Si(OCH2Fc)4: Synthesis, Electrochemical Behavior, and Twin Polymerization

Article abstract of DOI:10.1002/ejic.201500464

The preparation and twin polymerization of the twin monomer Si(OCH₂Fc)₄ [Fc = Fe(η⁵-C₅H₄)(η⁵-C₅H₅)] (2) by the reaction of FcCH₂OH (1) with SiCl₄ in the presence of pyridine was explored. The electronic properties of 2 were investigated by cyclic voltammetry, square-wave voltammetry, and UV/Vis/near-IR spectroelectrochemistry, which showed a redox separation caused by electrostatic repulsion. Thermally induced condensation of 2 is characteristic, as evidenced by differential scanning calorimetry (DSC) and thermogravimetry coupled mass spectrometry (TG-MS). Upon heating 2 to 210 °C, twin polymerization occurred and a hybrid material was formed that showed similarities with known systems derived from 2,2′-spirobi[4H-1,3,2-benzodioxasiline] (SBS), such as the nanopatterning of the formed silicon dioxide, which is characteristic for twin polymerization. Electron microscopy of this material revealed the absence of typical microstructuring found for other twin polymers, and hence, the herein presented system can be characterized as a borderline system if compared to known twin monomers such as SBS. The copolymerization of 2 and SBS afforded a hybrid material from which porous carbon or silica materials containing iron oxide nanoparticles could be obtained. The oxidation state of the incorporated particles was examined by M?ssbauer experiments, which confirmed that only Fe^III was incorporated within the porous carbon and silica materials, respectively. The preparation of iron oxide containing porous carbon capsules was achieved by applying a mixture of 2 and SBS to silicon dioxide spheres (d = 200 nm). After twin polymerization and carbonization, porous carbon capsules with incorporated iron oxide nanostructures were obtained. The straightforward preparation of iron-rich porous carbon and silica materials by twin polymerization of Si(OCH₂Fc)₄ [Fc = Fe(η⁵-C₅H₄)(η⁵-C₅H₅)] and 2,2′-spirobi[4H-1,3,2-benzodioxasiline] is reported; the electrochemical properties of Si(OCH₂Fc)₄ are discussed.

Full text of DOI:10.1002/ejic.201500464

FULL PAPER
DOI:10.1002/ejic.201500464
Si(OCH Fc) : Synthesis, Electrochemical Behavior, and
2
4
Twin Polymerization
[
a]
[a]
[b][‡]
Christian Schliebe, Ulrike Pfaff, Thomas Gemming,
Charles Lochenie,[
c][‡‡]
Birgit Weber,
[c][‡‡]
and Heinrich Lang*
[a]
Keywords: Hybrid materials / Nanoparticles / Iron / Superparamagnetism / Electrochemistry / Twin polymerization
The preparation and twin polymerization of the twin mono- sence of typical microstructuring found for other twin poly-
5
5
mer Si(OCH
2
Fc)
4
[Fc = Fe(η -C
5
H
4
)(η -C
5 5
H )] (2) by the re- mers, and hence, the herein presented system can be charac-
action of FcCH
2
OH (1) with SiCl
4
in the presence of pyridine
terized as a borderline system if compared to known twin
monomers such as SBS. The copolymerization of 2 and SBS
afforded a hybrid material from which porous carbon or silica
materials containing iron oxide nanoparticles could be ob-
was explored. The electronic properties of 2 were investi-
gated by cyclic voltammetry, square-wave voltammetry, and
UV/Vis/near-IR spectroelectrochemistry, which showed a
redox separation caused by electrostatic repulsion. Ther- tained. The oxidation state of the incorporated particles was
mally induced condensation of 2 is characteristic, as evi- examined by Mössbauer experiments, which confirmed that
III
denced by differential scanning calorimetry (DSC) and ther- only Fe was incorporated within the porous carbon and sil-
mogravimetry coupled mass spectrometry (TG–MS). Upon
heating 2 to 210 °C, twin polymerization occurred and a hy-
brid material was formed that showed similarities with
ica materials, respectively. The preparation of iron oxide con-
taining porous carbon capsules was achieved by applying a
mixture of 2 and SBS to silicon dioxide spheres (d = 200 nm).
known systems derived from 2,2Ј-spirobi[4H-1,3,2-benzo- After twin polymerization and carbonization, porous carbon
dioxasiline] (SBS), such as the nanopatterning of the formed
silicon dioxide, which is characteristic for twin polymeriza-
tion. Electron microscopy of this material revealed the ab-
capsules with incorporated iron oxide nanostructures were
obtained.
Introduction
tallic compounds, including ferrocene, which can be decom-
[
8]
posed to afford iron-doped carbon materials. Different
modifications of carbon materials can thereby obtained, for
example, uniform iron-rich carbon capsules through high-
The incorporation of iron or iron oxide nanoparticles
within porous carbon or silica matrices has gained great
interest, owing to the versatile catalytic[1] and magnetic[2]
applications of iron. The preparation of such materials can
be achieved by impregnation of activated carbon supports
with aqueous iron(III) nitrate solutions followed by high-
temperature treatment.[ Applications of these as-prepared
materials include their use as anode materials for lithium-
[9]
pressure pyrolysis. Carbon nanotubes can also be pre-
pared, for example, by subliming ferrocene and decompos-
[10]
ing the vapors in a second furnace at higher temperature.
