Metal(II) Formates (M = Fe, Co, Ni, and Cu) Stabilized by Tetramethylethylenediamine (tmeda): Convenient Molecular Precursors for the Synthesis of Supported Nanoparticles

Article abstract of DOI:10.1002/hlca.201800227

γ-Alumina supported 3d transition-metal nanoparticles are commonly used catalysts for several industrial reactions, such as Fischer-Tropsch, reforming, methanation, and hydrogenation reactions. However, the activity of such catalyst is often limited by the low metal dispersion and a high content of irreducible metal, inherent to the conventional preparation methods in aqueous phase. In this context, we have recently shown that [{Ni(μ²-OCHO)(OCHO)(tmeda)}₂(μ²-OH₂)] (tmeda=tetramethylethylenediamine) is a suitable molecular precursor for the formation of 1–2 nm large nanoparticles onto alumina. Here, we explore the synthesis of the corresponding Fe, Co, and Cu molecular precursors, namely [{Fe(μ²-OCHO)(OCHO)(tmeda)}₄], [{Co(μ²-OCHO)(OCHO)(tmeda)}₂(μ²-OH₂)], [Cu(κ²-OCHO)₂(tmeda)], which are, like the Ni precursor, soluble in a range of solvents, rendering them convenient metal precursors for the preparation of supported metallic nanoparticles on γ-alumina. Using a specific adsorption of the molecular precursor on γ-alumina in a suitable organic solvent, treatment under H₂ provides small and narrowly distributed Fe (2.5±0.9 nm), Co (3.0±1.2 nm), Ni (1.7±0.5 nm), and Cu (2.1±1.5 nm) nanoparticles. XAS shows that the proportion of MAl₂O₄ (M = Co, Ni, Cu) is small, thus illustrating the advantage of using these tailor-made molecular precursors.

Full text of DOI:10.1002/hlca.201800227

DOI: 10.1002/hlca.201800227
FULL PAPER
Metal(II) Formates (M =Fe, Co, Ni, and Cu) Stabilized by
Tetramethylethylenediamine (tmeda): Convenient Molecular
Precursors for the Synthesis of Supported Nanoparticles
Tigran Margossian,a Kim Larmier,a Florian Allouche,a Ka Wing Chan,a and Christophe Copéret*a
a
Department of Chemistry and Applied Biosciences, ETH Zurich Vladimir Prelog Weg 1–5, CH-8093 Zurich,
Switzerland, e-mail: ccoperet@ethz.ch
γ-Alumina supported 3d transition-metal nanoparticles are commonly used catalysts for several industrial
reactions, such as Fischer-Tropsch, reforming, methanation, and hydrogenation reactions. However, the activity of
such catalyst is often limited by the low metal dispersion and a high content of irreducible metal, inherent to the
conventional preparation methods in aqueous phase. In this context, we have recently shown that [{Ni(μ2-OCHO)
(OCHO)(tmeda)}2(μ2-OH2)] (tmeda=tetramethylethylenediamine) is a suitable molecular precursor for the
formation of 1–2 nm large nanoparticles onto alumina. Here, we explore the synthesis of the corresponding Fe,
Co, and Cu molecular precursors, namely [{Fe(μ2-OCHO)(OCHO)(tmeda)}4], [{Co(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2
)], [Cu(в2-OCHO)2(tmeda)], which are, like the Ni precursor, soluble in a range of solvents, rendering them
convenient metal precursors for the preparation of supported metallic nanoparticles on γ-alumina. Using a
specific adsorption of the molecular precursor on γ-alumina in a suitable organic solvent, treatment under H2
provides small and narrowly distributed Fe (2.5�0.9 nm), Co (3.0�1.2 nm), Ni (1.7�0.5 nm), and Cu (2.1�
1.5 nm) nanoparticles. XAS shows that the proportion of MAl2O4 (M =Co, Ni, Cu) is small, thus illustrating the
advantage of using these tailor-made molecular precursors.
Keywords: Metal formate, tetramethylethylenediamine, specific adsorption, metal(0) nanoparticles.
