B, [BMIm][PF6]
A, [BMIm][NTf2]
B, [BMIm][NTf2]
Ni
Ni
Ni
n.d.
Ni
5 (± 2)
5 (± 2)
7 (± 4)
8 (± 3)
10 (± 2)
11 (± 2)
Fig. 1a
Fig. S3.1
Fig. 1b, S2.1
Fig. S3.4
Fig. 1c, S2.3
Fig. S3.6
lauryl-pyridinium
and
1-n-octyl-tetrahydrothiophenium
in
combination with tetrafluoroborate [BF4]–, hexafluorophosphate
[PF6]– and bis(trifluoromethylsulfonyl)imide [NTf2]– anions
(Scheme 2). No additional capping agents were used during
particle synthesis.
Ni
A, [OMIm][PF6]
B, [OMIm][PF6]
A, [OMIm][NTf2]
Nid (NiF2)d
n.d.
Ni
7 (± 1)
10 (± 2)
10 (± 3)
Fig. 2, S2.7
Fig. S3.8
Fig. S2.11
A, [LMIm][BF4]
Nid (NiO)d
7 (± 2)
Fig. 3a, S2.14
Results and Discussion
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A, [LMIm][PF6]
B, [LMIm][PF6]
A, [LMIm][NTf2]
Nid (NiF2)d
9 (± 3)
15 (± 2)
10 (± 4)
Fig. 3b, S2.18
Fig. S3.11
Fig. 3c, S2.21
anions.
Ni
Nid
Pyridinium cations
A, [BPy][PF6]
A, [BPy][NTf2]
B, [BPy][NTf2]
A, [LPy][PF6]
Ni e
2 (± 1)
2 (± 1)
2 (± 1)
3 (± 1)
Fig. 7, S2.25
Fig. 7
Fig. S3.12
Fig. S2.27
Ni e
Ni e
Ni (NiO,d NiF2)
Tetrahydrothio-
phenium cation
A, [OTh][PF6]
n.d.
9 (± 1)
Fig. S2.31
a 1.0 wt.ꢀ% Ni-NP/IL dispersions obtained by MWI with 100 W for 10 min at
220 °C. b From PXRD unless noted otherwise (see d) based on positive
matching to Crystallography Open Database, COD;[48] Ni stands for Ni with
hcp packing; Ni(hcp) reference COD 9008509; NiO reference COD 1010095;
NiF2 reference COD 9007538; n.d. = not determined. c Average diameter (Ø)
and standard deviation (σ). Histograms are included in Fig. S2 and S3 in
Supp. Info. See experimental section for TEM measurement conditions. d
Supported by SAED measurement of selected nanocrystals. e Because of the
small size of the Ni-NPs no PXRD could be obtained and phase analysis is
based on HR-TEM.
(a)
Work by others had shown that decomposition of [Ni(COD)2] in
1-alkyl-3-methyl-imidazolium ILs with different alkyl chain
lengths and bis(trifluoromethylsulfonyl)imide anion gave Ni-NPs
[49]
with an average diameter of 2.4-5.4 nm diameter
nm.[16]
or 4.9-5.9
(b)
Prechtl et al. observed spontaneous decomposition of
[Ni(COD)2] in the ILs [EMIm]NTf2] and [BMIm][NTf2] with the
formation of 1,5- and 1,3-COD. It was concluded that
decomposition of [Ni(COD)2] in imidazolium ILs with short alkyl
chains involved attack on the acidic C2-H and formation of the
respective N-heterocyclic carbene (NHC) on the Ni-NP surface
was proposed.[47] Such surface-coordinated NHC formation was
also
reported
for
Ir(0)-NPs
formed
from
[(1,5-
COD)Ir(CH3CN)2]BF4 under H2 in the presence of Proton Sponge
(PS) in acetone in the IL [BMIm][NTf2].[50]
(c)
A combined TEM and energy dispersive X-ray spectroscopic
analysis (EDX, Fig. 4) shows only the expected nickel emission
lines. The peaks in the powder X-ray diffractogram (PXRD, Fig. 5)
are positively matched to metastable hexagonal close-packed (hcp)
nickel metal with space group P 63/mmc. The stable bulk nickel
metal phase is face-centered cubic (fcc) or body-centered cubic
(bcc).[2,51,52] Also, Ni-NPs (2 nm diameter) stabilized with ethylene
glycol[ 53 ] and Ni-NPs (10-36 nm diameter) stabilized with the
cationic surfactants CTAB/TC12AB had the fcc structure.[54] Nickel
nanoparticles with an hcp phase[55] were obtained with 62 ± 9 nm
diameter stabilized by trioctylphosphine oxide (TOPO) from the
decomposition of nickel acetate[56] or stabilized with citrate from a
sol-gel method.[57] Ni-NPs with the hcp structure and a diameter
between 13 and 25 nm are also formed in polyethylene glycol,
oleic acid and oleyl amine.[58] There are also other possibilities to
obtain nickel compounds in an hcp phase.[59,60,61]
Figure 1. TEM images of 1.0 wt% Ni-NP/IL dispersions from [Ni(AMD)2] for
[BMIm]+ cations with (a) [BF4]–, (b) [PF6]– and (c) [NTf2]– anions. For images of
Ni-NP/IL dispersion from [Ni(COD)2] see Fig. S3.1-S3.7 in Supp. Info.
