Magnetic core-shell nanoparticles as carriers for olefin dimerization catalysts

Article abstract of DOI:10.1002/ejic.201201547

We report the covalent support of functionalized nickel complexes on magnetic core-shell hybrid particles γ-Fe₂O₃/ SiO₂. Two completely different ways of connecting the particle with these nickel complexes were carried out. The first approach used the hydrosilylation method between the alkene-substituted nickel complex and a silane. In a second approach, the particles were connected with the complexes by means of click chemistry (copper-catalyzed Huisgen 1,3-dipolar cycloaddition). For this purpose, the nickel complexes were substituted with an alkyne moiety. Transmission and scanning electron microscopies, energy-dispersive X-ray diffraction, and FTIR spectroscopy were the methods employed to characterize the successful heterogenization of the nickel complexes. Copyright

Full text of DOI:10.1002/ejic.201201547

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DOI:10.1002/ejic.201201547
Magnetic Core–Shell Nanoparticles as Carriers for Olefin
Dimerization Catalysts
Thomas M. Ruhland,[a][‡‡] Julian R. V. Lang,[b][‡‡] Helmut G. Alt,*[b]
and Axel H. E. Müller*[a][‡]
Keywords: Heterogeneous catalysis / Nanoparticles / Core–shell structures / Nickel / Magnetic properties / Polymerization
We report the covalent support of functionalized nickel com-
plexes on magnetic core–shell hybrid particles γ-Fe2O3/SiO2.
Two completely different ways of connecting the particle
with these nickel complexes were carried out. The first ap-
proach used the hydrosilylation method between the alkene-
substituted nickel complex and a silane. In a second ap-
proach, the particles were connected with the complexes by
means of click chemistry (copper-catalyzed Huisgen 1,3-di-
polar cycloaddition). For this purpose, the nickel complexes
were substituted with an alkyne moiety. Transmission and
scanning electron microscopies, energy-dispersive X-ray dif-
fraction, and FTIR spectroscopy were the methods employed
to characterize the successful heterogenization of the nickel
complexes.
ucts, and the reuse of the catalyst. These problems are of
environmental and economic interest in terms of large-scale
synthesis. A possible heterogenization of the existing cata-
lysts could help to design nanocatalysts with excellent ac-
tivity, greater selectivity, and higher stability, and could be
an attractive solution to this problem. These characteristics
can easily be achieved by means of tailoring the size, shape,
morphology, composition, and thermal and chemical sta-
bility. The pioneering compounds of magnetochemistry are
superparamagnetic iron oxide particles, such as magnetite
(Fe3O4) and maghemite (γ-Fe2O3). In addition to these ox-
ide materials, pure metal nanomagnets made out of iron or
cobalt are producible on relevant scales.[23–26] Both blank
particles are vulnerable to the loss of magnetism in chemical
reactions. For further applications, the particles need to be
protected against chemical, thermal or mechanical influ-
ences. In general, there are three methods of stabilization:
(1) the addition of monomers such as carboxylates or phos-
phates,[27–30] (2) coating with inorganic materials such as
silica or gold metal,[31–36] and (3) coating with organic ma-
trices such as surfactants and polymers.[37–42] Silica plays an
important role in the preparation of core–shell nanoparticle
systems due to its excellent physical and chemical proper-
ties, its variable surface chemistry, and its drastically in-
creased chemical stability. It is optically transparent, easily
functionalized, and the simple and robust synthesis of silica
particles from the monomer tetraethoxysilane (TEOS) has
been established for many decades. Silica-coated γ-Fe2O3
magnetic nanoparticles (MNPs) combine the advantages of
magnetic cores and silica surfaces that can be function-
alized. Although classic heterogeneous catalysts are widely
used in industry, lower activities than homogeneous systems
are commonly detected.[42] A great proportion of these cata-
lysts are deep inside the supporting material and thus reac-
Introduction
Nanotechnology is a multidisciplinary platform to target
materials for a wide field of applications. Magnetic nano-
particles and hybrid core–shell particles of well-defined size
