Encapsulation of chiral Fe(salen) in mesoporous silica structures for use as catalysts to produce optically active sulfoxides

Article abstract of DOI:10.1039/c6cy00113k

Solid catalysts which are heterogeneous at the macroscopic scale but homogeneous at the microscopic level were prepared by the encapsulation of Fe(salen) by a "ship in a bottle" approach. This approach permits the synthesis of a "free" Fe(salen) complex inside the nanocages of SBA-16 and m-MCF, having conformational freedom and behaving as a complex in solution. These materials were used as catalysts for asymmetric oxidation of sulfides. The entrance sizes of the mesoporous materials SBA-16 and m-MCF were tuned by changing the synthesis parameters and by silylation of the silica surface with n-propyl groups, which resulted in materials with different Fe(salen) loadings. Chiral Fe(salen) trapped in m-MCF materials showed higher activity than the complex immobilized on SBA-16. The activity and enantioselectivity of the catalysts based on m-MCF were on a par with the homogeneous counterpart under specific conditions. The heterogenized catalysts presented a limited recyclability; however, they were clearly advantageous compared to the homogenous counterpart, where reutilization was not possible.

Full text of DOI:10.1039/c6cy00113k

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Encapsulation of chiral Fe(salen) in mesoporous
silica structures for use as catalysts to produce
optically active sulfoxides†
Cite this: DOI: 10.1039/c6cy00113k
ab
a
a
a
Rafael L. Oliveira, Tom Nijholt, Mozaffar Shakeri, Petra E. de Jongh,
c
a
Robertus J. M. Klein Gebbink and Krijn P. de Jong*
Solid catalysts which are heterogeneous at the macroscopic scale but homogeneous at the microscopic
level were prepared by the encapsulation of Fe(salen) by a “ship in a bottle” approach. This approach per-
mits the synthesis of a “free” Fe(salen) complex inside the nanocages of SBA-16 and m-MCF, having con-
formational freedom and behaving as a complex in solution. These materials were used as catalysts for
asymmetric oxidation of sulfides. The entrance sizes of the mesoporous materials SBA-16 and m-MCF
were tuned by changing the synthesis parameters and by silylation of the silica surface with n-propyl
groups, which resulted in materials with different Fe(salen) loadings. Chiral Fe(salen) trapped in m-MCF ma-
terials showed higher activity than the complex immobilized on SBA-16. The activity and enantioselectivity
of the catalysts based on m-MCF were on a par with the homogeneous counterpart under specific condi-
tions. The heterogenized catalysts presented a limited recyclability; however, they were clearly advanta-
geous compared to the homogenous counterpart, where reutilization was not possible.
Received 16th January 2016,
Accepted 22nd February 2016
DOI: 10.1039/c6cy00113k
www.rsc.org/catalysis
thesized an Fe(salan) complex that showed good yield and
stereoselectivity when H O was used as an oxidant.
2 2
Introduction
7
Asymmetric oxidation of sulfides is an important method to
produce optically active sulfoxides. These sulfoxides are
widely used as chiral auxiliaries in asymmetric synthesis and
Despite the catalytic performance of some chiral metal
complexes, most of them have not been applied on an indus-
trial scale yet. The major problems are the difficulty in sepa-
ration, recycling and contamination with the metal of the de-
sired products. Heterogeneous catalysts have attracted much
attention due to their advantages, easy separation from the
1
as bioactive molecules in the pharmaceutical industry, for ex-
ample, drugs for treatment of platelet aggregation, neurode-
2
generative disorders, peptic ulcer diseases and others. In the
8
last few decades, several methods to produce enantiopure
sulfoxides have been described; the use of biocatalysts and
products and recyclability. Thus, a lot of energy has been
spent on the immobilization of metal complexes on solid
3
9
chiral metal complexes are the most applied methodologies.
supports, such as silica, polymers and carbon. However,
Chiral metal complexes based on titanium, vanadium and
manganese have been extensively investigated. In contrast,
heterogeneous catalysts can suffer from degradation of the
immobilized complexes and metal leaching, limiting their re-
4
1
0
chiral iron complexes have been less explored for this reac-
tion. Recently, much effort has been done to produce iron
cyclability. For example, Basset et al. studied a silica-
supported zirconium complex applied on transesterification
of acrylates and observed a considerable deactivation of the
catalyst due to exchange of ligands during catalytic perfor-
5
complexes for asymmetric oxidation of sulfides since iron
shows advantages such as low toxicity and earth abundance.
Bryliakov et al. showed that Fe(salen) complexes can be used
as catalysts for the asymmetric oxidation of sulfides; however,
reasonable stereoselectivity was only obtained when iodo-
1
1
mance and also leaching of zirconium.