Introduction
formation of hardly reducible metal-aluminum mixed
Metal supported nanoparticles are the most common oxides in the case of alumina.[25–27] The reduction of
heterogeneous catalysts used in industry.[1–3] They are such metal incorporated into the support during the
used in a broad range of applications, such as the impregnation phase requires the use of high temper-
methane
conversion
processes,[4–9]
biomass ature treatment under reductive atmosphere, leading
transformation,[10,11] or fine chemical synthesis.[12,13] typically to severe sintering of the particles, with
Numerous parameters influence the performances of nanoparticle size possibly exceeding 10 nm for Cu, Ni,
these catalysts, such as the type of support,[14,15] the and Co, up to 80 nm for Fe.[23,28–32] Several approaches
metal nanoparticles size,[16,17] and their interaction have been undertaken to avoid these problems, for
with the support.[18–21] Decreasing the size of those instance, by modifying conventional preparation
supported nanoparticles allows the enhancement of methods by either complexing the initial molecular
catalyst performances.[7,16,18,22–24] While small particles precursor with a chelating ligand,[33–37] or by adding
and good dispersion can be obtained on various precious metal to the material.[17,38,39] This allows for a
supports, a significant fraction of metal can be lost drop of the reduction temperature and hence the
through its incorporation into the support through the decrease of particle size. We have recently introduced
a Ni formate – [{Ni(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)]
([Ni]) (tmeda=tetramethylethylenediamine) – as a
convenient molecular precursor for the synthesis of 1–
2 nm Ni(0) on a broad range of supports through a
Supporting information for this article is available on the
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Helv. Chim. Acta 2019, 102, e1800227
specific adsorption method in organic solvent fol-
lowed by a reduction step under H2.[7,18] We reasoned
that it would be valuable to broaden this approach to
other 3d transition metals, namely Fe, Co, and Cu,
because of their relatively low cost and use in
numerous applications.[40–44] Here, we describe the
synthesis of metal formate [{Fe(μ2-OCHO)(OCHO)(tme-
Table 1. Summary of the Bond Distances [Å] from X-ray
crystallography of the different complexes.
[Fe]
[Co]
[Ni]
[Cu]
M2+ radius[50]
M1À O1
0.92[a]
0.89[a]
0.83
0.87
2.095(2)
2.171(1)
2.295(2)
2.275(2)
2.075(2)
2.150(1)
2.192(2)
2.207(2)
2.049(2)
2.099(1)
2.168(2)
2.148(2)
2.499(2)
1.992(1)
2.028(2)
2.028(2)
M1À O2
M1À N1
da)}4] ([Fe]), [{Co(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)] M1À N2
([Co]), and [Cu(η2-OCHO)2(tmeda)] ([Cu]) as precursors
for the synthesis of the supported nanoparticles, here
using alumina as a prototypical support, commonly
used in industry[45–47] that typically suffers from
[a]High spin ionic radius, evidenced by UV/Vis for [Co] (Figure 3).
incorporation of the metal ions into its structure, À OH (except for [Fe]), using toluene for [Co] and [Ni]
hence the need of high temperature of reduction to versus acetonitrile for [Fe] and [Cu] in order to take
generate metal nanoparticles.
into account their specific solubility (Table S1). After
washing the excess of complex and drying, the metal
loading of the different specifically adsorbed precur-
sors was measured by elemental analysis. The metal
loading of Fe/Al2O3 is 0.82 wt-% for a measured
Results and Discussion
Fe(II), Co(II), and Cu(II) formate compounds – [Fe], surface area of alumina of 32 m2 gÀ 1, corresponding to
[Co], [Cu] – were prepared from readily available 2.9 metalnmÀ 2 for Fe/Al2O3. As shown in Figure 2, the
metal precursors through the same strategy as metal loading decreases for the other metals with
previously reported for [Ni] using tmeda[7] as a 2.4 metalnmÀ 2 for Co/Al2O3, 1.8 metalnmÀ 2 for Ni/
stabilizing ligand (Scheme 1). The yields are 34% and Al2O3 and 1 metalnmÀ 2 for Cu/Al2O3. It is noteworthy
75%, for [{Co(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)] that the precursor loading expressed in number of
([Co]) and [Cu(η2-OCHO)2(tmeda)]) ([Cu]), respectively, precursor per nm2 is similar for all system, close to