and shape with unique and advanced properties for a large
field of possible applications have received a great deal of
attention that range from biomedical applications, magnetic
separation media, and other high-functional devices[1–5] to
materials engineering[6–10] and catalysis.[11–17] A spherical
nanoparticle with a core–shell architecture is a viable way
to combine multiple functionalities on a nanoscopic
scale.[18–20]
Catalysis is becoming a strategic field of science because
it represents a new way to meet the challenges in many dif-
ferent fields. The concept of green chemistry, which makes
catalysis science even more important, has become a routine
part of sustainable chemistry and daily life. Nanocatalysts
with tunable size, shape, and composition have drawn tre-
mendous interest in both theoretical and technological
fields in recent years.[21,22] Although heterogeneous cata-
lysts are widely used in a variety of industries, it is often
difficult to isolate and separate the final product after the
reaction has completed. Homogeneous catalysis suffers
from the problematic separation of the catalysts and prod-
[a] Macromolecular Chemistry II, Universität Bayreuth,
95440 Bayreuth, Germany
E-mail: axel.mueller@uni-mainz.de
Homepage: www.mcii.de
[b] Inorganic Chemistry II, Universität Bayreuth,
95440 Bayreuth, Germany
E-mail: helmut.alt@uni-bayreuth.de
[‡] Current address: Institute of Organic Chemistry, Johannes
Gutenberg University Mainz,
55099 Mainz, Germany
[‡‡]These authors contributed equally to this work.
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tants have limited access to the catalytic sites.[43] By decreas- encapsulation and to either entrap several particles or indi-
ing the size of the support down to the nanometer scale, vidual ones and to precisely tailor the thickness of the silica
the surface area is increased and the support can be evenly shell.[10] The size of the γ-Fe2O3 particles is sufficiently con-
dispersed in solution, thereby forming a homogeneous trolled by the amount of oleic acid that is used in relation
emulsion.[13] In recent years, much attention has been fo- to the amount of [Fe(CO)5]. TEM images in Figure 1 (A)
cused on catalysis research, and magnetic nanoparticles show the γ-Fe2O3 cores with an average diameter of
have been employed in important reactions such as hydro- (9.9Ϯ1.8) nm, which is corroborated by dynamic light scat-
genation, hydroformylation, Suzuki–Miyaura and Heck tering (DLS) with a z-average hydrodynamic radius of
couplings, and olefin metathesis.[44–46]
6.6 nm and a polydispersity index (PDI) of 0.04. Clearly,
Herein, we combine the properties of core–shell nano- the oleic acid ligand shell is invisible in the TEM image,
particles with the catalytic character of certain complexes but contributes to the hydrodynamic distribution function
in hybrid core–shell–corona nanoparticles. The idea was to obtained by DLS, thus resulting in a higher value.[5] A thin
create a heterogeneous catalyst for facile product separation 20 nm layer of silica was coated on this core by means of a
for the catalytic conversion of olefins. An external magnet sol–gel process to give silica-coated γ-Fe2O3 magnetic nano-
can remove the magnetic particles from the solution, or particles with an average diameter of 52 nm (Figure 1, B).
generate two phases for subsequent product separation. We
report the application of nickel complexes supported on
magnetic nanoparticles incorporated into silica shells dur-
ing the catalyzed polymerization and dimerization of both
ethylene and propylene.
The series of catalysts (Scheme 1) was designed to in-
clude two methods of linkage with the core–shell support
material. Both methods contain simple reaction steps and
uncomplicated handling and anchoring of a triethoxysilane
group. As will be mentioned in the Experimental Section,
the first approach (Scheme 2/3) uses hydrosilylation to com-
bine the bis(imine)nickel complex with the supporting ma-
terial. The second approach (Scheme 4) describes the com-
bination of the azido-functionalized magnetic particles and
the alkyne end-functionalized phenoxyimine–nickel com-
plexes by using copper-catalyzed azide–alkyne Huisgen 1,3-
dipolar cycloaddition (CuAAC).[47,48] For both approaches,
the same magnetic core–shell particles that contain a γ-
Fe2O3 core coated with a silica layer were used.