Figure 2. TEM images of Ni-NP/IL dispersions from [Ni(AMD)2] in
[OMIm][PF6]. For images of Ni-NP/IL dispersion from [Ni(COD)2] see Fig. S3.8
in Supp. Info.
Concerning oxidation of nickel we note that traces of oxygen in the
EDX spectrum (signal at 0.52 eV) are ubiquitous and always
present, also on noble metal samples and pure carbon grids (from
surface oxidation). For Ni-NPs in ILs with [BF4]– or [PF6]– anions,
small amounts of NiO and NiF2 are obtained in three cases (see
Table 1) as identified by phase analysis in the powder X-ray and
electron diffractograms (Fig. S2.9, S2.16, S2.29). It is known in
literature that [BF4]– or [PF6]– anions in ILs hydrolyze or
decompose to fluoride, F– which then leads to the formation of
62
]
metal fluoride, here NiF2.[
Alternatively, reactive metal
nanoparticles may also abstract fluoride from [BF4]– or [PF6]–
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(a)
(b)
(c)
(d)
Figure 4. Example EDX spectrum of 1.0 wt% Ni-NPs in [BMIm][BF4] from
[Ni(AMD)2]. See Fig. S2.4 for EDX spectrum in [BMIm][NTf2].
Figure 5. Example PXRD of 1.0 wt% Ni-NPs in [BMIm][BF4] from [Ni(AMD)2];
Ni hcp reference peaks in red, COD 9008509 and possible Ni fcc reference
peaks in blue, COD 9009862. See Figures in Supp. Info. for additional PXRDs.
Small shifts in PXRD patterns are not uncommon for nanoparticles. A number
of effects can be considered for such shifts including range of stoichiometric
composition, partly inhomogeneous element distribution, defects such as
stacking and twin faults and nanosized crystalline domains being much
smaller than the bulk reference material causing lattice contraction or
expansion and strain.[63,64]
It is evident from Table 1 that variation in the IL cation and anion
influences the particle size and crystal phase.[34,65] The average
diameter of the Ni-NPs from both precursors increases slightly
Figure 3. TEM images of 1.0 wt% Ni-NP/IL dispersions from [Ni(AMD)2] for
[LMIm]+ cations with (a) [BF4]–, (b) [PF6]– and (c) [NTf2]– anions. For image of
Ni-NP/IL dispersion from [Ni(COD)2] see Fig. S3.11. (d) TEM images of 1.0
wt% Ni-NP/IL dispersions from [Ni(AMD)2] (left)and [Ni(COD)2] (right) with
addition of 5.0 wt% KF to [BMIm][NTf2].
66 ]
with the IL anion volume[
in the series [BMIm][BF4],
[BMIm][PF6], and [BMIm][NTf2] (Fig. 6). It might be assumed
that fluoride ions formed from [BF4]– and [PF6]– could have been
responsible for preventing the NP growth. To check or disprove
this assumption we have added 5 wt% KF as a fluoride source
(soluble in IL) to the Ni-precursor/[BMIm][NTf2] dispersion before
starting the MW heating. A TEM analysis (Fig. 3d) of the obtained
Ni-NPs in the presence of KF showed Ni-NPs from [Ni(AMD)2]
with average diameter of 10 ± 2 nm, and from [Ni(COD)2] with
average diameter of 11 ± 2 nm. This size and size dispersion is
identical to the Ni-NPs formed from [Ni(AMD)2] (10 ± 2 nm) and
[Ni(COD)2] (11 ± 2 nm) in [BMIm][NTf2] without the addition of
KF (Table 1). So it can be assumed that the formation of F– from
[BF4]– and [PF6]– does not prevent the NP growth. A change in
imidazolium cation or rather in 1-n-alkyl chain length does not
give a similar trend. Only in the case of the [PF6]– anion and
[Ni(COD)2] as precursor there is a steady increase from 1-butyl
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over 1-octyl to 1-lauryl.
9008509).
(a)
Figure 6. Correlation between Ni-NP size and the molecular volume of the IL
anion[66] for [BMIm]+ cations from [Ni(AMD)2] (black) and [Ni(COD)2] (red line).
Errors bars indicate the standard deviation (cf. Table 1).