Chiral complexes can be immobilized on solid matrixes
using various methods, such as grafting, electrostatic interac-
tion, sol–gel methods and encapsulation through a “ship in
6
sylbenzene was used as an external oxidant. Egami et al. syn-
1
2
the bottle” approach. In the “ship in the bottle” approach,
a
Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science,
metal complexes are usually synthesized inside porous mate-
1
3
Utrecht University, Netherlands. E-mail: k.p.dejong@uu.nl
rials such as zeolites. The potential advantage of this ap-
proach is that the complex does not suffer modification, hav-
ing the possibility to keep the same catalytic activity of the
homogeneous counterpart. However, zeolites present some
limitations as supports due to their small cage sizes and
b
Institute of Chemistry, State University of Campinas, Brazil
c
Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science,
Utrecht University, Netherlands
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c6cy00113k
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restricted window sizes, resulting often in materials with
lower activity and stereoselectivity. Ordered mesoporous sil-
ica (OMS) has been shown as an alternative due to its larger
entrance size and pore structure, permitting encapsulated
complexes with more conformational freedom and behaving
20 hours followed by 24 hours at different temperatures 50°,
60°, 70°, and 80 °C. The solid product was collected by filtra-
tion, washed with distilled water, dried at 60 °C for 24 h and
calcined at 550 °C in static air for 6 hours. The synthesized
and calcined materials will be referred to as ‘pristine mate-
rials’ in this paper. The synthesized materials were desig-
nated as MCF-X, where X is the hydrothermal treatment
temperature.
1
4
as a free complex in solution. The most applied OMS for
the “ship in the bottle” synthesis is SBA-16; nevertheless, it is
difficult to obtain this material with large cage sizes (>7 nm)
and small entrance sizes (∼2 nm). Recently, our group
reported the synthesis of a new silica material called modi-
fied mesocellular foam (m-MCF), which combines large cage
sizes (15–22 nm) and narrow entrance sizes (1.8–3.7 nm),
showing properties that are very interesting for application in
SBA-16 synthesis. The synthesis of SBA-16 was done, fol-
1
9
lowing the procedure described by Kim et al. The structural
properties of SBA-16 were tuned by changing the composition
of the mixture between F127 and P123, the hydrothermal
temperature and time. Briefly, an aqueous solution of poly-
mers was prepared in hydrochloric acid in distilled water
overnight. Then, TEOS was added to the mixture at 35 °C un-
der vigorous stirring for 15 minutes until it was completely
dissolved. After this, the mixture was placed in an oven for
24 h under static conditions at 35 °C and the mixture was
further maintained at 80 °C or 100 °C for hydrothermal treat-
ment during different periods. Subsequently, the samples
were calcined at 823 K for 6 hours. The samples are desig-
nated as SBA-16-X-T-t, were X (A, B, C) stands for the compo-
sition of the mixture of polymers, T is the hydrothermal tem-
perature and t is the hydrothermal treatment time.
1
5
the “ship in the bottle” approach.
In this contribution, we encapsulated an Fe(salen) com-
plex in modified m-MCF and SBA-16 materials with different
entrance and cage sizes. m-MCF and SBA-16 surfaces were
functionalized with n-propyl groups which allowed us to tune
their entrance and cage sizes. The obtained hybrid materials
were conveniently characterized to appraise the influence of
their different structures on the loading of the Fe(salen) com-
plex, and were evaluated as catalysts in asymmetric sulfo-
xidation, using the oxidation of thioanisole as a model
reaction.
The starting molar composition was 0.0040F127: 1.0TEOS:
Experimental section
Chemicals
4.0HCl: 130H
1
0
2
O
for SBA-16 A; 0.00084P1230: 0038F127:
.0TEOS : 4.2HCl : 137H O for SBA-16 B, and 0.0016P123:
.0037: 1.0TEOS : 4.4HCl: 144H O for SBA-16 C.
2
2
Triblock copolymer polyIJethylene oxide)–polyIJpropylene
oxide)–polyIJethylene oxide) (P123), Pluronic F-127, tetraethyl
orthosilicate (TEOS, 98%), n-propyltriethoxysilane (97%),
mesitylene (TMB, 99%), styrene (99.5%), ethylene glycol
Functionalization of the silica surface with n-propyl
Functionalization was conducted following the procedure de-
scribed by Shakeri et al.
(
99%), methyl phenyl sulfoxide (97%) and ironIJIII) chloride
hexahydrate (FeCl ·6H O, 97%) were purchased from Sigma
Aldrich. Hydrogen peroxide (H , 35 wt% in H O), acetoni-
16a
700 mg of m-MFC or SBA-16 was
3
2
dried for 5 h at 140 °C to remove physisorbed water. Then,
the solid was dispersed in 25 mL of dry toluene, and 2.5 mL
of n-propyl triethoxysilane was added to the mixture dropwise
over 5 minutes under vigorous stirring, followed by the addi-
tion of 1 mL of n-butylamine. The mixture was heated to 110
O
2 2
2
trile (CH CN, HPLC), thioanisole (PhSMe, 99%), and methyl
3
sulfoxide (DMSO, 99.7%) were purchased from Acros Or-
ganics. Benzyl phenyl sulfide (PhSCH Ph, 98%), 3,5-di-tert-
2
butyl-2-hydroxybenzaldehyde (99%) and 1-butylamine (99%)
were purchased from Alpha Aesar. (1R,2R)-Diamino-
cyclohexane (99%) was purchased from ABCR. Hydrochloric
acid (HCl, 37%) was purchased from Emsure. Methanol
°
C and stirred for 48 hours under nitrogen. The obtained ma-
terials were filtered off and washed with toluene, toluene :
ethanol (1 : 1) and ethanol, and dried at 60 °C for 24 hours.