Results and Discussion
Here we describe in detail the synthesis, characterization,
and catalytic behavior of the catalysts on magnetic core–
shell nanoparticles (NPs). The synthetic strategy towards
monodisperse hybrid catalysts with core–shell architecture
consists of the following steps. In a first step, we synthesized
monodisperse and hydrophobically functionalized super-
paramagnetic γ-Fe2O3 NPs by means of the thermal decom-
The first synthetic strategy involves a precursor that con-
position of [Fe(CO)5] in the presence of oleic acid, which sists of an ω-alkenyl diimine moiety coordinated to
serve as cores for the final hybrid particles (Figure 1, A). nickel(II) bromide and a silane that can react with the silica
Afterwards single NPs were encapsulated with a silica layer particles (Scheme 2). After the silane-ene reaction, the com-
by a dedicated microemulsion procedure, thus yielding plex with the triethoxysilane moiety can be coupled directly
monodisperse core–shell γ-Fe2O3/SiO2 particles. Precise ad- to the support material (Scheme 3). This hydrosilylation re-
justment of the conditions allows one to achieve a reliable action is simple and quick. After heating in toluene, com-
Figure 1. TEM images of (A) the iron oxide cores and (B) the magnetic nanoparticles with silica shell. (C) Intensity-weighted hydrody-
namic radii distribution (DLS) of γ-Fe2O3 (–) and γ-Fe2O3/SiO2 (---).
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Scheme 1. Prepared complexes 2–4 containing an alkyne moiety.
Scheme 2. Hydrosilylation reaction of the diimine nickel complex and triethoxysilane.
Scheme 3. Route of heterogenization for complex 1.
plex 1 was immobilized covalently on the silica support (1- TEM images in Figure 2 show the core–shell structure with
H). Following the product removal by centrifugation, the the modified surface of the particles. The surface of the
particles were washed with toluene, THF, and CH2Cl2. particles with connected complexes is distinctly coarser
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than the blank Fe2O3/SiO2 nanoparticles. This indicates
that a thin layer of supported catalyst surrounds the silica
particles.
Figure 2. TEM image of 1-H. The surface is distinctly coarser than
the blank particles.
Energy-dispersive X-ray diffraction (EDX) was used to
identify the chemical composition at the surface and of the
particles. The EDX spectrum in Figure 3 clearly shows the
typical peaks. A small peak from the iron core is clear. Sili-
Figure 3. EDX spectrum of the heterogeneous complex 1-H.
In the second approach (Scheme 4), we discuss the
con (shell) and oxygen (core and shell) are the dominant heterogenization of phenoxyimine–nickel complexes. Here,
peaks. This can be confirmed by peak integration of the Huisgen 1,3-dipolar cycloaddition is used to attach azido-
typical nickel and bromine peaks, which shows a 4.2 and functionalized γ-Fe2O3/SiO2 particles to the alkyne group
7.9% fraction, respectively. This represents an atomic ratio of the complex. This is a very adaptable method to attach
of 1:1.9, thus confirming the composition NiBr2. These re- any compound that carries an alkyne group to a desired
sults are in good agreement with the proposed structure and surface. Therefore, three different nickel complexes that
the conclusion drawn from the TEM image.
bear an alkyne group (Scheme 2) were synthesized and con-
Scheme 4. Route of heterogenization for complexes 2–4.