Influence of the IL-cation/anion size on nanoparticle diameters
67
]
were already reported before, e.g, for silver,[37] cobalt,[
(b)
rhodium,[42] and tungsten[41] where the NP diameter increased with
the IL anion volume (for [BF4]–, [PF6]–, triflate and [NTf2]–) with
[BMIm]+, [methyl-trioctyl-ammonium]+ and [1-butyl-3-methyl-
pyrrolidinium]+ as cation. Also, ruthenium nanoparticles from
[Ru(COD)(COT)]/H2 in various 1-alkyl-3-methyl-imidazolium
ionic liquids showed a relationship between the Ru-NP diameter
and the size of IL non-polar domains.[17] This suggested that the
size of the nanoparticles is governed by the degree of self-
organization of the imidazolium ionic liquid in which they are
generated, such that more structured ionic liquids, lead to smaller
metal particle size.[18] The dependence of nanoparticle size in
imidazolium-based ILs was also investigated for Au-NPs[ 68 , 69 ]
where the particles vary from 0.75 to 3.5 nm diameter for 1-ethyl-
3-methylimidazolium, 1-butyl-3-methylimidazolium and 1-methyl-
3-octylimidazolium cations and [BF4]– as counter ion.[68] Further,
decomposition of [Ni(COD)2] in 1-alkyl-3-methyl imidazolium
[NTf2]– ILs with different 1-alkyl chain lengths gave Ni-NPs with
an average diameter from 2.4 to 5.4 nm[49] or 4.9 to 5.9 nm [16] such
that the particle diameter increased with the cation size. A
correlation between the size of palladium nanoparticles and
different organic solvents was reported as well.[70]
(c)
Figure 7. TEM images of 1.0 wt% Ni-NP/IL dispersion from [Ni(AMD)2] for
[BPy]+ cations with (a) [PF6]– and (b,c) [NTf2]– anions, where the HR-TEM
image in (c) shows a well-oriented Ni (hcp) particle. For a further HR-TEM
image of single Ni-NP from [BuPy][NTf2] see Fig. S3.12 in Supp. Info.
Ni-NPs in thiophenium IL
Ni-NPs in pyridinium-based ILs
Ni-NPs obtained from [Ni(AMD)2] in the IL 1-octyl-thiophenium
hexafluorophosphate have an average particle diameter of 9 (±1)
nm which is in the range of imidazolium-based ILs. However, the
particles are strongly agglomerated (Fig. 8). Still, the crystallinity
based on the diffraction pattern of the interference levels can be
seen from the HR-TEM images. An additional PXRD which would
show the crystallinity of the particles could not be obtained even
after long measurement time possible because of amorphous
particles.
Ni-NPs from the decomposition of [Ni(AMD)2] and [Ni(COD)2] in
pyridinium-based ILs had an average diameter of 2-3 (±1) nm
which rendered them significantly smaller than the Ni-NPs
synthesized in imidazolium-based ILs (compare Fig. 7 and Fig. 1).
It can be assumed that the pyridinium cation leads to high
structuring through - self-organizing interactions,[71] as well as
to -anion interactions, which are very stable.[72] Apparently these
self-organizing interaction are retained to some extent even at 220
°C as we should have otherwise obtained larger nanoparticles as
from the imidazolium ILs.
The crystal structure of these small Ni-NPs had to be elucidated
by HR-TEM as no PXRD could be obtained. The distances
between the Ni atom columns in the high-resolution TEM image of
Fig. 7c correspond to those of the Ni(hcp) structure (COD
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powder form from the IL dispersion by precipitation with
acetonitrile, centrifugation, washing with acetonitrile and drying
under vacuum.
Figure 8. HR-TEM of 1.0 wt% Ni-NPs in IL from [Ni(AMD)2] in [OTh][PF6].
X-ray photoelectron spectroscopy (XPS) study of Ni-NPs in
[BPy][NTf2]
To determine the oxidation state of the nickel- nanoparticle sample
Ni/[BPy][NTf2], an X-ray photoelectron spectroscopic (XPS)
analysis was carried out. In a number of measurements, the binding
energy of the electron from the Ni2p3/2 orbital of the 2 (±1) nm
diameter Ni particles was shifted by about 0.3 eV to the higher
binding energy of EB = 853.2 eV compared to bulk Ni(0) (Fig. 9).
This is not typical for Ni-NPs or any other nickel compounds. An
instrumental error could be excluded by direct calibration with an
ultrapure nickel standard (99.99% Ni foil, polished and cleaned by
sputtering) with reference to argon Ar2p3/2 (Ar gas was used for
sputtering, added in traces as internal standard and could be found
physisorbed on the probe surface). We could also exclude an
influence from chemi- or physisorbed species as this shift in
binding energy was not seen for larger Ni(hcp) nanoparticles from
the same or related ILs.
Figure 10. Magnetization curves of an isolated Ni (2±1 nm) sample from
[BPy][NTf2] (black line) and an isolated Ni (10±4 nm) sample from [LMIm][NTf2]
(red line) at room temperature.