The functionalized materials are named by placing the letter
F in front of the silica material's name.
(MeOH, 100%), ethanol (EtOH, 100%), dichloromethane
(DCM, 99.5%), chloroform (CHCl , 99.5%) and toluene
(100%) were purchased from Interchema.
3
Functionalization of the silica surface with different
organosilanes (entrance size determination)
Synthesis of materials and heterogenized Fe-salen complexes
m-MCF synthesis. For m-MCF synthesis, 4 g of copolymer
Pluronic P123 was dissolved in an aqueous acidic solution
150 ml, 1.6 M HCl) in a 500 mL polypropylene bottle at room
In this procedure, 100 mg of m-MCF or SBA-16 was dried at
140 °C for 5 h to remove physisorbed water. Then, the solid
was dispersed in 3.5 mL of dry toluene and 1.72 mmol of
organosilane with different carbon lengths was added,
followed by the addition of 0.15 mL of n-butylamine. The mix-
ture was heated to 110 °C and kept under stirring for 48
hours under nitrogen. The obtained materials were filtered
off and washed with toluene, toluene : ethanol (1 : 1) and eth-
anol, and dried at 60 °C for 24 hours.
(
temperature overnight. Then, 3 grams of TMB (mesitylene)
was added to the reaction mixture at 35 °C dropwise and
stirred vigorously for 2 h. After this period, 17 grams of TEOS
−
1
was slowly added to the mixture (1.5 mL min ) and stirred
vigorously for 5 minutes. After this period, the mixture was
kept under static conditions in the closed bottle at 35 °C for
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Ship in a bottle synthesis. This procedure was conducted
via a method described by our group recently. The n-propyl
functionalized m-MCF or SBA-16 (700 mg) was dried at 120
Sulfoxidation procedure
1
5
Synthesis of iodosylbenzene (PhIO). The synthesis was
based on Saltzman et al. Briefly, 4.025 g (12.5 mmol) of (di-
acetoxyiodo) benzene was added to aqueous NaOH (3 M,
22
°
C for 6 hours under vacuum (−1.0 bar). Then, 150 mg of 3,5-
di-tert-butyl-2-hydroxybenzaldehyde was dissolved in 2 mL of
CH Cl and added to the solid. The mixture was kept under
1
8.75 mL) over a period of 5 min. The lumps of solids formed
were triturated over a period of 15 min and the reaction mix-
ture was stirred for another 45 min. Then, 12.5 mL of H
was added to the solution and the mixture was left to stir for
0–45 min. Büchner filtration was applied to obtain greenish
solids. The solids were returned to the beaker and triturated
in 50 mL of H O, followed by another Büchner filtration. A fi-
2
2
stirring for 2 days under reflux at 40 °C under a N2 atmo-
sphere. Then, 37 mg of (1R,2R)-diaminocyclohexane was
added to the mixture. The mixture was stirred again for 2
2
O
3
days at 40 °C under a N atmosphere. After this period, the
2
solvent was removed by static vacuum. Then, 175.21 mg (2
equiv.) of FeCl ·6H O was dissolved in 3 mL of methanol and
3 2
added to the solid. The system was stirred at RT for 24 h.
The mixture was centrifuged (4000 rpm, 5 min) and the sol-
vent was removed by decantation. The remaining solid was
2
nal purification was done by trituration of the solids in 18.25
mL of chloroform. Green solids were obtained by Büchner fil-
tration; the solids were dried under nitrogen.
washed with toluene (40 mL), methanol (40 mL) and CH Cl2
2
Sulfoxidation
(40 mL) to remove the unconfined Fe-salen complex. The
The FeIJsalen)-based catalyst (1 or 2 mol% relative to thio-
ether) was charged in a glass vial together with 4 mL of sol-
vent and 0.4 mmol of thioether, followed by the addition of
an oxidant (0.64 mmol). The mixture was stirred for the de-
sired time and temperature. After this period, the solids were
recovered by centrifugation (5 min, 4000 rpm), and the liquid
layer was tapped off and analyzed by GC to determine the
conversion; the ee values were obtained by HPLC analysis.
For recycle experiments, the solid catalysts were recovered by
filtration, washed with ethanol (35 mL) and dried under vac-
uum, and then, the new solution mixture described above
was added to the solid.
solids thus obtained were dried under vacuum at 40 °C. The
functionalized materials with Fe-salen inside their structure
are named by placing the term Fe in front of the silica mate-
rial's name. For example, Fe-F-m-MCF-60 is the functionalized
m-MCF-60 containing the Fe-salen complex in its cage.