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nected to the azido-functionalized support by following the The surface of the heterogeneous complexes is distinctly
procedure shown in Scheme 4. The respective ligands were coarser than the blank γ-Fe2O3/SiO2 particles. All these as-
synthesized according to the literature,[49] and the com- pects indicate that the heterogenization reaction was suc-
plexes were obtained through a known process.[50] The mag- cessful, and the IR results are confirmed.
netic particles Fe2O3/SiO2 were combined with an azide-
functionalized propyl(triethoxy)silane moiety through con-
densation. Huisgen 1,3-dipolar cycloaddition was used to
attach azido-functionalized γ-Fe2O3/SiO2 particles to the
alkyne group of the complex. Figure 4 shows the FTIR
spectra of the pure γ-Fe2O3/SiO2 particles, the nickel–phen-
oxyimine-connected γ-Fe2O3/SiO2 (2-H), and pure complex
2. The silica surface is characterized by two dominant
peaks: the nonbonded surface Si–OH groups with a band
in the range of 3300–3700 cm–1, and the characteristic Si–
O–Si vibration (1080 cm–1).
Figure 5. TEM image of 2-H. The surface is distinctly coarser than
the blank particles.
Catalytic Properties
The catalytic performance of the heterogeneous catalysts
1-H, 2-H, 3-H, and 4-H is described in the following sec-
tion. The bisimino–nickel complex 1-H was applied as cata-
lyst for the catalytic polymerization of ethylene. Recent
works have described the homogeneous polymerization of
ethylene with bisimino–nickel complexes after activation
with methylaluminoxane (MAO).[39] In our experiments, the
immobilized catalyst was stirred in toluene, and ethylene
was added to the solution under low pressure for 20 min.
After quenching with 2-propanol, the polymer particles
were washed and separated by means of centrifugation. The
particles were visualized by SEM (Figure 6). The images
show spherical particles with a rough polymer surface. A
broad polymer particle-size distribution with particle sizes
from 0.1 to 1.5 μm was observed. The images show that
polymerization happens at the catalysts on the particle sur-
face and that the particles grow independently.
Figure 4. FTIR spectra of the pure nanoparticles (MNP), the sup-
ported complex (2-H), and the corresponding pure complex (2).
The supported complex shows two peaks at 1620 and
1730 cm–1, which are typical for the C=N and C=C stretch-
ing bands. The created triazole group shows a typical peak
at 1450 cm–1 that is dominated by the peaks of the support.
This resonance is not very intense, but it is the only charac-
teristic resonance of the triazole group and confirms a suc-
cessful grafting. The phenoxyimine complex unit itself
shows no strong IR stretching frequency and it is likely
masked by the silica bands of the support. The pure com-
plex shows the very intense band of the C–H vibration of
Figure 6. SEM images of the polyethylene particles polymerized by
1-H.
the alkyne group (3280 cm–1).
After the click reaction with the alkyne complexes, the
particles were imaged by TEM. Figure 5 shows the core–
The polymerization test proves that covalently immobi-
shell structure with the modified surface of the particles. lized complexes on γ-Fe2O3/SiO2 particles are catalytically
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active and that catalysis happens at these functionalized Table 1. Results of the catalytic dimerization of propylene with the
heterogeneous complexes 2-H, 3-H, and 4-H.
particles. The polymerization experiments were carried out
as control experiments for the following dimerization ex-
periments, in which no change in the shape and size of the
catalytic active particles could be observed. During the
catalytic tests, we mainly focused on the role of the support 2-H
C6 [%]
C9 [%]
C12 [%]
Activity of the
heterogeneous catalyst
[kg(product)/molh–1]
75
92
87
25
6
7
–
2
6
264
405
380
3-H
in easing the product separation. Thus, all catalytic tests
4-H
were carried out using a heterogeneous catalyst in a closed
system (pressure Schlenk tube). Here, we used phenoxy-
imine–nickel complexes (2-H), which are normally highly
active for the dimerization of propylene after activation
All three heterogeneous catalysts are highly efficient for
the catalytic dimerization of propylene (Table 1). The cata-
lytic activities and the product distribution are in a range
that agrees with the literature.[41] The reaction products
were decanted after separating the particles by using a
strong magnet to hold the catalyst in the Schlenk tube.
with MAO.