while [Fe], a sensitive compound prepared under inert unity with 0.7 [Fe]nmÀ 2, 1.2 [Co]nmÀ 2, 0.9 [Ni]nmÀ 2,
conditions, is obtained in 33% yield. All syntheses can and 1.0 [Cu]nmÀ 2, in line with approximately
be carried out on a multi-gram scale to give pure 2 OHnmÀ 2 per precursor (Figure 2, red columns).[51]
crystalline complex soluble in a range of solvent from
The structure of the different adsorbed species onto
water to toluene (Table S1). While they all display γ-alumina was thus investigated by a combination of
similar structural features, in particular a +2 oxidation spectroscopy methods (IR, UV/Vis, and X-ray absorption
state and the presence of octahedral metal sites, they spectroscopy (XAS)). The IR spectra of the different
differ by their nuclearity: mono and tetra-nuclear for specifically adsorbed complexes onto Al2O3-500 recorded
[Cu] and [Fe] versus dinuclear for [Co] as found for the under argon are shown in Figure S3. We observe that the
previously reported [Ni] complexes. The tmeda ligand specific adsorption of the different precursors induces a
is connected in the same η2 mode for the four shift of the AlÀ OH bands to lower wavenumbers and to
different structures. The M1À N1(or M1À N2) bond dis- the appearance of C(sp2)À H, C(sp3)À H, and CÀ O bands
tances decrease from 2.295(2) Å for [Fe] to 2.192(2) Å associated with the formate and tmeda ligands, with
for [Co], 2.168(2) Å for [Ni], and 2.028(2) Å for [Cu] bands at 2980 cmÀ 1, 2849 cmÀ 1, 2731 cmÀ 1, 1610 cmÀ 1,
following the decrease size in metal ionic radius for and 1470 cmÀ 1. According to XAS, the edge energy
the Fe2+, Co2+, and Ni2+ (Table 1).[48] The N1À M1À N2 decreases from 7120.0 eV for [Fe] to 7112.0 eV for Fe/
°
°
angle also increases from 80.04(8) to 87.13(8) on Al2O3 (measured from the first inflection point of the
going from [Fe] to [Cu]. Overall, their solid-state spectra), which indicates reduction of Fe centers. How-
structure follows the size of the metal: with Fe2+, the ever, obtaining information on the coordination environ-
larger cation of the series, the compound adopts a ment was not possible due to poor data quality of the
tetrameric structure (Figure 1,a), while the structures extended X-ray absorption fine structure (EXAFS). Linear
are dimeric or monomeric with smaller M2+ ions combination fitting of the Fe/Al2O3 XANES spectra shows
(M =Co, Ni, and Cu) as shown in Figure 1,b–1,d.
45% of [Fe] (Fe2+), 34% of Fe2O3 (Fe3+), and 21% of Fe
Specific adsorption of the different precursors onto foil (Fe0, Figure S4). This complex mixture of Fe species
dehydroxylated γ-alumina was conducted with an lead us to hypothesize that [Fe] partially dispropor-
equimolar quantity of precursor with respect to surface tionate to iron (0) metal and iron oxide (III) upon contact
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Figure 1. Crystal structure and synthesis path of complex a) [Fe], b) [Co], c) [Ni], and d) [Cu]. Thermal ellipsoids were set at 50%
probability level and except for μ-H2O for complex [Co] and [Ni], hydrogen atoms and the co-crystallized water molecule are
omitted for clarity.[49]
coordination of [Co] onto the surface hydroxy groups
of alumina as previously observed for Ni.[7,25,35] While a
change of oxidation state and/or of the geometry
could also explain this observation,[36,52] the XANES
spectra of [Co] and Co/Al2O3 (Figure S5) show similar
edge energies (7719.8 eV and 7720.3 eV, respectively),
consistent with the conservation of the Co(II) oxidation
state upon dispersion in this case. Furthermore, the
absence of a pre-edge feature in both spectra
indicates that Co remains octahedral upon adsorption.
EXAFS spectra and fits for [Co] and Co/Al2O3 are
shown in Figure S6. In the case of the molecular
precursor, the fit includes scattering paths of four
closest oxygen atoms, two nitrogen atoms, nine
carbon atoms from the second sphere, and the other
cobalt center. The bond distances obtained from the
Figure 2. Metal uptake (right column, black) onto Al2O3-500 and
precursor uptake (left column, red) onto Al2O3-500, which
corresponds to the metal loading divided by the number of
metal atoms per molecular complex.
onto the alumina surface according to an equation of fit of [Co] are consistent with its X-ray structure
the type: 3 Fe2+!Fe0 +2 Fe3+
.