Fe(salen) complex synthesis. The synthesis of the salen li-
gand was done following the procedure described by
2
0
Jacobsen et al. Briefly, a 100 mL three-necked flask was
equipped with a cooler, and 78.30 mg (0.686 mmol) of
2 3
(1R,2R)-diaminocyclohexane, 189.55 mg (1.37 mmol) of K CO
and 2.5 mL of distilled H O were added. The mixture was
2
stirred until the dissolution of the chemicals, and then 9.5
mL of EtOH was added to the flask. The mixture was heated
under stirring to 77 °C, then 321.39 mg (1.37 mmol) of 3,5-di-
tert-butyl-2-hydroxybenzaldehyde was dissolved in 5 mL of
EtOH, added dropwise to the mixture and stirred for 2 h. Af-
ter that, 2 mL of water was added and the reaction mixture
was cooled to a temperature around 5 °C over 2 hours and
kept at that temperature for additional 1 hour. Crude solids
were obtained by vacuum filtration and washed twice with 2
mL of EtOH. The solids were collected and dissolved in 8 mL
Instrumentation
GC analysis. A small fraction of the liquid samples was
taken and diluted with toluene and analyzed by GC-FID. GC
analysis was performed using a PerkinElmer Clarus 500,
equipped with a 30 m capillary column with 5% phenyl/95%
methylpolysiloxane as the stationary phase (AT5), using the
following parameters: initial temperature 50 °C, temperature
−
1
ramp 10 °C min , final temperature 250 °C, injection volume
0.5 μL.
of DCM, and washed twice with 5 mL of H
2
O and once with
2
mL of brine. The organic layer was dried over Na SO ; the
HPLC analysis. All of the chiral HPLC experiments were
carried out using a Perkin Elmer Series 200 pump/diode array
detector. A chiralcel OD column of 250 × 4.6 mm was used,
with a bulk stationary phase consisting of 10 μm particles of
the silica support physically coated with the polymeric chiral
selector cellulose trisIJ3,5-dimethylphenylcarbamate). A flow of
2
4
final product was obtained after removal of the solvent using
a rotavap. Yield: 72.2% (270.97 mg). IR (KBr pellet): 2960
cm ((CH ) C–), 2864 cm (C–H), 1631 cm (CN), 1595
−
1
−1
−1
3
3
−
1
cm (CC Aromatic). ESI-MS found m/z: 547.42.
Complexation of salen with iron was done following a pro-
2
1
−1
cedure reported previously. 104.61 mg (0.387 mmol) of
FeCl ·6H O dissolved in 4 ml of MeOH were added to 211.79
1 mL min of 80% hexane and 20% isopropanol was used
for all experiments.
3
2
mg (0.387 mmol) of ligand dispersed in 2 mL of MeOH. The
mixture was stirred for 24 h at room temperature. After this
period, a purple solid was collected and washed with MeOH,
Electron microscopy. The morphology and sizes of the sil-
ica particles were determined with a Tecnai FEI XL 30SFEG
scanning electron microscope (SEM). Transmission electron
microscopy was performed using a FEI Tecnai20F, operated
at 200 kV and equipped with a CCD camera.
The samples were embedded in epoxy resin (Epofix, EMS)
and cured at 60 °C overnight. Then, they were cut into thin
sections with a nominal thickness of 60 nm using a Diatome
Ultra knife, 4 mm wide and 35° clearance angle, mounted on
dissolved in dichloromethane (DCM) and dried over Na
2 4
SO .
The solution was left to stand at room temperature and crys-
−
1
tals were obtained. IR (KBr pallet): 2960 cm ((CH
3 3
) C), 2866
−
1
−1
cm (C–H), 1602 cm (CN). UV-vis: λ (nm) = 229, 274 (Ar,
π–π*), 351 (CN, π–π*), 514 (Fe, d–d). ESI-MS found m/z:
6
38.71.
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a Reichert-Jung Ultracut E microtome. The sections floating
on water after cutting were picked up and deposited onto a
carbon coated polymer grid and left to dry.
Gas physisorption. N and Ar physisorption measurements
2
were performed at 77 K using a Micromeritics Tristar 3000.
The samples were dried before the measurement under an
N flow at 250 °C for at least 12 h. The functionalized sam-
2
ples were dried at 130 °C for at least 12 h. The micropore
and mesopore volumes (V ) were determined using the t-plot
p
method. The pore size distribution of the mesoporous silica
supports was calculated from the adsorption branch of the
isotherm by BJH analysis. The maximum of the pore size dis-
tribution was taken as the average pore diameter.
Elemental analysis. Fe contents of silica was determined
by inductively coupled plasma-atomic emission spectroscopy
(ICP-AES) using a Metrohm IC Plus 883.
UV-vis and IR spectroscopy. UV-vis experiments were car-
ried out on a Varian Cary 500 Scan UV-vis-NIR spectropho-
tometer with the configuration in the range of 200–800 nm
−
1
with a data interval of 1 nm and at a rate of 600 nm min .
KBr pellets were prepared by mixing 15 mg of sample with
2
50 mg of KBr; this mixture was ground into a fine powder,
after which it was introduced to a pellet press. The solid was
pressured by a 15.011 SPECAC hydraulic press to obtain the
KBr pellets. The IR spectroscopic experiments were carried
out on
a Perkin Elmer Spectrum One FT-IR infrared
spectrometer.
TGA analysis. Thermogravimetric analysis (TGA) was car-
ried out using a Perkin-Elmer Pyris 1, with a typical sample
Fig. 1 Nitrogen adsorption isotherms for pristine SBA-16 (top frame)
and m-MCF (bottom frame). The isotherms were offset vertically by
300, 600 and 900 for the SBA-16 materials and by 300, 600, and 1100
for m-MCF.