Table 2. Data obtained from the magnetization curves at room temperature.
a
a
b
c
d
sample
χini
χini
Ms
Mr/Ms
Hc
[A·m–1
]
[105 m3 kg–1
]
105/Ms
[A·m2·kg–1
]
[m A–1
0.11
]
Ni (2±1 nm)
sample from
[BPy][NTf2] e
Ni (10±4 nm)
sample from
[LMIm][NTf2] f
0.23
2.08
0.14
0.09
11500
8400
0.08
0.08
0.99
a χini = initial susceptibility; b Ms = saturation magnetization; c Mr/Ms =
remanence magnetization; d Hc = coercivity; e sample amount 0.97 mg; f
sample amount 0.4 mg.
Both samples show slightly hysteretic curves at room temperature.
On the time scale of the experiment (approximately 1 hour), no
oxidation is observed (the saturation magnetization is unchanged
for positive and negative branches, see Figure 10). The
characteristic values calculated on the base of the magnetization
curves are shown in Table 2. The saturation magnetization Ms of a
Ni (2±1 nm) sample from [BPy][NTf2] is 2.08 A·m2·kg–1, and of a
Ni (10±4 nm) sample from [LMIm][NTf2] it is 0.99 A·m2·kg–1.
These values are comparable with the values for hcp Ni found in
literature, varying from 0.82 A·m2·kg–1 for ~60 nm nanoparticles
[76]
to 7.4 A·m2·kg–1 for micrometer-sized clusters.[77] Remanence
Figure. 9 Shift (ΔEB) of the Ni2p3/2 binding energy in small Ni particles from
magnetization (Mr/Ms) and coercivity (Hc) values were calculated
as an average between the positive and the negative values. Both
samples show a narrow hysteresis (Hc < 15 kAm–1) with a
moderate relative remanence Mr/Ms ~ 0.1. This value is thus
clearly below the expected relative remanence for an ensemble of
magnetically blocked Stoner-Wohlfarth-like particles (Mr/Ms)SW =
0.5, but clearly deviating from pure superparamagnetic behavior
((Mr/Ms)sp = 0). For purely non-interacting Ni nanoparticles of the
present size regime, a superparamagnetic behavior would be
[BPy][NTf2] in the X-ray photoelectron spectrum.
In the literature, this shift in binding energy of the photoelectrons
to higher values is described for very small clusters.[73,74] For very
small silicon clusters (Si4 to Si720) Ganteför et al. could show a
cluster-size dependent shift in binding energy of the photoelectron
from the outer layers.[73,74] It could also be shown that the
electronic structure of a free cluster varies with every atom and that
also the position of an atom in the cluster has an influence on the
binding energy shift of the corresponding photoelectron spectrum.
78
]
expected above the blocking temperature.[
However, in
temperature-dependent measurements in the temperature range
between 150 and 300 K, performed on samples synthesized under
the same conditions, no indication for a magnetic transition in the
temperature range is identified. The blocking temperature is thus
expected to be below 150 K. Based on these observations,[78] the
Magnetic properties of Ni(hcp)-NPs
Magnetic measurements of two isolated nickel nanoparticle
samples from [BPy][NTf2] and from [LMIm][NTf2]) were carried
out for their magnetic properties.[75] The Ni NPs were isolated in
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slight hysteresis is ascribed to a significant contribution of particle
interaction to the quasi-static magnetic behavior, caused by the
strong magnetic moment inherent to single domain nickel
nanostructures.
The total average atom number NT of 383 is between the total
atom number of Nm = 309 for an icosahedron with m = 5 shells and
Nm = 561 for an icosahedron with m = 6 shells (Nm = (1/3)(2m –
1)(5m2 – 5m +3).[84] The number of surface atoms for a 5-shell
icosahedron is NS = 162 and for a 6-shell icosahedron NS = 252 (NS
= 10m2 – 20 m + 12).[84] In our rough approximation we take here
NS = 200 for the number of surface atoms for NT = 383. This gives
a fraction of exposed or surface atoms NS / NT ≈ 200 / 383 of 0.52
or 52% on average.
The total average atom number NT of 64000 is between the total
atom number of Nm = 62062 for an icosahedron with m = 27 shells
and Nm = 69354 for an icosahedron with m = 28 shells. The number
of surface atoms for a 27-shell icosahedron is NS = 6762 and for a
28-shell icosahedron NS = 7292. In our approximation we take here
NS = 6900 for the number of surface atoms for NT = 64000. This
gives a fraction of exposed or surface atoms NS / NT ≈ 6900 / 64000
of ~0.11 or 11% on average.
Thus, when the number or rather fraction of surface atoms is
taken into account the activity (TOF) of the larger 11 nm particles
from [BMIm][NTf2] is about four times higher than the activity of
the smaller 2 nm Ni particles from [BPy][NTf2]. This finding is
similar to results by Campbell et al. for Ru-NPs with different
nanoparticle size in [BMIm][NTf2] in the catalytic hydrogenation
of 1,3-cyclohexadiene and cyclohexene.[85]
Catalytic hydrogenation of alkenes with Ni nanoparticles
The hydrogenation of a C=C double bond to a C-C single bond is
an important reaction in industrial and synthetic organic chemistry.