A different approach is given by using polymers containing
metallocenes, and hence, a variety of ferrocene and ferro-
cene-related compounds can be functionalized with side
3]
[
3]
ion batteries with improved cycling stability. Another iron
[11,12]
chains and then polymerized under various conditions.
source is, for example, iron(III) chloride[
4–7]
or organome-
The group of Manners et al. focused on strained silicon
ansa-ferrocenes and demonstrated the superior polymeriza-
[
a] Technische Universität Chemnitz, Faculty of Natural Sciences,
Institute of Chemistry, Inorganic Chemistry,
9107 Chemnitz, Germany
[
13]
tion abilities of such species. The versatile processability
[
13–17]
0
of the obtained materials was investigated.
Iso-
E-mail: heinrich.lang@chemie.tu-chemnitz.de
https://www.tu-chemnitz.de/chemie/anorg/
b] IFW Dresden,
Helmholtzstrasse 20, 01069 Dresden, Germany
E-mail: t.gemming@ifw.dresden.de
c] Universität Bayreuth, Institute of Chemistry, Inorganic
Chemistry II,
structural compounds were also prepared by using different
main group elements, for example, Sn, Se, and S, and or-
[
[
[18–20]
ganic groups such as CH =CH .
2
2
The recently introduced concept of twin polymerization
allows the preparation of organic–inorganic twin poly-
95440 Bayreuth, Germany
[21]
mers
by an acid-catalyzed reaction of, for example, the
[
[
‡] Author to whom correspondence pertaining to STEM studies
should be addressed.
‡‡] Authors to whom correspondence pertaining to magnetic
characterization should be addressed.
twin monomer 2,2Ј-spirobi[4H-1,3,2-benzodioxasiline]
SBS).[22,23] During the polymerization process, strong
(
mechanistic coupling between the formation of the organic
and inorganic phase leads to dispersed silicon dioxide clus-
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500464.
Eur. J. Inorg. Chem. 2015, 3850–3860
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ters on a nanometer scale within the organic polymer.[24] terized by a minor shift of the CH2 protons from δ =
After carbonization and removal of the inorganic phase 4.32 ppm in 1 to δ = 4.47 ppm in 2. The respective C H
5
4
[
41]
with a boiling NaOH solution, highly microporous materi- protons show, as expected, an AAЈXXЈ spin system with
als are obtained. Carbonization followed by careful oxid- JHH = 1.58 Hz, whereas for the C H group a singlet was
ation of the organic phase results in mainly mesoporous observed (δ = 4.12 ppm). In the C{ H} NMR spectrum
[
24]
5
5
1
3
1
[
25]
silicon dioxide.
This novel methodology was also used of 2, the CH carbon atom is shifted from δ = 60.7 ppm in
2
to coat spherical hard templates to obtain porous carbon 1 to δ = 62.0 ppm in 2. Similar behavior is found for the
[
26,27]
capsules.
Recently, it was shown that the functionalization of the
organic part of the twin monomer has a significant effect was detected, which is typical for Si(OR) species.
remaining C H and C H carbon atoms. Considering the
5 4 5 5
29
Si-DEPT spectrum, a resonance signal at δ = –82.60 ppm
[
21]
4
on the twin polymerization itself.[
28,29]
Moreover, modifica-
The IR spectrum of 2 is characterized by two very dis-
tion of the inorganic component leads to the formation of tinct vibrations, which appear at ν˜ = 1104 (νC–O) and
[
30,31]
–1
diverse oxide materials, for example, the oxides of Sn,
1036 cm (νSi–O). These absorptions can be used to moni-
[
32]
[33,34]
[35]
[35]
[36]
Ge,
Ti,
Zr,
Hf,
and W.
In another study, it tor the progress of the reaction of 1 with SiCl to give 2.
4
was demonstrated that the incorporation of silver nanopar-
ticles during the twin polymerization process is possible ular ion [M] at m/z = 888.0360, which proves the forma-
High-resolution ESI mass spectrometry shows the molec-
+
with the addition of silver(I) carboxylates of the [AgO - tion of 2.
2
c
CCH C H S(PPh )] type to SBS, which give homoge-
2
4
3
3
neously distributed silver nanoparticles in a porous carbon
or silica matrix.
[
37]
Electrochemical Studies of 1 and 2
We herein report a straightforward synthetic method for
the preparation of iron oxide containing porous carbon and
silica materials by the use of a polymerizable ferrocene-
based monomer. In addition, the polymerization of the twin
To obtain insight into the electronic properties of 1 and
2
, electrochemical [cyclic voltammetry (CV) and square-
wave voltammetry (SWV)] (Figure 1) and UV/Vis/near-IR
NIR) spectroelectrochemical (Figure 2) measurements
(
5
5
monomer Si(OCH Fc) [Fc = Fe(η -C H )(η -C H )] (2)
2
4
5
4
5
5
were performed. For the measurements, dichloromethane
and its copolymerization with SBS will be reported. Also,
the preparation of iron oxide filled carbon capsules under
the addition of a silicon dioxide template is in scope of this
article.
–1
solutions containing 1 or 2 (1.0 mmolL ) and [nBu N]-
4
–1 [42–44]
[
2
B(C F ) ] (0.1 molL )
as supporting electrolyte at
6
5 4
5 °C were used. All potentials are referenced to the FcH/
+
5
[45]
FcH redox couple [E°Ј = 0 mV, FcH = Fe(η -C H ) ].
5
5 2
In the CV curve of 1, as expected, a single reversible one-
electron redox event is observed (E° = 6 mV), whereas for
Ј
1
Results and Discussion
2
four redox processes in a close potential range are charac-
teristic (Figure 1). For better resolution of these events,
Synthesis and Characterization of Si(OCH Fc)
2
4
SWV was performed, and thus, the redox potentials for all
5
5
°Ј
The synthesis of Si(OCH Fc) [Fc = Fe(η -C H )(η - four ferrocenyl termini could be determined (E = –76 mV,
2
4
5
4
1
°
Ј
°Ј
°Ј
C H )] (2) was achieved according to Scheme 1. Reduction
E = 29 mV, E3 = 144 mV, E4 = 237 mV) (Figure 1). The
5
5
2
of commercially available ferrocenecarbaldehyde gave integrated peak area of 1:1:1:1 verifies the presence of four
[
38–40]
alcohol 1,
which upon subsequent treatment with closely spaced one-electron processes (Figure 1). The redox
°Ј
°Ј
°Ј
SiCl in the presence of pyridine afforded Si(OCH Fc) (2, separation (ΔE = 105 mV, ΔE = 115 mV, ΔE = 93 mV)
4
2
4
1
2
3
see the Experimental Section). After appropriate workup, of the symmetrically equal ferrocenyl moieties is caused
the title compound was isolated as a yellow solid in 67% either by intramolecular electronic interactions between the
yield (Scheme 1).
ferrocenyl/ferrocenium units or by electrostatic repulsion
Si(OCH Fc) (2) was characterized by IR, 1H NMR, between the oxidized ferrocenyl groups. The latter would
[46]
2
4
1
3
1
29
1
C{ H} NMR, and Si{ H} NMR spectroscopy and ESI be more favorable, because a π-conjugated bridging unit be-
1
mass spectrometry. The H NMR spectrum of 2 is charac- tween the redox-active Fc groups, which is beneficial for
2 4
Scheme 1. Preparation of Si(OCH Fc) (2).
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–1
4 6 5 4
Figure 1. (Left) Voltammograms of a 1.0 mmolL solution of 1 (top) and 2 (bottom) in dichloromethane containing [nBu N][B(C F ) ]
–1
–1
as the supporting electrolyte (0.1 molL ) at 25 °C (scan rate: 100 mVs ). (Right) Square-wave voltammogram (step height: 25 mV, pulse
width: 5 s, amplitude: 5 mV).
tributed to the structural properties of 2 excluding π conju-
gation and validates that mainly electrostatic interactions
are responsible for the observed redox splitting between the
n+
equally charged redox centers in 2 (n = 2, 3, 4). Therefore,
compound 2 can be classified as a class I system according
[
52]
to Robin and Day.
Thermal Characterization of 2
Ferrocene twin monomer 2 was characterized by
differential scanning calorimetry (Figure 3). Melting of 2
occurs as an endothermic process at T = 128 °C, followed
by a strong exothermic process at T = 210 °C. The nature
Figure 2. UV/Vis/NIR spectra of 2 at increasing potentials versus
Ag/AgCl (–200 to 700 mV). Measurement conditions: 25 °C,
dichloromethane, 0.1 molL–1 [nBu
4 6 5 4
N][B(C F ) ] as supporting
electrolyte (arrows indicating increasing potential).
intramolecular electron transfer, is nonexistent.[47] To prove
this, spectroelectrochemical UV/Vis/NIR experiments were
performed in an optically transparent thin layer electro-
[
48]
chemistry cell.
Stepwise increase in the potential (–200
n+
to 700 mV) allowed the in situ generation of cationic 2 (n
=
1, 2, 3, 4) from neutral 2 (Figure 2).
As expected, neutral 2 shows no absorption in the NIR
region (λ = 1000–3000 nm). Within successive oxidations,
increasing absorptions below λ = 750 nm (UV/Vis region)
were observed, including d–d transitions of the ferrocenyl
[
49]
substituents and ligand-to-metal charge transfer from the
[46]
cyclopentadienyl substituent to the iron atom. Addition-
–
1
–1
ally, a very weak excitation (ε Ͻ100 Lmol cm ) at λ = Figure 3. Thermogravimetric (black) and differential scanning
2
500 nm was detected, which can be assigned to the forbid- calorimetric (red) traces of 2 showing an exothermic process at T
=
210 °C with simultaneous weight loss (see the Supporting Infor-
den metal-centered ligand-field electronic transition (Fig-
mation for the complete DSC and TG traces). TG–MS (green)
study performed with 2 showing the evaporation of water (m/z =
ure 2). This excitation is a typical indication of the forma-
tion of a 17-valence electron ferrocenium species.[
50,51]
The
1
8) during the exothermic process starting at T = 210 °C (40–
–
1
–1
absence of an intervalence charge-transfer absorption is at- 800 °C, 10 Kmin , Ar 20 mLmin ).
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of the latter event was investigated by thermogravimetric formed raster electron microscopy/energy dispersive X-ray
analysis, which revealed the release of volatile materials spectroscopy (SEM/EDX) and transmission electron micro-
with Δm = 4.23%. This weight loss can be explained by a scopy (TEM) studies to determine the amount of incorpo-
thermally induced condensation reaction. The volatile rated silicon. It is clear that silicon is homogeneously incor-
products, which were formed during the heating process of porated within this hybrid material, which can be seen from
2, were identified by TG–MS coupling experiments (Fig- the EDX pattern (Figure 5). Given that the oxygen signals
ure 3). Between T = 200 and 230 °C, the evaporation of observed are homogeneously distributed over the total sam-
water (m/z = 18) was observed, which is indicative of the ple, we suggest that a silicon dioxide matrix is formed. The
condensation of 2.
TEM images of the examined polymer particle (Figure 5)
show overall homogeneous texturing with particles exhibit-
ing a higher contrast than the surrounding matrix. This is
most probably attributed to the formed silicon dioxide clus-
ters during the twin polymerization process. In comparison
with other twin polymers, for example, 2,2Ј-spirobi[4H-
Homopolymerization of 2
[
25]
Homopolymerization experiments were performed by 1,3,2-benzodioxasiline], the nanostructuring is more dis-
suspending 2 in diisopropylnaphthalene and by heating this tinctive, owing to the higher amount of silicon within this
mixture to 230 °C. After passing the melting point of 2 twin monomer.
(128 °C) a clear solution formed, which darkened just below
200 °C. While reaching the condensation temperature of 2
(
(
210 °C), water evaporated and a brown solid precipitated
see the Experimental Section for a detailed procedure).
The obtained hybrid material was further investigated by
1
3
1
solid-state C{ H} cross polarization magic angle spinning
CP-MAS) NMR spectroscopy (Figure 4). The signals for
the ferrocenyl C H and C H moieties are observed at
(
5
5
5
4
about δ = 90 and 70 ppm. In contrast to monomeric 2, no
resonance signal at δ = 62 ppm for the CH OSi groups
2
could be detected, which verifies complete conversion of 2.
The new signal at δ = 29 ppm can be assigned to the forma-
tion of a bridging FcCH Fc motif. A similar NMR reso-
2
Figure 5. (a) High-angle annular dark field scanning transmission
nance signal for this structural motif was observed for di- electron microscopy (HAADF-STEM) image of the obtained hy-
[
53]
brid material from 2 by thermal polymerization, and (b) the SEM
image and (c) EDX analysis showing the homogeneous distribution
of silicon within the sample.
ferrocenylmethane.
Copolymerization of 2 with 2,2Ј-Spirobi[4H-1,3,2-
benzodioxasiline]
The copolymerization of 2 and SBS was performed un-
der acidic conditions with MeSO H, as thermal initiation
3
was not suitable owing to the higher polymerization tem-
[
54]
perature of SBS (240 °C). Therefore, SBS and 2 were dis-
solved in dichloromethane, and the yellow mixture was
heated to reflux. Then, the initiator dissolved in dichloro-
methane (1 mL) was added, and the solution became turbid
and a pale green solid precipitated.
After appropriate workup, the obtained hybrid material
was investigated by 13C{ H} CP-MAS NMR and Si{ H}
CP-MAS NMR spectroscopy. All expected resonance sig-
nals for the phenolic resin were observed, as typical for a
1
29
1
Figure 4. 13C{ H} CP-MAS NMR spectrum of the hybrid material
obtained by thermal twin polymerization of [black line,
3 4
00.6 MHz, 20 kHz, reference: Si(SiMe ) ] compared to the twin polymer derived from SBS (see the Supporting Infor-
1
2
1
1
3
1
C{ H} NMR spectrum of 2 in CDCl
mental Section for detailed assignment of the signals).
3
(red line) (see the Experi-
[25]
29
1
mation). The Si{ H} CP-MAS NMR spectrum showed
Q and Q signals, which are characteristic of the formed
3
4
[
25]
SiO network.
However, the carbon signal of the ferro-
2
The solid-state NMR characterization of the incorpo- cenyl component could not be seen, as during the twin po-
rated SiO network was not possible, as the amount of sili- lymerization process paramagnetic species were formed
2
con within 2 was too small (3.2%). Accordingly, we per- from 2. These paramagnetic impurities have an influence
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on proton decoupling, which results in spectra with lower 7.1%, silica compound: 13.1%). The iron distribution
[
55,56]
resolution.
The incorporation of ferrocenyl units dur- within the carbon and silica matrices is homogeneous, as
ing the copolymerization process was confirmed by atomic EDX analysis showed no agglomeration. The incorporated
absorption spectroscopy (AAS) measurements and an iron nanoparticles were detected by TEM studies (Figure 7). The
content of 3.8% was found. This value was also supported HAADF-STEM images of the carbon material exhibit z
by EDX experiments (3.6%) and a homogeneous distribu- contrasting, which is caused by heavier elements such as
tion over the total sample was achieved (see the Supporting iron.
Information).
Preparation of Iron Oxide Containing Porous Carbon/Silica
Materials
The hybrid material obtained by acid-initiated copoly-
merization of 2 and SBS was carbonized under a constant
flow of argon and then oxidized in air to obtain the inor-
ganic silicon dioxide matrix or it was heated at reflux with
a 5 m sodium hydroxide solution to remove the inorganic
SiO network to give a porous carbon material (see the Sup-
2
porting Information for a detailed procedure).
The porous nature of the carbon and silica material was Figure 7. HAADF STEM image of (a) the carbon and (b) the silica
material. Bright-field TEM images of the specific materials are dis-
investigated by N2 adsorption/desorption isotherms. An
played as insets (scale bar inset a: 20 nm, scale bar inset b: 100
nm).
IUPAC type I isotherm[ for the carbon material was ob-
57]
tained, which indicates a mainly microporous nature of this
sample (Figure 6). The silica material exhibits a type IV iso-
therm according to IUPAC[57] with a small hysteresis, which
is mostly caused by capillary condensation within mesopo-
Within standard bright-field TEM images (inset, Fig-
ure 7) some darker structures are observed, however, with
no separation to the surrounding material, which makes
particle-size determination inaccurate. The same observa-
tion was made for the silica material. The bright-field TEM
images exhibit a homogeneous pattern, and the HAADF
images reveal some separated areas with a higher z contrast,
which could probably refer to iron oxide rich areas. As no-
ticed above for the nanoparticles incorporated within the
porous carbon material, particle-size determination is also
not possible for the silica material, but the observable struc-
tures are Յ10 nm in both materials.
rous structures. The surface areas and pore volumes were
calculated by using DFT methods.[58] The measured values
are given in Figure 6 and are lower than those of the pure
carbon and silica materials obtained from the SBS twin
polymer. This behavior can be explained by the incorpo-
rated iron oxide nanoparticles, as they are mostly situated
within the porous structure and, therefore, lower the surface
[
37]
area.
The oxidation state of the incorporated iron could not be
determined by X-ray powder diffraction (XRPD) studies, as
the particles are amorphous (see the Supporting Infor-
mation). Therefore, Mössbauer experiments were per-
formed, and the results thereof are shown in Figure 8. The
fitting of both signals was done by applying Lorentzian
curves to the experimental data. The nanoparticles within
the matrix are rather small, as indicated by TEM, and have
a non-negligible surface area, which causes line broadening
(different iron sites on the surface that contribute to the
overall spectrum). Therefore, the herein presented fits are
[
59]
approximations.
However, comparing the obtained iso-
–1
mer shifts of the carbon sample (0.32 mms ) with literature
results, they would fit to either Fe C (Fe C = 0.29 mms )
or Fe O (0.37 mms ). With the obtained parameters
a quadrupole splitting of 0.71 mms was determined) it is
–1
Figure 6. N
2
adsorption/desorption isotherm of the porous carbon
3
3
[
60]
–1
material measured at 77 K. (Inset) Table with the obtained surface
areas and pore volumes for the as-prepared carbon and silica mate-
rials (see the Supporting Information for further isotherms and
pore-size distributions).
2 3
–1
(
[61,62]
not possible to distinguish between Fe O and Fe O .
2
3
3
4
Hence, no conclusion can be made if the incorporated
AAS experiments proved an iron content of 7.1% for the nanoparticles are Fe O , Fe O , and/or Fe C.
2
3
3
4
3
carbon material and an iron content of 15.0% for the silica
Owing to preparation under an oxidizing atmosphere
material. These values were confirmed by EDX measure- and at elevated temperatures, the incorporation of iron ox-
ments (see the Supporting Information, carbon material: ides (Fe O or Fe O ) within the sample is more likely.
2
3
3
4
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Figure 8. Results of the Mössbauer experiments for the carbon (left) and silica sample (right) and the fitting of the experimental data
IS: isomer shift, QS: quadrupole splitting, FWHM: full width at half maximum). The very wide line widths of the Mössbauer spectra
(
are due to the very small size of the nanoparticles.
Figure 9. Magnetization curves obtained by SQUID measurements for the carbon (left) and silica samples (right, measured at 300 K).
The magnetic behavior of these materials was investi- ticles (200 nm in diameter) were functionalized with meth-
gated by superconducting quantum interference device anesulfonic acid and were suspended in toluene. After the
(
SQUID) experiments (Figure 9). The silica material addition of the monomer mixture, the substrate suspension
[63,64]
showed superparamagnetic behavior.
In contrast to changed its color to green, and upon stirring overnight the
this, the curve progression of the carbon material indicates coated templates precipitated. The thus-prepared material
paramagnetic behavior that does, however, not follow the was carbonized under an argon atmosphere at 800 °C for
Brillouin function. Because superparamagnetic iron carbide 2 h, and after cooling it to ambient temperature, the incor-
nanoparticles were obtained by carbonization of ferro- porated SiO was removed by heating the carbonized mate-
2
[
8]
[65]
cene or FeCl3, this is an indication that the silica mate- rial at reflux in 5 m NaOH (Scheme 2, see also the Support-
rial contains Fe C. On the basis of the Mössbauer results, ing Information).
3
no explanation can be given for the differences in magnet-
The obtained carbon capsules were characterized by N2
ism. The different possible iron species cannot be distin- adsorption/desorption isotherms. At first glance, the ob-
guished and the formation of superparamagnetic nanopar- served isotherm looks like a type I isotherm according to
[
57]
ticles is possible in all cases. Furthermore, the thus-pre- IUPAC (Figure 10), which is caused by the mainly micro-
pared particles are not crystalline, which will also influence porous carbon obtained from the twin polymer of SBS.
their magnetic behavior.[
8,65]
However, considering the desorption part of the observed
isotherm, a small hysteresis typical for type IV isotherms[
can be noticed. This is probably caused by cavities formed
between connecting capsules.
57]
Preparation of Iron Oxide Containing Porous Carbon
Capsules
2
–1
The calculated surface area (915 m g ) and the obtained
3
–1
The coating abilities of 2 and SBS upon spherical silica pore volume (0.42 cm g ) are smaller than the values for
templates were investigated.[ Therefore, plain silica par- the reference material (surface area = 1180 m g , pore vol-
27]
2 –1
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2 3
Scheme 2. Preparation of Fe O -filled porous carbon capsules.
Figure 10. (Left) N
2
adsorption/desorption isotherms of the obtained carbon capsules. (Right) Bright-field TEM images showing the
prepared porous carbon capsules with the incorporated nanoparticles.