−
1
quantity of 5 mg and an air flow of 10 ml min . A heating
−
1
rate of 5 °C min from 50 °C until 700 °C was used.
Results and discussion
Synthesis and pretreatment of mesoporous materials
SBA-16-C-373-24 showed a small fraction of entrance sizes
around 3.8 nm (see material designations in the Experimen-
tal section).
2
The N physisorption isotherms of the pristine ordered meso-
porous materials (m-MCF and SBA-16) are displayed in Fig. 1.
The isotherms can be classified as type IV following the
IUPAC classification since they show one step capillary con-
densation in the adsorption branch (Fig. 1), corresponding to
A method based on the post-synthetic functionalization of
the silica surface with alkoxysilanes with different carbon
chain lengths was applied for determining the entrance size
of the synthesized materials. After this functionalization,
the obtained hybrid materials were examined by nitrogen
1
6
the filling of the uniform mesopores by N molecules. In all
2
cases, ink-bottle hysteresis was observed, showing that liquid
physisorption to study the accessibility of the pores for N
2
N remained trapped by the narrow entrances of the pores,
resulting in delayed desorption at a relative pressure around
molecules; see the ESI† for details (Fig. S1 and S2 show the
isotherms of the synthesized materials after the
functionalization with different organosilanes) and the de-
rived values of entrance size in Table 1.
2
0
.45. This fact does not permit the exact determination of the
entrance sizes of these materials since liquid nitrogen is not
stable below a relative pressure of 0.45.
Table 1 shows the structural properties of the synthesized
silica materials (m-MCF and SBA-16). The increase of cage
size resulted almost always in an increase of the entrance size
especially for the SBA-16 materials. However, we were able to
reduce the entrance sizes of the SBA-16 materials by decreas-
ing the hydrothermal treatment time, in good agreement
Argon physisorption at 77 K is an alternative technique to
study the entrance size of these materials due to the capillary
evaporation of argon which takes place at lower relative pres-
sures than with nitrogen thus extending its use to analyze en-
trance sizes down to 3.8 nm. The argon physisorption iso-
therms of the synthesized materials are shown in Fig. 2.
From the obtained hysteresis of the synthesized m-MCF and
SBA-16, it is possible to conclude that the majority of en-
trance sizes are smaller than 3.8 nm. However, the sample
1
4a
16b
with the results reported by Li et al.
and Jaroniec et al.
The obtained materials were examined by SEM (ESI† Fig. S3
and S4) and TEM (Fig. 3) to assess the ordered structure of
pristine SBA-16 and m-MCF.
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Fig. 3 TEM images of pristine host materials: (A) SBA-16-B-353-24, (B)
SBA-16-C-373-24, (C) MCF-50, and (D) MCF-70. All scale = 50 nm.
Preparation and characterization of catalysts
A post synthetic functionalization with n-propyl groups of sil-
ica materials (Table 2) was carried with the intention of
narrowing the entrance sizes and allowing a better dispersion
of solid materials in organic solvents (ESI† Fig. S5) due to the
increase of hydrophobicity (organic solvents are commonly
used as solvents for salen complex synthesis and as solvents
for sulfoxidation reactions). Moreover, the functionalization
decreases the acidity of silica that might cause the degrada-
tion of the trapped salen complex. After the functionali-
zation, N
2
physisorption was used to characterize these
materials (Table 2), showing a decrease of the porosity, sur-
face area and pore size compared to pristine SBA-16 and
m-MCF (isotherms in the ESI,† Fig. S6).
IR spectroscopy was used to assess the presence of alkyl
Fig. 2 Argon adsorption isotherms for pristine SBA-16 (top frame) and
m-MCF (bottom frame). The isotherms were offset vertically by 300,
−
1
groups on silica. The peaks at 2850–2970 cm which corre-
spond to the C–H stretch vibrations of alkyl groups were ob-
500 and 700 for the SBA-16 materials and by 300, 700, and 1000 for
m-MCF.
−1
served. Moreover, the intensity of the peak at 3740 cm
which corresponds to the stretching mode of free silanol
groups almost disappeared (ESI,† Fig. S7), showing the suc-
cessful grafting of the organosilanes on the silica surface.
TGA analyses were used to confirm the decomposition of al-
kyl groups grafted on the silica surface (ESI,† Fig. S8),
suggesting the successful functionalization of the external
and internal walls of the silica (the functionalized materials
are designated as F-m-MCF and F-SBA-16).
2
Table 1 Structural properties of pristine materials from N physisorption
and surface modification; for material designations see the Experimental
section
Cage
a
Surface area
Pore volume
size
Entrance
size (nm)
2
−1
3
−1
b
Material
(m g
)
(cm g
)
(nm)
A salen ligand was selected as a model which allows us to
obtain reasonable loadings in the cages of different
SBA-16-A-373-5.5
SBA-16-A-373-24
SBA-16-B-353-24
SBA-16-C-373-24
m-MCF-50
m-MCF-60
m-MCF-70
m-MCF-80
769
875
634
911
619
623
685
689
0.49
0.56
0.43
0.77
0.65
0.66
0.92
0.96
5.0
5.0
6.5
7.5
16
16.5
20.8
21.2
2.0
2.5
2.8
3.2
2.0
2.0
2.5
2.8
1
4a,14b,15
materials.