Freshly synthesized Ni/IL nanoparticle dispersions (1.0 wt% in
metal) were tested for hydrogenation of 1-hexene and benzene in a
Büchi high-pressure, stainless-steel autoclave within IL phase
under organic-solvent-free conditions. In a typical catalytic
procedure, the high-pressure autoclave was charged with the Ni/IL
colloid dispersion (0.1 g) and the substrate (2.0 g). The reaction
mixture was heated to 100 °C. After reaching the reaction
temperature the autoclave was pressurized with 5 bar of H2. After a
selected time the reaction was stopped, cooled down and a 0.5 g
sample was analyzed for its content by GC and NMR.
Expectedly, the Ni/IL dispersion proved to be an active catalyst
for hydrogenation reactions.[40] The catalytic activity did not
significantly depend on the nanoparticle size. At first sight, the
activity of smaller Ni nanoparticles (Ni/[BPy][NTf2], 2±1) was
very similar (insignificantly higher) to the larger Ni nanoparticles
(Ni/[BMIm][NTf2], 11 (±2)).
The activity as turnover frequency (TOF) for metal nanoparticle
catalyst is often calculated on the number of moles of the metal
used. Obviously, only the surface atoms of the metal nanoparticle
can be catalytically active. Thus, an activity which is based only
the surface atoms can be one order of magnitude larger than the
TOF referenced to the total amount of metal. Further, as only a
fraction of the surface atoms will be catalytically active, such as
corners, edges or defect sites even the TOF based on surface atoms
will still underestimate of the true activity of the active sites.[ 79]
From TEM we obtained an average Ni-NP diameter of 2 nm for
Ni/[BPy][NTf2] (from both [Ni(AMD)2] and [Ni(COD)2]) and 11
nm for Ni/[BMIm][NTf2] (from [Ni(COD)2]). From these average
diameters D the total number of metal atoms (NT) in the
nanocrystal can be calculated according to eq. 1, 2 or 3:[79,80,81,82]
After catalysis the Ni nanoparticles had increased in size to
21±6 nm but were still separate nanoparticles (Fig. 11). EDX
showed nickel and a low content of oxygen.
(a)
4
퐷
푁 휌푉 and 푉 = 3 휋(2)3
(1)
퐴
푁푇
=
퐴푟
with NA = Avogadro’s number (6.022 · 1023 mol–1),  = metal
density, Ar = relative atom mass (in g/mol).[79]
푁푇 = 4휋 (2퐷푏)3 /3푉
(2)
푔
with b = 1.105 for closed packed (cp), face-centered cubic or
hexagonal-cp (fcc or hcp) crystallographic structures, Vg = volume
of a single metal atom according to Vg = (4/3)r3, r = atomic
(metal) radius. The constant b account for the free space between
the atom spheres in the metal packing.
퐷
3
푁푇 = (2푟푏
)
(3)
With the values for nickel ( = 8.90 g/cm3, Ar = 58.69 g/mol, b =
1.105 for Ni with hcp packing, Vg = 0.00810 nm3, r = 0.1246 nm)
eq. 1, 2 or 3 give NT = 383 atoms for Ni-NPs of 2 nm mean
diameter and NT = 63745, rounded to 64000, for the 11 nm Ni-
NPs.
(b)
Figure 11. (a) TEM, (b) EDX (averaged over 10 particles) of Ni/[BMIm][NTf2]
Under assumption that Ni-clusters have a icosahedral shell
structure [83] it is possible to calculate the number of surface atoms
(NS) from the number of shells m which can be derived from the
total number of atoms (NT).[84]
dispersion after catalytic hydrogenation of 1-hexene at 100 °C and 60 min.
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Table 3 Catalytic data for the hydrogenation of 1-hexene or benzene to hexane or cyclohexane, respectively, with Ni/IL dispersions.a
c
Ni-precursor,a
A = [Ni(AMD)2]
B = [Ni(COD)2]
Catalyst
(av. particle size/nm)
Substrate
t [h]
T [°C]
Conversion [%]
TOF b
TOFsurface
B
B
B
B
B
Ni/[BMIm][NTf2] (11)
Ni/[BMIm][NTf2] (11)
Ni/[BMIm][NTf2] (11)
Ni/[BMIm][NTf2] (11)
Ni/[BMIm][NTf2] (11)
benzene
benzene
benzene
1-hexene
1-hexene
2
10
60
1
100
100
100
100
60
15
31
91
93
89
113
47
23
1302
1246
1027
427
209
11836
11327
1
--
--
--
--
blind, [BMIm][NTf2]
blind, [BMIm][NTf2]
blind, [BPy][NTf2]
blind, [BPy][NTf2]
1-hexene
benzene
benzene
1-hexene
2
10
10
2
100
100
100
100
1.5
0.5
0
10
1
----
2
1
A
B
A
A
Ni/[BPy][NTf2] (2)
Ni/[BPy][NTf2] (2)
Ni/[BPy][NTf2] (2)
Ni/[BPy][NTf2] (2)
benzene
benzene
1-hexene
1-hexene
2
10
1
100
100
100
60
18
34
95
90
136
51
1330
1260
262
98
2560
2420
1
a In a typical catalytic test reaction 0.1g Ni/IL dispersion (1.0 wt% = 0.001 g in total metal, 17 µmol Ni) and 2.0 g of degassed dry substrate (benzene: 25.6 mmol,
1-hexene: 23.8 mmol) were stirred under 5 bar H2 at 60-100 °C for 1-60 h.