(Table S2 and Table 1). The best fit of Co/Al2O3 is
The adsorption of [Co] onto alumina surface was obtained by keeping the octahedral geometry of the
studied with a similar approach. The UV/Vis spectrum initial precursor and by adding one CoÀ Al scattering
of the [Co]-complex shows a unique band at 510 nm path with two aluminum neighbors accounts for two
that can be assigned to the d-d transition of an features at 2.86 Å, consistent with a surface coordina-
octahedral Co2+(d7).[36] However, Co/Al2O3 shows one tion complex. A cobalt-cobalt path allows to fit the
red-shifted bands at 558 (Figure 3,a) by comparison to feature centered at 3.61 Å (Figures S6–S8). This sug-
what is observed for [Co]. This is consistent with the
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Figure 3. Background subtracted UV/Vis spectra of materials a) Co/Al2O3 and [Co] in toluene solution, b) Ni/Al2O3 and [Ni] in
toluene solution, c) Cu/Al2O3 and [Cu] in acetonitrile solution.
Figure 4. Surface coordination complex for a) Fe/Al2O3, b) Co/Al2O3, c) Ni/Al2O3, and d) Cu/Al2O3.
gests that [Co] retains its dimeric structure at the Cu700 (Figure 5,d). Smaller and narrower particle size
surface of alumina (Figure 4).
distribution can also be achieved in the case of copper
The UV/Vis spectrum of (Cu/Al2O3) shares similar- when decreasing the reduction temperature down to
°
ities with that of [Cu] (Figure 3,c), showing bands at 500 C with an average size of 2.1�1.5 nm (Cu500
,
270 nm and 650 nm that can be assigned to charge Figure S14).
transfer and d-d of a distorted octahedral Cu2+(d9).[53]
Depending on the metal precursor specifically
The d-d band of Cu/Al2O3 is red-shifted in comparison adsorbed on the surface, formation and size of the
to the [Cu]-complex, which is ascribed to a weaker particles are influenced by the reduction temperature
field ligand environment at the metal such as and hence, the reducibility of the different systems
Al2O3.[7,54] Despite the [Cu]-complex and Cu/Al2O3 was estimated by XANES analysis at the respective
°
cannot be studied by XAS due to the reduction of Cu metal edge after reduction of the samples at 700 C
centers under the X-Ray beam, [Cu] likely retains its (Figure 6). The XANES spectra of Co700, Ni700, and
octahedral structure after adsorption on alumina sur- Cu700 (Figure 6,a, 6,b, and 6,c) are very similar to that
face, based on the UV/Vis analysis.
of their respective metal foil reference, displaying
When the different metal formate complexes are similar pre-edge and edge positions. In contrast, the
specifically adsorbed onto γ-alumina surface, they XANES spectrum of Fe700 contains a white line with a
retain their octahedral conformation for [Co] and [Cu] normalized adsorption value that is 50% higher than
as previously observed[7] and re-confirmed for [Ni] by that of the Fe foil reference, indicative of the presence
UV/Vis spectroscopy (Figure 3). In addition, like for Ni, of oxidized iron species. Linear combination fitting of
the Co complex retains its dimeric structure upon the XANES data for Fe700 using the Fe foil, FeAl2O4,
chemisorption onto γ-alumina surface (Figure 4,a and and Fe2O3 references (Figure 6,a) showing that only
°
4,b).
40% of total iron was reduced at 700 C under an H2
°
Following the specific adsorption of the different flow. While reducing at 900 C would likely help to
metal precursor onto the alumina surface, reduction at fully reduce the oxidized iron, it has a detrimental
°
700 C in hydrogen flow yields nanoparticles with effect on particle size, which increases from 2.5 to
particle size distribution of 2.5�0.8 nm for Fe700 7.8 nm. LCF analysis of the XANES data for the three
°
(Figure 5,a), 3.0�1.2 nm for Co700 (Figure 5,b), of 1.7� other systems reduced at 700 C shows that the
0.5 nm for Ni700 (Figure 5,c), and of 4.5�2.2 nm for metalð0Þ content increases, while moving from left to
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Figure 5. Particle distribution function of the particle size associated to the TEM pictures for the alumina-based catalyst.
Figure 6. XANES spectrum for the different reduced samples. a) Fe K-edge: Fe700, FeAl2O4, Fe2O3, and Fe0 reference, b) Co K-edge:
Co700, CoAl2O4, and Co0 reference, c) Ni K-edge: Ni700, NiAl2O4, and Ni0 reference, d) Cu K-edge: Cu700, CuAl2O4, and Cu0 reference.
right in the periodic table with 80% for Co700, 92% for
Conclusions
Ni700, and 98% for Cu700. This is directly in line with
the Temperature Programmed Reduction study (Fig- We have developed the synthesis of 3d metal(II)
°
ure S17). After reduction at 700 C, the proportion of formate precursors that are convenient for the
fully reduced metal increases from iron to copper preparation of small supported nanoparticles using
(Figure 7 and Table 2).
γ-alumina as a prototypical support. The synthesis of
the metal formate complexes can be easily carried
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out on gram scale. Upon specific adsorption of these temperature to generate Fe(0) nanoparticles at the
different metal(II) complexes onto dehydroxylated expense of suffering from particle sintering.
alumina, the metal centers retain their coordination
environmental (octahedral) and nuclearity, and this
approach provides access to high metal loading of Experimental Section
approximately one precursor nmÀ 2 for all systems.
General Procedure
After treatment under hydrogen flow at high tem-
peratures, small and narrowly distributed nano- Iron powder (99%), nickel carbonate (99%), copper
particles are formed: iron (2.5�0.8 nm), cobalt formate (99%), and formic acid (99%) were purchased
(3.0�1.2 nm), nickel (1.7�0.5 nm), and Cu (2.1� from Sigma-Aldrich and Alfa Aesar. Co and Cu
1.5 nm), but with their different specificities that complexes were prepared without particular precau-
follows the reducibility of the metal: 1) the more tions, while the synthesis of the Fe complex was
easily reducible Cu provides Cu(0) nanoparticles at performed using Schlenk technique due to the
500 C, which increases in size upon increasing the sensitivity of Fe(OCHO)2 ·2 H2O towards O2.[55] De-ion-
°
temperature of thermal treatment, 2) then Ni ized water was purified using a Purilab instrument (>
provides small Ni(0) particles in a broader range of 10 MΩcm). Deionized water and formic acid solutions
temperature along with some residual Ni(II) sites, 3) were degassed for 2 h under flow of argon. Extra dry-
Co follows with an increasing amount of irreducible acetonitrile™ (99.9%) was purchased from Acros.
Co(II) sites, and 4) finally Fe, the more difficult metal Toluene was purified using double MBraun SPS
to reduce in the series, requires much higher alumina columns and degassed by argon-bubbling for
at least 15 min prior to use. The solvents were stored
over molecular sieves. Specific adsorptions were
carried out using a Schlenk line with Ar (grade 4.5) and
10À 3 mbar vacuum.
Support Preparation and Characterization
Alumina nanospheres (NanoSphere) were purchased
from Alfa Aesar, NanoDur® with a particle size ranging
from 40 nm to 50 nm. The powder XRD pattern shows
the presence of a γ-alumina crystalline phase (Fig-
ure S2). The BET surface area was estimated at
32 m2 gÀ 1 for NanoSphere material. Prior to use, 6 gÀ o1f
°
°
powder was calcined at 500 C for 12 h (5 Cmin )
under synthetic air flow (100 mLminÀ 1), degassed
under high vacuum (10À 5 mbar) at room temperature
for 15 min and stored in a glovebox. As previously
°
Figure 7. Metal(0) content [%] at 700 C for the different
samples determined from LCF of the XANES spectra at the
respective metal edge.
Table 2. Characterization of the as prepared supported nanoparticle catalysts. The alumina supports were dehydroxylated at
°
500 C.
Precursor
Solvent
Impregnated
material
Reduced
Metal[b]
(wt-%)
Metal density
Particle size[c]
[nm]
XANES LCF analysis[d] [%]
material[a]
[MetalnmÀ 2
]
M(0)
MAl2O4
[Fe]
[Fe]
[Co]
[Ni]
[Cu]
[Cu]
CH3CN
CH3CN
Toluene
Toluene
CH3CN
CH3CN
Fe/Al2O3
Fe/Al2O3
Co/Al2O3
Ni/Al2O3
Cu/Al2O3
Cu/Al2O3
Fe700
Fe900
Co700
Ni700
Cu500
Cu700
0.82
0.82
0.80
0.58
0.34
0.34
2.9
2.9
2.4
1.8
1
2.5�0.9
7.8�3.0
3.0�1.2
1.7�0.5
2.1�1.5
4.5�2.2
40
20(40)[e]
n.d.[f]
80
n.d.