Scheme 1 shows the reaction (Schiff base
reaction) which took place inside the cage structure of F-m-
MCF and F-SBA-16 (for more details, see the Experimental
section). IR spectroscopy was used to confirm the formation
of the salen ligand inside m-MCF-60, as a new peak at 1632
−
1
cm appeared which corresponds to CN bond formation
−
1
a
b
(Fig. 4B) and the absence of the peaks at 1582 cm and 1760
Determined by BJH analysis from the adsorption branch. Determined
by post-synthetic treatments.
−
1
cm
which correspond to primary amine and carbonyl
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Table 2 Structural properties of functionalized mesoporous materials
bands were observed, suggesting no encapsulation of the
complex, due to the large entrance size. A higher concentra-
tion of salen complex inside the materials is observed when
the entrance sizes are smaller than 2 nm (F-m-MCF-50, F-m-
MCF-60 and F-SBA-16-A-5.5). Table 3 shows the iron content
in the materials measured by inductively coupled plasma
from N
2
physisorption (designated as F-m-MCF and F-SBA-16 materials)
(
ICP) which is in line with the UV-vis spectra results. In gen-
eral, a higher concentration of Fe(salen) complexes is encap-
sulated in the m-MCF materials than in the SBA-16 materials.
For example, m-MCF-50 contained 4 times more iron than
F-SBA-16-A-373-5.5, showing the advantages of m-MCF for this
approach.
a
Surface area
Pore volume
Cage size
(nm)
2
−1
3
−1
Material
(m g
)
(cm g
)
F-SBA-16-A-373-5.5
F-SBA-16-A-373-24
F-SBA-16-B-353-24
F-SBA-16-C-373-24
F-m-MCF-50
F-m-MCF-60
F-m-MCF-70
F-m-MCF-80
368
455
341
500
332
313
379
376
0.23
0.29
0.24
0.40
0.43
0.41
0.64
0.65
4.3
4.5
5.3
6.8
14
16
21
21
The IR spectrum of F-m-MCF-60 after complexation of
iron is shown in Fig. 4A. It is known that the band which cor-
−
1
responds to CN displays a red-shift of 3–12 cm after com-
plexation, which corresponds to the participation of the
azomethine and phenolic oxygen of the ligand binding to Fe
a
17a
Determined by BJH analysis from the adsorption branch.
ions.
However, only a part of this band shows this shift in
the studied case, indicating an incomplete complexation of
Fe with the salen ligand; in other words, not all salen ligands
present in m-MCF-60 participate in the complexation of iron.
groups of aldehyde in the spectrum, confirming the success-
ful formation and encapsulation of the salen ligand in m-
MCF-60.
Asymmetric oxidation of sulfides
Upon complexation with iron, a color change from yellow
to red was observed in the solid material, giving a strong in-
dication that the Fe(salen) complex was formed inside func-
tionalized m-MCF and SBA-16. The UV-vis spectra of all mate-
rials (m-MCF and SBA-16) are shown in Fig. 6. In almost all
cases, four bands were observed; the bands at 220 and 280
nm are attributed to π–π* located predominantly on the phe-
The asymmetric oxidation of thioanisole was used as a model
reaction (Table 4). This reaction was run in different solvents,
using different oxidants, and Fe(salen) immobilized on F-m-
MCF-60 or pure Fe(salen) as catalysts. In all cases, a high se-
lectivity to methylsulfinyl-benzene (Table 4, 1a) was observed
with a very small fraction of the substrate being further oxi-
dized to methylsulfonyl-benzene (1b). Iodosylbenzene (PhIO)
was the only oxidant which gave rise to enantioselectivity, in
agreement with the results reported by Talsi et al. that the
Fe(salen) complex is able to form specific active sites with
PhIO by a Lewis acid activation of iodosylbenzene to form
17,9f
nyl ring and the peak at 312–350 nm to CN.
The
expected d–d transition at 522–565 nm of Fe(salen) was also
observed, further confirming its formation inside the silica
materials (the materials were designated as Fe-F-m-MCF and
Fe-F-SBA-16). The homogeneous complex was also synthe-
sized and physically mixed with m-MCF-60; this mixture
showed the same peaks as those observed for materials syn-
thesized by the “ship in the bottle” approach (Fig. 5). The iso-
6
the oxygen-transfer species. A blank reaction was also run
under the same conditions reported in Table 4, entry 12,
using F-MCF-60 without the iron complex inside the cages;
only 3% of thioanisole was converted.
therms from N
F-m-MCF-60 and Fe-F-m-MCF-60 are shown in the ESI†
Fig. S9).
The intensities of the characteristic bands of Fe(salen) de-
2
physisorption at 77 K of pristine m-MCF-60,
(
creased with the increase of the pore entrance sizes. These
intensities are related to their concentration inside the silica
structures (Fig. 6). In the case of F-SBA-16-C-373-24 which has
an entrance size around 3.2 nm, no characteristic UV-vis
Scheme 1 Schiff base reaction which takes place inside the cages of
Fig. 4 IR spectra of: A) Fe-F-m-MCF-60 and B) the salen ligand
the synthesized materials.
immobilized on F-m-MCF-60.