b TOF = [mol product x (mol Ni)–1 x h–1
]
c TOFsurface = [mol product x (mol Ni)–1 x h–1] / (NS / NT) with the fraction of surface atoms NS / NT = 0.11 for Ni/[BMIm][NTf2] and 0.52 for Ni/[BPy][NTf2] (see
estimation in text).
diisobutylaluminiumhydride (DIBAL-H) according to literature
]
procedures.[89
The ionic liquids were synthesized by reacting 1-
Conclusions
methylimidazole, tetrahydrothiophene or pyridine with 1-halo-
We describe here a simple, reproducible, and broadly applicable
alkanes to yield first the alkylated iminum halide salts which were
microwave-induced decomposition of nickel amidinate,
further reacted with HBF4, NH4PF6 or LiNTF2 to give the
[Ni(AMD)2], or [Ni(COD)2] for the synthesis of small and uniform
respective ionic liquid. The ILs were dried under high vacuum
(10−7 mbar) at 80 °C for several days. Quantitative anion exchange
Ni-NPs in different ionic liquids. The results nicely show the
advantages of metal-organic precursor concepts together with non-
and, thus, IL-anion purity was assessed to be mostly near or over
conventional ionic liquid solvents and microwave-assisted
99% from analysis by ion chromatography (Dionex ICS-1100, with
pyrolysis[ 86 ] for a controlled synthesis of rather small metal
IonPac® AS22, 4x250 mm column), e.g., [BMIm][BF4] 99.05%,
nanoparticles without additional surfactants and reducing agents
[BMIm][PF6] 98.87%, [BMIm][NTf2] 99.29%, [OMIm][PF6]
99.10%, [OMIm][NTf2] 99.21%, [BPy][PF6] 99.28%, [BPy][NTf2]
The Ni-NPs are mostly formed in the rarely occurring
99.75%, [LPy][PF6] 98.85%.
(e.g. hydrogen) for soft wet-chemical nanoparticle formation.
metastable hexagonal closed-packed, hcp structure. Magnetic
measurements of two Ni nanoparticle samples showed very similar
Thermogravimetric analysis (TGA) was carried out with a
Netzsch TG 209 F3 Tarsus, equipped with Al-crucible using a
heating rate of 5 K min–1 under inert atmosphere (N2).
magnetic properties.
The obtained Ni nanoparticles were active in the [BMIm][NTf2]
or [BPy][NTf2] IL dispersion for the catalytic alkene or benzene
hydrogenation with over 90% conversion.
Water content was measured by coulometric Karl Fischer
titration (ECH/Analytik Jena AQUA 40.00), e.g., [BMIm][BF4] <
10 ppm, [BMIm][PF6] < 30 ppm, [BMIm][NTf2] < 300 ppm,
[OMIm][PF6] < 10 ppm, [OMIm][NTf2] < 10 ppm, [BPy][PF6] <
Experimental Section
300 ppm, [BPy][NTf2] < 300 ppm, [LPy][PF6] < 20 ppm.
NMR analyses of selected ILs before (and after) 10 min MW
heating at a power of 100 W to an approximate temperature of 220
°C (see Figures 5.1-5.9 in Supp. Info. for the NMR assignment
compare ref. 90):
[BMIm][NTf2] (300.13 MHz, CD3CN): δ/ppm=8.39 (s, 1H), 7.37
(s, 1H), 7.34 (s, 1H), 4.21 (t, 3J=7.3 Hz, 2H), 3.81 (s, 3H), 1.80
(quintet, 3J=7.5, 2H), 1.32 (sextet, 3J=7.36, 2H), 0.94 ppm (t,
3J=7.4 Hz, 3H). – 13C-NMR (75 MHz, CD3CN): δ/ppm=13.6, 19.9,
32.5, 36.8, 50.3, 123.2, 124.6, 136.8.
[OMIm][NTf2] (300.13 MHz, CD3CN): δ/ppm=8.38 (s, 1H), 7.36
(d, 3J=3.6 Hz, 1H), 7.32 (d, 3J=3.6 Hz, 1H), 4.10 (t, 3J=7.10 Hz,
3H), 3.81 (s, 3H) 1.81 (m, 2H), 1.30 (m, 10H), 0.89 (t, 3J=6.45 Hz,
3H). – 13C-NMR (75 MHz, CD3CN): δ/ppm=14.3, 23.3, 26.6, 29.7,
32.4, 36.8, 123.2, 124.6, 136.7, 120.9 (q, 1J=318 Hz, CF3S).