3
–1
ume: 0.48 cm g ) derived by using only 2,2Ј-spirobi[4H- This is most probably permitted by the finite thickness of
,3,2-benzodioxasiline] as the twin monomer. the carbon hull, as within the bulk material the incorpo-
TEM studies reveal the formation of spherical carbon rated nanoparticles are even better separated. An iron
1
capsules, but otherwise, as expected, the incorporated nano- content of 5% was determined by AAS measure-
particles are found inside the capsules and not in the hull. ments.
–
1
–1
Figure 11. Results of the Mössbauer experiments with fitting of the experimental data (IS = 0.31 mms , QS = 0.72 mms , FWHM =
0
–1
.30 mms ) for the iron-containing carbon capsules (left) and the respective SQUID experiments (right).
Eur. J. Inorg. Chem. 2015, 3850–3860
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Additional Mössbauer and SQUID experiments showed catalytically active metal nanoparticles, such as Pd and Pt,
that the incorporated nanoparticles are of the same nature will be studied. Such materials are of interest as catalysts
as those obtained by copolymerization of 2 with SBS and, in, for example, C–C cross-coupling reactions.
therefore, also exhibit paramagnetic behavior (Figure 11).
Experimental Section
Conclusions
General: All reactions were performed under an atmosphere of ar-
gon by using standard Schlenk techniques. Dichloromethane was
purified by distillation from calcium hydride. n-Hexane was taken
from an M. Braun SBS-800 purification system (stationary drying
with columns filled with molecular sieves, 3 Å). In addition, pyr-
The preparation and characterization of a ferrocene-con-
5
taining twin monomer of the Si(OCH Fc) [Fc = Fe(η -
2
4
5
C H )(η -C H )] (2) type was presented. Compound 2 was
5
4
5
5
2
synthesized by reaction of FcCH OH (4 equiv.) with SiCl4
in the presence of pyridine. Silicon orthoester 2 was electro- idine was dried by heating with potassium hydroxide at reflux and
chemically characterized and showed four reversible redox was then distilled and stored over molecular sieves (3 Å). Ferro-
°
Ј
°Ј
°Ј
processes (E = –76 mV, E = 29 mV, E = 144 mV, and E
cenecarbaldehyde and diisopropylnaphthalene were purchased
1
2
3
°
Ј
[39]
from commercial suppliers; alcohol 1 and 2,2Ј-spirobi[4H-1,3,2-
=
237 mV). Further investigation by UV/Vis/near-IR
4
benzodioxasiline][ were prepared according to published pro-
25]
spectroelectrochemistry proved that the redox separation
was caused be electrostatic repulsion of the oxidized species.
The thermal behavior of 2 was investigated by differential
scanning calorimetry (DSC), thermogravimetry (TG), and
coupled TG–mass spectrometry (TG–MS). DSC revealed a
strong exothermic peak at 210 °C, which could be verified
cedures. 1H NMR (500.3 MHz) and C{ H} NMR (125.8 MHz)
spectra were recorded with a Bruker Avance III 500 spectrometer
at 298 K. Chemical shifts are reported in parts per million using
13
1
6
undeuterated solvent residues as an internal standard ([D ]benzene:
1
13
1
13
1
H, 7.16 ppm and C{ H}, 128.06 ppm). Solid-state C{ H} NMR
experiments were conducted by using cross polarization (CP) and
by TG and TG–MS experiments as thermally induced con- magic-angle spinning (MAS) with spinning frequencies of 12.5 and
densation. As a consequence thereof, 2 was polymerized un- 20 kHz. Infrared spectra were recorded with a FTIR Nicolet 200
der thermal conditions in a high-boiling solvent (220 °C, spectrometer. The melting point of 2 was determined with a Gallen-
kamp MFB595010M melting point apparatus. Microanalysis was
diisopropylnaphthalene) to give a brown solid material. In-
1
3
1
performed with a Thermo FLASHEA 1112 Series instrument. The
mass spectrum of 2 was recorded with a Bruker micrOTOF QII
system. SEM images and EDX analyses were performed with a
Phillips NanoNovaSEM. TEM images were recorded with a Fecnai
FC20 equipment with 200 kV accelerating voltage. Gas adsorption/
desorption measurements were performed by using an Autosorb
IQ2 system. Thermal analyses were performed with a Mettler To-
vestigation of this material by solid-state C{ H} cross po-
larization magic angle spinning NMR spectroscopy re-
vealed the transformation of the orthoester moiety of the
monomer (CH OSi) into a FcCH Fc motif, as indicated by
2
2
the resonance signal at δ = 29 ppm. The incorporated sili-
con dioxide network, which was formed during the polyme-
rization process, could not be observed by solid-state NMR ledo TGA/DSC1/1600 system with a MX1 scale. Thermal carbon-
techniques, as a result of its low concentration but was con- izations and oxidations were performed in a Carbolite Tube Fur-
firmed by energy dispersive X-ray spectroscopy measure- nace MTF 12/38/400.
ments. As shown by transmission electron microscopy
images, the typical microstructuring of the twin polymers
was not observed, which is also a consequence of the low [nBu N][B(C F ) ] as supporting electrolyte were conducted under
Electrochemistry: Measurements on 1.0 mmolL–1 solutions of the
analyte in anhydrous dichloromethane containing 0.1 molL of
–1
[
25]
4
6 5 4
amount of silicon. However, 2 underwent thermally in- a blanket of purified argon at 25 °C by utilizing a Voltalab 10 elec-
duced twin polymerization, and we suggest that this mono- trochemical laboratory. A three electrode cell, which included a Pt
auxiliary electrode, a glassy carbon working electrode (surface area
mer type should be considered as a borderline system for
2
+
–1
3
0.031 cm ), and an Ag/Ag (0.01 molL AgNO ) reference elec-
twin polymerization. The copolymerization of 2 and 2,2Ј-
spirobi[4H-1,3,2-benzodioxasiline] (SBS) under acidic con-
ditions was investigated, which produced a pale-green solid
material. After carbonization and oxidation or removal of
trode, mounted on a Luggin capillary was used. The working
electrode was pretreated by polishing on a Buehler microcloth
first with 1 μm and then 1/4 μm diamond paste. The reference elec-
trode was built from a silver wire inserted into a solution of
SiO with sodium hydroxide, a porous carbon or silica ma-
–1
–1
2
3 4 6 5 4
0.01 molL [AgNO ] and 0.1 molL [nBu N][B(C F ) ] in aceto-
terial was obtained. Both materials have a rather high sur-
nitrile in a Luggin capillary with a Vycor tip. This Luggin capillary
2
–1
2 –1
face area (858 m g , carbon; 555 m g , silica). Attempts was inserted into a second Luggin capillary with a Vycor tip
–
1
were made to determine the identity of the incorporated filled with a 0.1 molL dichloromethane solution of [nBu
iron-containing nanoparticles by Mössbauer studies, which [B(C F ) ].
6 5 4
4
N]-
[43,44,47,66–71]
Successive experiments under the same ex-
perimental conditions showed that all formal reduction and oxid-
showed that this was not possible for both materials. Super-
conducting quantum interference device studies revealed
the superparamagnetic behavior of the nanoparticles incor-
porated in the silica material, whereas the carbon com-
pound remained paramagnetic. The monomeric mixture of
ation potentials were reproducible within Ϯ5 mV. Experimental po-
+
tentials were referenced against an Ag/Ag reference electrode, but
[
72,73]
the results are presented referenced against ferrocene
[FcH/
+
+
FcH couple = 220 mV vs. Ag/Ag , ΔE
p
= 61 mV; FcH =
5
[74]
Fe(η -C
decamethylferrocene [Fc* = Fe(η -C
standard, the experimentally measured potential was converted
5 5 2
H ) ] as an internal standard, as required by IUPAC. If
2
and SBS was further used for the incorporation of iron-
containing nanoparticles within porous carbon spheres.
The thus-accessible magnetic carbon materials will be into E vs. FcH/FcH+ (under our conditions the Fc*/Fc* couple
5
5
5 2
Me ) ] was used as an internal
+
+
[72]
further investigated, and specifically the incorporation of was at –614 mV vs. FcH/FcH , ΔE
p
= 60 mV). Data were then
Eur. J. Inorg. Chem. 2015, 3850–3860
3857
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
manipulated on a Microsoft Excel worksheet to set the formal
redox potentials of the FcH/FcH couple to E°Ј = 0.00 V. The cy-
clic voltammograms were taken after typical two scans and are con-
sidered to be steady-state cyclic voltammograms in which the signal
pattern differs not from the initial sweep.
in dichloromethane (0.8 mL) was added by syringe in a single por-
tion. The solution became turbid and a pale-green solid precipi-
tated after heating for 12 h at reflux. The solid material was col-
lected by filtration and was washed with dichloromethane
(2ϫ20 mL). After drying in vacuo, a pale-green-colored solid
+
(
3.1 g, 74%) was obtained.
Template Coating with 2 and SBS: The spherical SiO
1.0 g, 16.6 mmol) was suspended in dichloromethane (200 mL)
UV/Vis/NIR Spectroelectrochemistry: The spectroelectrochemical
measurements of compound 2 (2.0 mm) in anhydrous dichloro-
methane containing [nBu N][B(C F ) ] (0.1 m) as the supporting
4 6 5 4
2
template
(
and was sonicated until a homogeneous suspension was formed.
Afterwards, methanesulfonic acid (0.16 mL, 2.5 mmol) was added
in a single portion, and the mixture was stirred for 15 min at ambi-
ent temperature. Then, the solvent was evaporated in vacuo, and
the remaining solid was suspended in toluene (200 mL) and soni-
cated until a homogeneous suspension was formed. Then, a mix-
ture of SBS (2.0 g, 7.3 mmol) and 2 (0.8 mg, 0.9 mmol) dissolved
in toluene (50 mL) was added dropwise. After 15 min of stirring
at ambient temperature the suspension turned pale green, which
indicated the beginning of the twin polymerization. Stirring was
conducted for 8 h, whereby the hybrid material-coated templates
precipitated. They were collected by filtration and washed with
dichloromethane (3ϫ50 mL) and were then dried in vacuo to yield
a pale-green solid (3.7 g, 96%).
electrolyte were performed in an optically transparent thin-layer
electrochemistry cell with quartz windows (UV/Vis/NIR)[
75]
at
2
5 °C. Between the spectroscopic measurements, the applied poten-
tials were stepwise increased by using step heights of 25, 50, and
00 mV. At the end of the measurements, the analyte was reduced
1
at –500 mV for 15 min and an additional spectrum was recorded
to prove the reversibility of the oxidations.
57Fe Mössbauer Spectrometry: 57Fe Mössbauer spectra were re-
corded in transmission geometry at a constant acceleration by
using a conventional Mössbauer spectrometer with a 50 mCi
5
7
Co(Rh) source. The samples were sealed in the sample holder
under an argon atmosphere. The spectra were fitted by using Recoil
76]
1
.05 Mössbauer Analysis Software.[ The isomer shift values are
given with respect to an α-Fe reference at room temperature.
2
Carbonization and SiO Removal: In a typical procedure, the spe-
Magnetic Measurements: Magnetization data were collected with a
Quantum Design MPMSR-2 SQUID magnetometer over an ap-
plied field range of –3 to 3 T at 300 K in the settle mode. The
samples were placed in gelatin capsules held within a plastic straw.
The data were corrected for the diamagnetic magnetization of the
sample holder.
cific compound (SBS and 2, or respective coated templates, 2.0 g)
was placed in a tube furnace and carbonized at 800 °C (50 Kmin )
for 15 min under continuous flow of argon. Removal of the incor-
–1
porated SiO
achieved by heating the carbonized material in 5 m NaOH (50 mL)
at reflux for 8 h. Afterwards, the SiO -free compounds were filtered
2
(from the twin monomer and the template) was
2
off and washed with hot water (300 mL, until pH≈ 7 was reached)
and then with deionized water (50 mL) followed by acetone
Synthesis of Tetraferrocenylmethoxysilane (2): A mixture of 1
(
3.01 g, 13.95 mmol) and pyridine (1.47 g, 18.6 mmol, 1.5 mL) was
dissolved in dichloromethane (20 mL). This mixture was cooled to
°C with an ice/brine bath. Afterwards, silicon tetrachloride
0.39 g, 2.3 mmol, 0.27 mL) diluted in dichloromethane (10 mL)
(
30 mL) and diethyl ether (50 mL). The respective carbon materials
were dried in vacuo.
0
(
General Oxidation Procedure: In a typical experiment, the appro-
priate hybrid material was transferred to an aluminum oxide cruci-
ble and placed in a quartz tube furnace. Under continuous flow of
was added dropwise with a syringe within 15 min whilst stirring.
The formed suspension was warmed to ambient temperature,
stirred for 1 h, and then all of the volatiles were removed in vacuo.
The resulting yellow solid was transferred to a Soxhlet extractor
and was extracted with absolute n-hexane until the extract was col-
orless. The remaining solution was then cooled to –35 °C for 12 h
and the precipitating microcrystalline product was collected and
–
1
Ar, the material was heated to 800 °C (50 Kmin ) and kept at this
temperature for 15 min. After cooling to ambient temperature, the
sample was heated again under continuous flow of air to 450 °C
–
1
(50 Kmin ) and kept at this temperature for 48 h, which resulted
in an orange-colored solid.
washed with n-hexane (3ϫ10 mL) to give a bright yellow solid
Supporting Information (see footnote on the first page of this arti-
cle): Detailed synthetic procedures and additional analytical data,
additional solid-state NMR spectra, TG traces, NIR spectra, TEM/
1
(
1.36 g, 67%), m.p. 121 °C. H NMR (CDCl
3
c
): δ = 4.47 (s, 2 H,
c
CH
2
), 4.24 (pt, 2 H, C
5
H
4
), 4.14 (pt, 2 H, C
) ppm. C{ H} NMR (CDCl ): δ = 86.15 (ipso C
), 68.38 (cC
), 62.02 (OCH
Si{ H}-DEPT NMR (CDCl ): δ = –82.60 ppm. IR (KBr): ν˜ =
074 w [ν(CHarom.)], 2945 w [νas(CH
5 4
H ), 4.12 (s, 5 H,
c
13
1
c
C
5
H
5
3
5
H
4
), 69.11
2
REM-EDX images, and N adsorption/desorption measurements.
c
c
(
C
5
H
4
), 68.59 ( C
5
H
5
5
H
4
2
) ppm.
2
9
1
3
3
2
)], 2883 w [ν
s
(CH
)], 1467 m Acknowledgments
2
–1
[
δ(CH
for C44
888.30): calcd. C 59.49, H 4.99; found C 59.25, H 4.98.
2
)], 1104 m [ν (C–O)], 1036 s [ν (Si–O)] cm . HRMS: calcd.
+
H
44Fe
4
O
4
Si [M] 888.0406; found 888.0360. C44
H
44Fe
4
O
4
Si The authors are grateful to the Deutsche Forschungsgemeinschaft
(DFG) (FOR 1497 Organic-Inorganic Nanocomposites by Twin
Polymerization) for generous financial support. T. Jagemann and
(
Thermal Polymerization of 2: Twin monomer
2 (0.72 mg,
Prof. Dr. M. Hietschold are thanked for SEM imaging of different
samples, Dr. C. Georgi and J. Noll are thanked for carrying out
the thermogravimetric measurements, B. Kempe and Dr. R. Busch-
beck are thanked for mass spectrometry measurements, Dr. A. Sei-
fert is thanked for the solid-state NMR spectroscopy analysis, and
A. Kirillova is thanked for preparing the Stöber particles.
0.82 mmol) was suspended in diisopropylnaphthalene (20 mL) and
upon heating a clear solution was obtained. Heating was continued
to 230 °C, and after passing 210 °C a brown solid precipitated. The
mixture was kept at this temperature for 15 min and then cooled
to ambient temperature. The formed solid was filtered off and
washed with dichloromethane (3ϫ30 mL) and dried in vacuo to
yield the product (0.39 mg, 54%).
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Published Online: July 14, 2015
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