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Table 3 Iron loading in the synthesized materials
Material
Fe loading (w/w)%
Fe-F-SBA-16-A-373-5.5
Fe-F-SBA-16-A-373-24
Fe-F-SBA-16-B-353-24
Fe-F-SBA-16-C-373-24
Fe-F-m-MCF-50
Fe-F-m-MCF-60
Fe-F-m-MCF-70
Fe-F-m-MCF-80
0.19
0.14
0.06
<0.005
0.85
0.70
0.50
0.26
Fig. 5 UV-vis spectra of the salen ligand encapsulated or Fe(salen)
encapsulated in m-MCF-60 and the physical mixture of Fe(salen) with
F-m-MCF-60.
conversions were observed as was reported before for sulfo-
xidation of thioanisole using a homogeneous Fe(salen) com-
plex as a catalyst.
6
The comparison between the activity of the Fe(salen) com-
plex immobilized on F-m-MCF and F-SBA-16 was also ex-
plored. Despite the same entrance sizes, higher conversion
values were observed when the Fe(salen) complex was
immobilized on F-m-MCF (Table 5, entries 2–9), probably the
large cage sizes of these materials allow more freely moving
Fe(salen) complexes with concomitant higher activities. The
conversions observed with the larger substrate benzyl phenyl
sulfide were considerably lower in the case of F-SBA-16, F-m-
MCF-50 and F-m-MCF-60 when compared to F-m-MCF-70 and
F-m-MCF-80. Probably, the combination of larger cages and
in particular entrance sizes of F-m-MCF-70 and F-m-MCF-80
facilitates the diffusion of this substrate and PhIO, producing
a more suitable catalyst. Moreover, even for the catalyst based
on F-m-MCF-70 and F-m-MCF-80, the oxidation of benzyl phe-
nyl sulfide was slightly lower than the conversion of thio-
anisole. This data shows that the conversion values can be
optimized through tuning the silica structure (entrance size
and cage size).
The homogeneous Fe(salen) complex was also tested as a
catalyst for comparison (Table 4, entries 1, 7, and 9). The
homogeneous counterpart showed only slightly higher activ-
ity than the immobilized complex in the cages of F-m-MCF-
6
0 for thioanisole as a substrate. This is much more favor-
1
8
able than some results previously reported; for example,
Ogunwumi et al. reported that an immobilized Mn salen
complex on zeolites suffered large reduction of the activity
13c
when compared with homogeneous counterparts.
Table 4
also shows that the homogeneous catalyst presented some-
what higher enantioselectivity when compared to
immobilized systems.
Fig. 6 UV-vis spectra of the Fe(salen) complex immobilized on cage-
like materials based on F-SBA-16 and F-m-MCF.
To determine the nature of the catalytic species for the re-
action, a hot filtration test was performed for the oxidation
of thioanisole using Fe(salen) immobilized on F-m-MCF-60 as
a catalyst; after 1 hour of reaction the liquid and solid phases
were separated. The liquid phase was kept under the same re-
action conditions, showing that the filtrate was only able to
provide a slight additional conversion of around 5% (Fig. 7).
This gives a strong indication that the vast majority of the re-
action is catalyzed by iron (salen) inside the m-MCF struc-
ture. A hot filtration test was also run using Fe-F-m-MCF-70
with benzyl phenyl sulfide as a substrate (ESI,† Fig. S12).
If acetonitrile and dichloromethane were used as solvents,
low conversions of thioanisole were observed when PhIO was
used as an oxidant. On the other hand, the conversion was
higher using H O (Table 4, entries 2, 4, 5, and 8), which is
probably related to the limited solubility of PhIO in these sol-
vents. Thus, methanol was selected as a solvent due to the
higher solubility of PhIO. In this case, a remarkable increase
of the conversion was observed (Table 4, entry 12). Sodium
hypochlorite was also used as an oxidizing agent, but low
2
2
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Table 4 Oxidation of thioanisole using Fe(salen) or Fe(salen) immobilized on F-m-MCF-60 with different solvents and oxidants
a
b
c
Entry
Solvent
Oxidant
PhIO
H O
2 2
NaOCl
PhIO
Conversion (%)
Selectivity (%)
ee (%)
d
1
CH
CH
CH
CH
3
CN
3
CN
3
CN
3
CN
68
65
8
11
40
8
65
10
92
94
20
90
65
99
99
98
99
99
99
99
99
98
95
98
98
99
64
0
0
55
0
0
60
56
66
0
0
57
62
2
3
4
5
6
CH
CH
CH
CH
Cl
2 2
Cl
2 2
Cl
2 2
Cl
2 2
2 2
H O
NaOCl
PhIO
PhIO
PhIO
d
7
8
9
d
CH
CH
CH
CH
CH
3
OH
3
OH
3
OH
3
OH
3
OH
10
11
12
13
2 2
H O
NaOCl
PhIO
PhIO
e
a
e
b
c
d
Determined with GC-FID. Selectivity to 1a. Determined by chiral HPLC analysis. Homogeneous FeIJIII) salen complex used as a catalyst.
The reaction was run at 0 °C for 10 hours. Reaction conditions: 4 mL of solvent, 0.4 mmol of thioanisole, oxidant (0.65 mmol) and catalyst
(1 mol% Fe), 20 °C, 4 hours.