All synthesis experiments were carried out with Schlenk
techniques under nitrogen or argon since the nickel amidinate is
hygroscopic and air sensitive. The solvents (acetone, acetonitrile,
n-hexane, toluene, tetrahydrofuran) were dried using the MBraun
solvent purification system. 1,3-Diisopropylcarbodiimide (>99%),
nickel(II)-chloride (> 99%), methyllithium, 1-chlorobutane
(> 99%), 1-chlorooctane (> 99%), 1-iodolaurate (> 99%), 1-
methylimidazole(> 99%), pyridine (> 99%), tetrahydrothiophene
(> 98%), were obtained from Sigma–Aldrich and used without
further purification.
[Ni{MeC(NiPr)2}2] ([Ni(AMD)2]) was synthesized by
deprotonation and methylation of 1,3-diisopropylcarbodiimide with
methyllithium. The resulting lithium amidinates were then reacted
with nickel(II) chloride according to literature procedures.[87 88
,
]
Ni(COD)2
was
synthesized
by
suspending
bis-
(acetylacetonato)nickel in THF and adding 1,5-cyclooctadiene and
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[LMIm][PF6] (300.13 MHz, CD3CN): δ/ppm= 8.36 (s, 1H), 7.35 (d, selection was achieved with a round aperture placed in the first
3J=3.4 Hz, 1H), 7.32 (d, 3J=3.4 Hz, 1H), 4.09 (t, 2H), 3.80 (s, 3H),
1.80 (m, 2H), 1.27 (m, 18H), 0.86 (t, 3J=7.4 Hz, 3H).
13C-NMR (75 MHz, CD3CN): δ/ppm=13.1, 22.1, 25.3, 28.8, 29.0,
31.31, 49.2, 121.94, 123.3, 135.7.
intermediate image plane with a corresponding diameter of 0.64
µm in the object plane. For each acquisition a sample region with a
significant amount of material was placed inside the aperture. The
objected was illuminated with wide-spread parallel beam obtaining
focussed diffraction patterns. The diffraction images were
calibrated with Debye-Scherrer patterns recorded from a Gold
reference sample (S106, Plano GmbH, Wetzlar, Germany). Several
diffraction patterns were recorded for each of the investigated Ni
samples and averaged after compensating for unavoidable small
diffraction shifts occurring between successive acquisitions. In
order to compare the recorded diffraction signal to reference
structure data, the signal from Debye-Scherrer pattern were
accumulated along the azimuthal direction.
Metal nanoparticle (M-NP) synthesis: In a typical reaction, the
nickel amidinate powders (58 mg, 0.17 mmol [Ni{MeC(NiPr)2}2]
or Ni(COD)2 (49 mg, 0.83 mmol) were dissolved or suspended (≈
12 h) under a nitrogen atmosphere at room temperature in 1.0 g of
dried and deoxygenated ionic liquid to give a 1.0 wt.ꢀ% total
metal/IL dispersion. For the decomposition, the mixture was placed
in a microwave (CEM, Discover) under an inert nitrogen
atmosphere and the conversion was finished within 10 min at a
microwave power of 50 W and a temperature reading of 220 °C.
Each decomposition reaction was carried out at least twice.
The five samples investigated by SAED were found to contain
Ni(hcp), NiO and NiF2 species in different amounts (cf. Fig. S2.10,
S2.17, S2.20, S2.24, S2.30). According to the calibration with the
gold sample, the measured Ni(hcp) lattice constant is decreased by
approximately 1% compared to bulk material,[ 92 ] whereas the
lattice constants of NiO and NiF2 are increased by about 2-3%
X-ray photoelectron spectroscopy, XPS-(ESCA-)measurement
was performed with a Fisons/VG Scientific ESCALAB 200X xp-
spectrometer, operating at room temperature, a pressure of 1.0·10–8
bar and a sample angle of 30°. Photoelectron spectra were recorded
using polychromatic Al-Kα excitation (14 kV, 20 mA) and an
emission angle of 0°. Calibration of the XPS was carried out by
recording spectra, using Al Kalpha X-rays, from clean samples of
copper, silver and gold, at 20 eV and 10 eV pass energies and
comparison with reference values.