20
8
n.d.
2
92
n.d.
98
1
[a] Fe700 correspond to Fe/Al2O3 reduced at 700 C. [b] Metal loading determined by elemental analysis. [c] Measured by HAADF/STEM.
°
[d] Determined by Linear Combination Fitting of the XANES spectrum using the corresponding spinel (MAl2O4) and foil references; [e]
[f]
for Fe700 LCF, an additional Fe2O3 reference was used with (value). n.d., not determined.
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reported by Wischert et al., Alumina dehydroxylated at water (100 mL). The mixture was stirred under reflux
2 [51]
°
500 C has a OH surface concentration of 2 pernm .
for 4 h, affording a pink solution. The solution was
taken to dryness, treated with absolute ethanol
(100 mL), and dried under vacuum (10À 2 mbar) to yield
Physisorption of N2
Co(OCHO)2 ·2 H2O
(7 g,
86%,
37 mmol).[57]
°
Nitrogen isotherms at À 196 C were measured on a Tetramethylethylenediamine (7.6 mL, 50 mmol) was
Bel-Mini apparatus from Bel-Japan. Before measure- added to a clear solution of Co(OCHO)2 ·2 H2O in
ment, the samples were outgassed under vacuum (ca. deionized water (4.0 g, 22 mmol, 100 mL), leading to a
[56]
10À 3 mbar) at 350 C for 2 h. The BET method
applied to calculate the total surface area.
was pink solution. The mixture turned dark blue while a
flocculent precipitate appeared, before finally turning
red-brown. The mixture was taken to dryness and
extracted with toluene (150 mL). Concentration of the
°
X-Ray Diffraction (XRD)
°
filtrate and crystallization at À 38 C yielded large pink-
The XRD measurements were performed using a purple crystals of [Co] (1.5 g, yield 24.3%) suitable for
Bruker-AXS ‘D8 Advance’ diffractometer using Cu Kα X-ray crystallography. CCDC-1834517 contains the
monochromatic radiation (λ=1.5418 Å). The XRD supplementary crystallographic data. Two successive
°
patterns were recorded between 10–80 (2θ with a recrystallization steps from toluene lead to the for-
°
0.033 step). The samples were analyzed in the form of mation of 2.1 g (yield 34%, 7.5 mmol). Coordinating
a finely crushed powder.
solvents such as tetrahydrofuran and diethyl ether
decoordinates the tetramethylethylenediamine from
the metal center. IR (KBr): 3015, 2981, 2888, 2843,
2818, 2800, 2781, 2733, 2697, 1645, 1563, 1465, 1366.
Precursor Complexes Preparation
The [{Ni(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)] precursor Anal. calc. for C16H38Co2N4O9 (548.36): C 35.04, H 6.98,
[Ni] was prepared as previously reported.[7]
N 10.22; found: C 34.95, H 6.73, N 9.93.
[{Fe(μ2-OCHO)(OCHO)(tmeda)}4] ([Fe]). To the iron
[Cu(η2-OCHO)2(tmeda)] ([Cu]). Tetramethylethyl-
powder (2 g, 0.036 mol) was added in 85 mL of enediamine (1.4 mL, 9.2 mmol) was added to a 30-mL
aqueous formic acid (1.55 mol/L, 0.13 mol). After blue solution of Cu(OCHO)2 ·4 H2O in deionized water
stirring the mixture under reflux for 2 h, it was filtered (0.98 g, 4.3 mmol), leading to a dark blue solution. The
to afford a light green solution to which formic acid mixture was taken to dryness and extracted with
(5 mL, 0.13 mol) was added. The mixture was taken to acetonitrile (30 mL). Concentration of the filtrate and
°
dryness under vacuum to afford Fe(OCHO)2 ·2 H2O crystallization at À 38 C yielded large blue crystals of
(5.25 g, 80%, 37 mmol).[55] Tetramethylethylenedi- [Cu] (0.85 g, 3.2 mmol, yield 75%) suitable for X-ray
amine (1.82 mL, 12 mmol) was added to a clear crystallography. CCDC-1834516 contains the supple-
solution of Fe(OCHO)2 ·2 H2O (1 g, 5.5 mmol) in abso- mentary crystallographic data. IR (KBr): 3157, 3019,
lute ethanol (30 mL), leading to a color change to 3004, 2984, 2929, 2855, 2723, 2616, 1590, 1563, 1461,
yellow. The mixture was stirred for 5 min, before being 1368. Anal. calc. for C8H18CuN2O4 (269.78): C 35.62, H
taken to dryness. It was extracted with toluene 6.72, N 10.38; found: C 35.34, H 6.56, N 10.11.