With two ligands coordinating the central metal, the
phenoxyimine complexes have two sides for heterogeniz-
ation (Scheme 5). After activation with MAO, one ligand
leaves the complex and leaves a free coordination side. After
activation, a single complex is fixed on a single side to a
particle. The catalytic dimerization of propylene was per-
formed with all three immobilized catalysts. Whereas bis-
imine–nickel catalysts are known for their catalytic polyme-
rization of ethylene,[39] phenoxyimine–nickel complexes are
highly active for the dimerization of propylene.[41]
Conclusion
We have demonstrated the successful heterogenization of
phenoxyimine–nickel and bis(imine)–nickel complexes by
means of an azide–alkyne click reaction and hydrosilylation
reaction. Uniformly nanosized silica-coated magnetic nano-
particles were synthesized and used as support material.
The complexes were covalently bound to the silica shell,
according to EDX, TEM, and FTIR observations. Polyme-
rization of ethylene after activation with MAO was success-
ful and SEM images clearly show spherical particles with a
rough polymer surface. Dimerization of propylene showed
the excellent catalytic properties of the heterogenized phen-
oxyimine–nickel complexes. The compared use under
homogeneous “free” conditions resulted in the same prod-
uct distribution. The facile preparation and the catalytic
properties show the high potential of this heterogeneous
system. Catalysts can be separated by means of a strong
magnet, and ease the problematic product separation. The
application of this catalytic system combines homogeneous
activities and selectivities, and the separation of the product
and the catalyst phase in heterogeneous systems.
Scheme 5. Phenoxyimine–nickel complexes before and after acti-
vation with MAO. The square represents the free coordination
site.[51]
The results in Table 1 show products of the dimerization
reactions of the supported catalysts 2-H, 3-H, and 4-H after
activation with MAO. All three heterogeneous catalysts
show high selectivities of up to 92% for dimeric products.
In addition to some trimeric products, nearly no higher
oligomers are formed. Aside from all the general advan-
tages of heterogeneous catalysts that have already been
mentioned, the activity of our heterogeneous catalysts is
comparable to their homogeneous analogues.[50] Table 1
shows the dimerization results of the heterogeneous cata-
lysts. The catalytic activities are in the range of 264 to 405,
which represents turnover numbers (TONs) of 3150 (2),
4520 (4), and 4820 (3). The gas chromatograms of the di-
merization experiments show typical distribution. The
branched methylpentenes t-4-MP-2 (trans-4-methyl-pent-2-
ene) and 2-MP-2 (2-methylpent-2-ene) are the main prod-
ucts, with 4-MP-1 (4-methylpent-1-ene), c-4-MP-2 (cis-4-
methylpent-2-ene), t-2-Hex (trans-2-hexene), and c-2-Hex
(cis-2-hexene) as byproducts.
Experimental Section
General Considerations: Air- and moisture-sensitive reactions were
carried out under an atmosphere of purified argon using conven-
tional Schlenk or glovebox techniques. The dimerization reactions
were performed with pressure Schlenk tubes.
Materials: All solvents were purchased as technical grade and puri-
fied by distillation over Na/K alloy under an argon atmosphere.
Iron(0)pentacarbonyl {[Fe(CO)5]; 99.9%}, oleic acid (90%), dioctyl
ether (Ͼ 99%), poly[oxyethylene(5)]nonylphenyl ether (Igepal CO-
520), ammonium hydroxide (NH4OH; 28% in H2O), and tetraeth-
ylorthosilicate (TEOS; Ͼ98%) were purchased from Sigma Ald-
rich. All other chemicals were purchased commercially from Ald-
rich or Acros or were synthesized according to literature pro-
cedures. The MAO solution (30 wt.-% in toluene) was obtained
from Albemarle, USA.
Procedure for the Synthesis of γ-Fe2O3 Nanoparticles: The synthesis
of γ-Fe2O3 nanoparticles was adapted from the literature.[5,10]
A
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250 mL two-neck round-bottomed flask, connected to a reflux con-
denser, was charged with dioctyl ether (120 mL) and oleic acid
(29.0 mL, 25.76 g, 91.2 mmol) and degassed with N2 for 15 min.