,
]
when compared to the respective bulk parameters.[93 94
The magnetization curves were monitored at room temperature
with an ADE Magnetics Vibrating Sample Magnetometer EV7 up
to maximum field strength of 1600 kA m-1. A typical experiment
consists of a virgin curve, followed by a full hysteresis loop. The
Ni NPs were isolated in powder form from the 1.0 wt% IL
dispersion by precipitation with 2 mL acetonitrile, centrifugation,
washing twice with 2 mL acetonitrile each with intermittent
centrifugation and finally drying under vacuum. The isolated Ni
(2±1 nm) sample from [BPy][NTf2] was measured in a sealed
Teflon vessel, placed on a glass sample holder between two poles
of an electromagnet, and vibrated at a frequency of 75 Hz. The
isolated Ni (10±4 nm) sample from [LMIm][NTf2] was measured
under similar conditions in a sealed Teflon vessel in a nitrogen
flow. The raw data was corrected by subtraction of the signal of the
paramagnetic material. Saturation magnetization (Ms), magnetic
moment (m), remanence magnetization (Mr), coercive field (Hc)
and initial susceptibility (χini) were calculated. Remanence
magnetization (Mr) and coercive field (Hc) were calculated as an
average between the positive and the negative values.
Powder X-ray diffraction, PXRD data were measured at ambient
temperature on a Bruker D2 Phaser using a flat, low-background
sample holder with a dome for exclusion of air and moisture and
Cu-Kα radiation (λ = 1.54182 Å, 35 kV). Samples had been
precipitated with acetone from the NP/IL dispersion and washed
with acetonitrile. PXRDs were measured for 2-12 h.
GC/MS data were recorded on a Thermo Finnigan Trace DSQ;
ionization method: EI, 70 eV, 200°C.
NMR spectra were recorded on a Bruker Avance DRX 200 (1H,
300.13 MHz) and Avance DRX 500 (13C, 125.57 MHZ) at 298 K
in C6D6, CD3CN and CDCl3 and the chemical shifts are referenced
to the residual proton solvent peaks against TMS.
Temperature-dependent magnetization (M) measurements were
performed in temperature range from 150 K to 300 K at constant
field strength of 800 kA m–1. The lower temperature value was
limited by the liquid nitrogen cooling setup. A typical Zero Field
Cooled (ZFC) measurement consisted in cooling of the sample to a
minimum temperature in zero field, followed by applying the
magnetic field and slow heating at a rate of 1 K/min. In a Field
Cooled (FC) measurement the magnetic field was applied during
slow cooling of the sample (1 K/min). Samples were measured in
the same vessels, as during the room temperature measurements.
High-resolution transmission electron microscopy (HR-TEM)
micrographs were taken at room temperature on a Tecnai FEI G20
TEM operating at an accelerating voltage of 200 kV. Samples were
deposited on 200 µm carbon-coated gold grids. The size
distribution was calculated from a manual diameter determination
over a minimum of 50 isolated particles.
TEM with energy-dispersive X-ray spectrometry, TEM-EDX:
FEI Tecnai f20, 136 kV, exposure time for individual EDX spectra
3 min. EDX analysis always shows traces of oxygen. Preparation
on the grids was done in a glove-box. However, there is an
unavoidable 30 sec air contact upon placing the grid in the TEM
chamber before evacuation. A low intensity oxygen signal at 0.52
eV is ubiquitous and always present also on noble metal samples
and pure carbon grids (from surface oxidation).
Catalysis: A Büchi stainless-steel autoclave with a glass inlet
was charged with 0.1 g of a freshly synthesized Ni/IL dispersion
(1.0 wt% in total metal, 17 µmol Ni). 2 g of degassed, dry substrate
was added. For benzene the Ni : substrate ratio was 1:1505 and for
1-hexene the Ni : substrate ratio was 1:1400. The reaction mixture
was heated to 60-100 °C (see Table 3). After reaching the reaction
temperature the autoclave was pressurized with H2 to 5 bar (Büchi
pressflow gas controller, bpc) which was kept constant by the
Powder TED measurements: Selected area electron diffraction
(SAED) patterns have been recorded with an FEI Titan 80-300
]
TEM,[
operated at 300 kV accelerating voltage. The area
91
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10.1002/chem.201605251
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Büchi bpc. After the chosen time (1-60 h) the reaction was stopped,
cooled down and a 0.5 g sample was analyzed for its content by
GC/MS and NMR. Conversion and selectivity were determined by
Shimadzu GC2014 for Ni/IL samples from [Ni(COD)2] [column
Ultra2, crosslinked 5% PhMe silicone, 25 m x 0.2 mm x 11 µm] or
by PerkinElmer headspace GC 8500 [column HS-6B, FID
detector]. A 1H NMR spectrum was recorded by dissolving 0.1 mL
of the sample in 0.65 mL deuterated chloroform.
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Acknowledgements
Authors are thankful to the Deutsche Forschungsgemeinschaft
(DFG) for financial support in the priority project SPP 1708
through grant Ja466/31-1 and Fi502/32-1, and in the core-facilities
program through grant MA 1280/40-1.
[20] E. T. Silveira, A. P. Umpierre, L. M. Rossi, G. Machado, J. Morais, G. V.
Soares, I. J. R. Baumvol, S. R. Teixeira, R. F. P. Fichtner, J. Dupont,
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Keywords: nickel nanoparticles · ionic liquids · soft wet-chemical
synthesis · hexagonal close packing, hcp · nickel amidinate
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