ð3 � 10 mLÞ, concentrated to reach saturation of the
°
solution and crystallized at À 38 C to afford large light
Specific Adsorption of [Fe], [Co], [Ni], and [Cu] onto
yellow crystals of [{Fe(μ2-OCHO)(OCHO)(tmeda)}4]
Alumina
(0.3 g, 1.2 mmol, yield 22%) suitable for X-ray crystal-
lography. CCDC-1834512 contains the supplementary [{Fe(μ2-OCHO)(OCHO)(tmeda)}4]/Al2O3-500 (Fe/Al2O3). The
crystallographic data. Two successive recrystallization compound [Fe] (120 mg, 0.12 mmol, 0.3 equiv.) was
steps in toluene lead to the formation of 0.45 g (yield dissolved in acetonitrile (40 mL) and contacted with
33%, 1.8 mmol). Anal. calc. for C28H68Fe4N8O16 2.0 g of NanoSphere Al2O3-500 (0.36 mmol AlOH,
(996.26): C 33.76, H 6.88, N 11.25; found: C 33.60, H 1 equiv.). Upon reaction, a gas release was observed in
6.92, N 10.95.
the solution. The mixture was gently stirred for 2 h at
room temperature. The solid was washed three times
[{Co(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)] ([Co]). with 20 mL of acetonitrile and dried under vacuum
Formic acid (10 mL, 277 mmol) was added to a (10À 5 mbar) for 16 h to yield a white solid. IR: 3510,
solution of Co(CO3) (5.1 g, 43 mmol) in deionized 3013, 2976, 2876, 2845, 2805, 2735, 1665, 1368.
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Elemental analysis: Fe 0.82, C 0.60, H 0.18, N<0.2, nanoparticles and the number in subscript to the
which corresponds to the molar ratio of N/Fe>1 and reduction temperature.
C/Fe=3.4.
Scanning Transmission Electron Microscopy (TEM/STEM)
Study
[{Co(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)]/Al2O3-500
(Co/Al2O3). The compound [Co] (172 mg, 0.3 mmol,
0.83 equiv.) was dissolved in toluene (20 mL) and Prior to TEM analysis, all samples were oxidized by the
contacted with 2.0 g of NanoSphere Al2O3-500 slow diffusion of air to the catalysts under argon. After
(0.36 mmol AlOH, 1 equiv.). The mixture was gently oxidation, the materials were dispersed in ethanol and
stirred for 2 h at room temperature. The solid was a droplet of the suspension was deposited on a lacey
washed three times with 20 mL toluene and dried carbon foil supported on a copper grid. Scanning
under vacuum (10À 5 mbar) for 16 h to yield a magenta transmission electron microscopy images were re-
solid. FT-IR: 3556, 3016, 2980, 2892, 2848, 2732, 1643, corded on an aberration-corrected Hitachi HD2700CS
1610, 1470, 1370. Elemental analysis: Co 0.80, C 0.72, H microscope with a high-angle annular dark field
0.18, N 0.26, with molar ratio of N/Co=1.4 and C/Co= detector (HAADF-STEM). Particle size distribution (PSD)
4.4.
was determined by analyzing 100 nanoparticles and
fitting a normal distribution (particle size=mean value
[{Ni(μ2-OCHO)(OCHO)(tmeda)}2(μ2-OH2)]/Al2O3-500 (Ni/ � standard deviation). X-Ray spectra were measured
Al2O3). The compound [Ni] (189 mg, 0.35 mmol, with an energy-dispersive detector (EDX) attached to
0.97 equiv.) was dissolved in toluene (20 mL) and the HD2700CS. Particle size distributions and EDX
contacted with 2.0 g of NanoSphere Al2O3-500 spectra are shown in the supplementary information
(0.36 mmol AlOH, 1 equiv.). The mixture was gently (Figures S9–S15). The metal nanoparticles appear
stirred for 2 h at room temperature. The solid was bright in the HAADF-STEM images as Ni has the
washed three times with 20 mL of toluene and dried highest atomic number (Z contrast) among the
under vacuum (10À 5 mbar) for 16 h to yield a light elements present, which was also confirmed by EDX
green solid. FT-IR: 3669, 3548, 2980, 2881, 2849, 2731, spectroscopy.