The reaction mixture was heated to 100 °C under an N2 atmo-
sphere before [Fe(CO)5] (4 mL, 30.4 mmol) was added. Sub-
sequently, the resulting mixture was heated to reflux and was kept
for 1.5 h until the solution turned black. After cooling to room
temperature, the reaction mixture was stirred under air to initiate
the oxidation process of iron to achieve γ-Fe2O3 nanoparticles. The
resulting particles were precipitated with ethanol and separated by
means of an NbFeB magnet. The particles were immediately redis-
persed in toluene, THF, or n-hexane. For further purification, the
precipitation and separation process was repeated.
magnetic particles and the alkyne end-functionalized phenoxyim-
ine–nickel complexes by using the copper-catalyzed azide–alkyne
Huisgen 1,3-dipolar cycloaddition (CuAAC).
Approach 1: To obtain 1-H, complex 1 (630 mg, 1.28 mmol) was
dissolved in a minimum amount of toluene. SpeierЈs catalyst
(H2PtCl6; 5 mg, 12 μmol) was added. Triethoxysilane (233 mg,
1.42 mmol) was added dropwise over a period of 30 min. After 24 h
of stirring, the complex precipitated by the addition of pentane
(Scheme 2). The solution was removed, and the magnetic core–shell
nanoparticles (100 mg) in toluene (25 mL) was added. The suspen-
sion was stirred for 3 d at 85 °C. The slight blue solution was sepa-
rated from the brown-green solid, and the particles were thoroughly
washed with toluene, THF, and CH2Cl2 (Scheme 3).
Procedure for the Synthesis of γ-Fe2O3/SiO2 Core–Shell Particles:
The synthesis is based on the experimental procedure mentioned
in the literature.[10] Herein, we present detailed experimental condi-
tions. Polyoxyethylene(5)nonylphenyl ether (0.23 g, 0.54 mmol, Ige-
pal CO-520) was dispersed in a 10 mL small flask that contained
cyclohexane (4.5 mL) in a ultrasound bath for 10 min. Next, γ-
Fe2O3 nanoparticles dispersed in cyclohexane (400 μL,
0.5 mgmL–1) were added to the flask and vortexed for 5 min at
200 min–1. The rapid addition of ammonium hydroxide (29.4%,
40 μL) formed a reverse brownish microemulsion, and the subse-
quent addition of tetraethylorthosilicate (30 μL) started the growth
of the silica shell. The nanocomposite particles were aged for 48 h
by using a shaking incubator at room temp. and at 100 rpm as the
oscillation rate (soft agitation). Then they were purified by several
cycles of centrifugation at 4500 rpm for 10 min and cautious redis-
persion in ethanol. The magnetic core–shell particles could also be
magnetically collected. The final product was redispersed in milli-
pore water.
Approach 2:[47–50] To obtain 2-H, 3-H, and 4-H, chloropropyl(tri-
ethoxy)silane (8.8 g) and sodium azide (2.4 g) were stirred in
DMSO (100 mL) for 4 d. A white solid precipitated and was re-
moved by filtration. The magnetic particles (200 mg) were added
to a third of the solution and stirred for 4 d at 80 °C. The particles
were separated and suspended in toluene with an excess amount of
the respective catalysts (1 g). Copper chloride (2 mg) and sodium
ascorbate (4 mg) were dissolved in distilled water (5 mL) and added
to the stirred mixture. After a reaction time of 4 d, the particles
were separated by means of centrifugation and washed thoroughly
with toluene, ethanol, and pentane.
Polymerization of Ethylene: The supported catalyst (25 mg) was
placed in a pressure Schlenk tube with toluene (10 mL) and MAO
solution (12 mL, 10 wt.-%). Ethylene was added under pressure
(2 bar) for 20 min at room temperature. The particles were carefully
washed with 2-propanol and toluene, and separated from the solu-
tion and dried under vacuum.
Procedure for the Synthesis of the Complexes
Dimerization of Propylene: The supported catalyst (25 mg) was
placed in a 400 mL pressure Schlenk tube with toluene (5 mL) and
activated with MAO solution (10 mL; Ni/Al 1:500). The pressure
Schlenk tube was filled with liquid propylene (50 mL) and closed,
warmed to room temperature with an external water bath, and
stirred. After a reaction time of 1 h, the Schlenk tube was opened
and the solution was analyzed by GC.