Table 5 Sulfoxidation using Fe(salen) and Fe(salen) immobilized on F-m-MCF and SBA-16
a
b
c
Entry
Material
Substrate
Conversion (%)
Selectivity (%)
ee (%)
1
2
3
4
5
6
7
8
9
Fe-salen complex
Fe-F-m-MCF-50
Fe-F-m-MCF-60
Fe-F-SBA-16-A-373-5.5
Fe-F-m-MCF-70
Fe-F-SBA-16-A-373-24
Fe-F-m-MCF-80
Fe-F-SBA-16-B-353-24
Fe-F-SBA-16-C-373-24
Fe-salen complex
Fe-F-m-MCF-50
Fe-F-m-MCF-60
Fe-F-SBA-16-A-373-5.5
Fe-F-m-MCF-70
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
92
92
90
60
91
65
90
67
4
94
39
43
25
80
40
82
60
2
98
97
98
98
97
98
98
98
98
98
94
93
98
95
98
95
96
99
66
58
57
55
55
56
56
57
0
74
60
62
60
63
61
64
65
0
10
11
12
13
14
15
16
17
18
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
2
SPh
2
SPh
2
SPh
2
SPh
2
SPh
2
SPh
2
SPh
2
SPh
2
SPh
Fe-F-SBA-16 A-373-24
Fe-F-m-MCF-80
Fe-F-SBA-16-B-353-24
Fe-F-SBA-16-C-373-24
a
b
c
Determined with GC-FID. Selectivity to the corresponding sulfoxide. Determined by chiral HPLC analysis. Reaction conditions: 4 mL of
MeOH, 0.4 mmol of substrate, PhIO (0.65 mmol), catalyst (1 mol% Fe), 20 °C, 4 hours.
The recyclability of FeIJsalen)F-m-MCF-70 was investigated
in the asymmetric oxidation of benzyl phenyl sulfide using
PhIO as an oxidant and methanol as solvent (Table 6). This
reaction was selected due to the higher enantioselectivity
obtained when compared to the values obtained for thio-
anisole. The catalyst was recovered by filtration and washed
thoroughly with ethanol. The catalyst could be recycled two
times without loss of activity and chemical selectivity, but a
slight decrease of enantioselectivity was apparent. However, a
slight loss of catalytic activity was observed during the third
cycle. The content of iron was determined to investigate the
possible explanation for this deactivation; however, the
leaching of iron was insignificant. The recyclability of thio-
anisole was also run and is shown in Table S2 (ESI†).
UV-vis spectroscopy was used to study any possible change
in the Fe(salen) complex structure inside F-m-MCF-70. Fig. 8
shows that the reused catalyst (after three cycles) still pre-
sents two bands at 200–300 nm which correspond to π–π* of
the phenyl ring; however, the band at 350 nm which is attrib-
uted to CN cannot be seen anymore, giving a strong
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cles, since the presence of these particles was suggested by
XRD analysis as a broad peak was observed around 50° two-
theta (ESI,† Fig. S10).
Conclusions
A chiral Fe(salen) complex was encapsulated in the nano-
cages of n-propyl modified SBA-16 and m-MCF. The silica
structure (entrance and cage sizes) was important to deter-
mine the final loadings of Fe(salen) inside these materials. In
general, the structure of m-MCF materials provides a higher
loading of Fe(salen) complexes compared to SBA-16, probably
due to their larger cage sizes. The catalytic activity of the ma-
terials could be tuned by changing the entrance and cage di-
mensions of the silica structure, and adjusting them to the
specific substrate used. For the catalytic performance, the
enantioselectivity of sulfoxidation reactions was just observed
when PhIO was used as an oxidant as previously reported for
Fig. 7 Time–conversion plot and leaching test as evidence for the
nature of the catalyst for the sulfoxidation of thioanisole.
Table 6 Recyclability of Fe(salen) immobilized on F-m-MCF-70 for the
oxidation of benzyl phenyl sulfide
6
the homogenous complex. Moreover, Fe(salen) trapped in
the mesoporous materials showed almost the same activity
and enantioselectivity as the homogeneous counterpart. Dur-
ing recyclability tests, the immobilized complex showed deac-
tivation after the second cycle, related to the decomposition
of the encapsulated Fe(salen) complex and the formation of
iron oxide particles.
a
b
c
Run
Conversion (%)
Selectivity (%)
ee (%)
1
2
3
4
5
80
79
61
60
52
95
97
98
98
98
63
55
0
0
0
Acknowledgements
The authors acknowledge support from European Research
Council (ERC) advanced grant no. 338846, the Dutch National
Research School Combination Catalysis (NRSCC) and São
Paulo Research Foundation grant no. 2015/07773-0. The au-
thors would also like to thank Allan Ribeiro da Silva for NMR
measurement.
a
b
Determined with GC-FID.
Selectivity to the corresponding
Determined by chiral HPLC analysis. Reaction
conditions: 4 mL of MeOH, 0.4 mmol of benzyl phenyl sulfide, PhIO
0.65 mmol), catalyst (2 mol%), 20 °C, 4 hours.
c
sulfoxide.
(
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