1662, 1610, 1470, 1394, 1368. Elemental analysis: Ni
0.58, C 0.70, H 0.19, N 0.26, with molar ratio of N/Ni=
1.7 and C/Ni=5.6.
UV/Vis Spectroscopy
UV/Visible spectra of the metal salt solutions were
[Cu(η2-OCHO)2(tmeda)]/Al2O3-500 (Cu/Al2O3). The recorded with a resolution of 1 nm in the transmission
compound [Cu] (100 mg, 0.38 mmol, 1.05 equiv.) was mode on an Agilent Technologies, Cary Series using
dissolved in acetonitrile (20 mL) and contacted with the corresponding solvent as a reference. UV/Visible
2.0 g NanoSphere of Al2O3–500 (0.36 mmol AlOH, spectra of the solids were recorded in the reflectance
1 equiv.). The mixture was gently stirred for 2 h at mode (1-nm resolution) on the same spectrometer,
room temperature. The solid was washed three times using KBr as a reference. The spectra were collected in
with 20 mL of acetonitrile and dried under vacuum an air-free cell which was loaded inside the glovebox.
(10–5 mbar) for 16 h to yield a light green solid. FT-IR: The diffuse reflectance spectra were submitted to the
3543, 3018, 2979, 2899, 2854,1687, 1636, 1608, 1471. Kubelka-Munk transform.
Elemental analysis: Cu 0.36, C 0.43, H 0.18, N<0.2,
with molar ratio of N/Cu<2 and C/Cu=6.
X-Ray Absorption Spectroscopy (XAS)
XAS experiments were performed at the Swiss-Norwe-
Reduction of the Impregnated Materials to Give
gian Beamlines (SNBL, BM31) at the European Synchro-
Supported Nanoparticles
tron Radiation Facility (ESRF). XAS spectra were
The dried samples were reduced under a flow of pure collected at the Fe, Co, Ni, and Cu K-edges using a
À 1
°
°
°
°
hydrogen at 500 C, 700 C, 900 C (1 Cmin ) for 8 h. double-crystal Si (111) monochromator (continuous
After outgassing under high vacuum (10À 5 mbar), the scanning in transmission mode). Calibration of the
catalysts were stored in an Ar atmosphere in a solvent- beamline was performed using metal reference foil
free glovebox. The code MetalTemperature refers to the and MetalAl2O4 reference. XAS transmissions mode
type of metal nanoparticle of the reduced supported spectra were detected using ion chambers filled with
HeÀ N2 gas mixtures. Inside the glovebox, 20 mg of
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Helv. Chim. Acta 2019, 102, e1800227
powder sample were pressed into 1 cm ring and radiation beam time at beamline BM31 (31-01-6) of
sealed into a 4 cm×4 cm aluminum bag. EXAFS data the SNBL and would like to thank Dr. Wouter Van Beek
were fitted in R-space (1–3.7 Å) after a Fourier trans- for assistance. We thank Dr. Nikolaos Tsakoumis for
form (k=2–10.5 ÅÀ 1) using a k weight of 1, 2, and 3 providing reference material for XAS measurements.
for the molecular precursors and the grafted species. We thank Dr. Frank Krumeich for the STEM measure-
XANES spectra of the supported nanoparticles were ment and ScopeM (ETH Zürich) for providing measur-
fitted using a linear combination fitting of Metal foil ing time.
and MetalAl2O4 references for cobalt, nickel, and
copper-based samples. In the case of iron-based
samples, Fe/Al2O3 was fitted with [Fe]-complex, Fe foil,
and Fe2O3 references. Fe700 was fitted with Fe foil,
Author Contribution Statement
FeO, and Fe2O3 references.
All authors have equally contributed to this work.
Temperature-Programmed Reduction (TPR) Studies
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opening to the machine, the entry and outlet tubings
were purged with Argon for 30 min using a by-pass
system. The TPR decomposition measurement was
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Received December 4, 2018
Accepted February 5, 2019
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