Complex 1 was prepared and characterized elsewhere.[52] For com-
pounds 2, 3, and 4, the respective ligand precursor (10 mmol) in
ethanol (100 mL) was mixed with Ni(OAc)2·xH2O (5 mmol)
(Scheme 1). The solution was stirred under reflux for 24 h. The
volume was reduced and the complexes precipitated by the addition
of pentane. The products were filtered through a glass frit and
washed with pentane. The complexes were obtained as green solids
with yields in the range of 73–82%. Similar complexes were first
described by Dieck et al. and are known as polymerization catalysts
in combination with MAO.[53]
Characterization: The products of the dimerization experiments
were characterized with a gas chromatograph (Agilent 6890) and
GC/MS instrument (FOCUS DSQ Thermo Scientific). Mass spec-
tra were recorded with a Varian MAT CH7 instrument (direct inlet
system, electron impact ionization 70 eV). Elemental analyses were
performed with a VarioEl III CHN instrument. Acetanilide was
used as standard.
MS data for 2: m/z (%) = 608 [M·+] (68), 357 (9), 333 (72), 236
(100). C32H34Cl2N2NiO2 (608.25): calcd. C 63.19, H 5.63, N 4.61;
found C 63.08, H 5.52, N 4.53.
MS data for 3: m/z (%) = 485 [M·+] (37), 214 (100). C28H31N2NiO2
(486.28): calcd. C 69.16, H 6.43, N 5.76; found C 69.64.08, H 6.31,
N 5.47.
MS data for 4: m/z (%) = 610 [M·+] (41), 510 (41), 335 (27), 276
(100). C38H36N2NiO2 (611.42): calcd. C 74.65, H 5.93, N 4.58;
found 74.17, H 5.34, N 4.81.
Bright-field TEM was performed with Zeiss CEM 902 and LEO
922 OMEGA electron microscopes operated at 80 and 200 kV,
respectively. Data evaluation and processing was carried out with
Soft Imaging Viewer and Image Tool. SEM was performed with a
LEO 1530 Gemini instrument equipped with a field-emission cath-
ode with a lateral resolution of approximately 2 nm. The accelera-
tion voltage was chosen between 0.5 and 30 kV for energy-disper-
sive X-ray analysis (EDX). EDX spectra were measured with a
LEO 1530 Gemini instrument and an Oxford EDX INCA 400 de-
vice. The obtained spectra were analyzed with the microscope soft-
ware and the present elements in the sample were detected by their
corresponding X-ray absorption peaks.
Synthesis of the Supported Complexes: These core–shell particles
contain an γ-Fe2O3 core coated with a silica layer. The silica shell
has plenty of hydroxy groups for potential derivatization, on which
the catalysts can be successfully immobilized. The series of catalysts
was designed to include two methods of linkage with the core–shell
support material. Both methods contain simple reaction steps and
uncomplicated handling and anchoring of a triethoxysilane group.
The first approach (Schemes 2 and 3) uses hydrosilylation to com-
bine the bis(imine)–nickel complex with the supporting material.
The second approach (Scheme 4) describes the combination of the
FTIR was carried out with a Spectrum 100 FTIR spectrometer
from Perkin–Elmer. For measurements, a U-ATR unit was used.
The dried samples were directly placed on top of the U-ATR unit
for measurements.
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The authors acknowledge financial support from ConocoPhillips,
Bartlesville, USA. André H. Gröschel is thanked for his help with
Schemes 4 and 5, and Alexander Majewski for providing the γ-
Fe2O3 nanoparticles. This work was supported by the Deutsche
Forschungsgemeinschaft (DFG) within the SFB 840 program (TP
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Received: December 21, 2012
Published Online: February 18, 2013
Eur. J. Inorg. Chem. 2013, 2146–2153
2153
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim