Magnetic Mesoporous Silica Nanocomposite Functionalized with Palladium Schiff Base Complex: Synthesis, Characterization, Catalytic Efficacy in the Suzuki–Miyaura Reaction and α-Amylase Immobilization

Article abstract of DOI:10.1007/s10562-019-02913-5

Abstract: Magnetic mesoporous silica nanocomposite, Fe₃O₄-MCM-41, was functionalized with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPS) and then condensed with 5,5′-methylene bis(salicylaldehyde), followed by N(4)-phenylthiosemicarbazide to produce a ONS Schiff base grafted nanocomposite. Finally, by adding palladium(II) acetate, the palladium Schiff base complex was immobilized on magnetic nanocomposite. The characterization of new nanocomposites was carried out by means of several techniques such as FT-IR, XRD, FE-SEM, HRTEM, EDS, BET, VSM, XPS, DRS and TGA. The new nanocatalyst, Fe₃O₄@MCM-41-SB-Pd, was used in synthesis of symmetrical and unsymmetrical biaryl compounds via the Suzuki–Miyaura cross-coupling of phenylboronic acid with aryl halides. This catalyst was easily recovered by applying an external magnetic field and reused for several times without significant loss of its catalytic activity. Also the ability of synthesized mesoporous nanocomposites for enzyme immobilization was investigated and results showed that they efficiently immobilized α-amylase enzyme. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02913-5

Catalysis Letters
https://doi.org/10.1007/s10562-019-02913-5
Magnetic Mesoporous Silica Nanocomposite Functionalized
with Palladium Schif Base Complex: Synthesis, Characterization,
Catalytic Eꢀcacy in the Suzuki–Miyaura Reaction and α‑Amylase
Immobilization
1
1
1
2,3
Ameneh Ahmadi · Tahereh Sedaghat · Roya Azadi · Hossein Motamedi
Received: 21 April 2019 / Accepted: 23 July 2019
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Magnetic mesoporous silica nanocomposite, Fe O -MCM-41, was functionalized with N-(2-aminoethyl)-3-aminopropyltri-
3
4
methoxysilane (AEAPS) and then condensed with 5,5′-methylene bis(salicylaldehyde), followed by N(4)-phenylthiosemicar-
bazide to produce a ONS Schiꢀ base grafted nanocomposite. Finally, by adding palladium(II) acetate, the palladium Schiꢀ
base complex was immobilized on magnetic nanocomposite. The characterization of new nanocomposites was carried out
by means of several techniques such as FT-IR, XRD, FE-SEM, HRTEM, EDS, BET, VSM, XPS, DRS and TGA. The new
nanocatalyst, Fe O @MCM-41-SB-Pd, was used in synthesis of symmetrical and unsymmetrical biaryl compounds via the
3
4
Suzuki–Miyaura cross-coupling of phenylboronic acid with aryl halides. This catalyst was easily recovered by applying an
external magnetic ꢁeld and reused for several times without signiꢁcant loss of its catalytic activity. Also the ability of syn-
thesized mesoporous nanocomposites for enzyme immobilization was investigated and results showed that they eꢂciently
immobilized α-amylase enzyme.
Graphic Abstract
Keywords Magnetic mesoporous · Suzuki–Miyaura reaction · Enzyme immobilization · Schiꢀ base
1
Introduction
The nanoporous materials have holes in nanoscale and large
volume that forms empty spaces in their structure. Based on
the classiꢁcation of IUPAC, these materials are divided into
three categories: microporous (pore size less than 2 nm),
*
Tahereh Sedaghat
tsedaghat@scu.ac.ir
Extended author information available on the last page of the article
Vol.:(0
1
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A. Ahmadi et al.
mesoporous (pore size between 2 and 50 nm) and macropo-
rous (pore size greater than 50 nm) [1]. Since the discovery
of mesoporous silica materials in 1992, these materials have
attracted a great deal of attention due to features such as
tunable pore size, controlled size, extremely high surface
area, chemical, thermal and mechanical stability and also
ability of surface modiꢁcation with various organic and
inorganic groups. These materials are used as adsorbents,
sensors, molecular sieves, catalysts, catalyst supports, ion-
exchangers, immobilization of enzymes, drug delivery and
in many other related applications [2–11]. Mesoporous
materials have a very high surface area and are used as sub-
strates for diꢀerent catalysts, so the design of mesoporous
nanomaterials that have catalytic active centers on their sur-
face, has always been of interest to researchers [10, 12–16].
Catalysts have an important role in the producing process for
the synthesis of medicines, commercial chemicals, energy
resources, fuels, biofuels and are divided into two general
categories, heterogeneous and homogeneous. Homogene-
ous catalysts, due to their solubility in the reaction envi-
ronment which increase the availability of catalytic site for
the substrate, display higher catalytic activities than their
heterogeneous counterparts. But, there are drawbacks in
the separating and recycling of the homogeneous catalysts-
product mixture, which may contaminate the ꢁnal product
and greatly limit their applications in practice. To overcome
these problems, heterogenization of homogeneous catalysts
has been suggested. Among several methods recommended
for this subject, the immobilization of homogeneous cata-
lysts onto solid supports such as silica materials, zeolites,
carbon materials, organosilica materials and layered com-
pounds, is one of the most eꢀective methods for prepara-
tion of heterogeneous catalysts with improved recovery and
recyclability [11, 17, 18]. Since the surface of catalyst plays
an important role in the catalytic reactions, materials hav-
ing high surface area, such as mesoporous materials, are
promising candidates as support for homogeneous catalysts.
Heterogeneous catalysts usually are separated from the reac-
tion medium through ꢁltration and centrifugation. But, these
procedures are tedious, time-consuming and ineꢂcient. The
use of magnetic nanoparticles as support for homogeneous
catalyst immobilization is a suitable method for the facile
separation and recycling of the catalyst from the reaction
media by applying an external magnetic ꢁeld [19–25].
The Suzuki–Miyaura cross-coupling reaction is an
increasingly popular approach for the synthesis of biaryl
compounds. The Suzuki reaction is catalyzed by palladium,
which is one of the most expensive metals and cannot be
eꢂciently isolated from the reaction medium. Therefore,
immobilization of palladium complexes onto a solid support
such as mesoporous silica leads to formation of heterogene-
ous catalyst and thus reducing Pd leaching into solution and
improvement the separating and recycling of the catalyst
[26–29].
Due to the tunable and uniform pores of mesoporous
materials, they have been also promising candidates for
enzyme immobilization in recent years [30–35]. The use
of mesoporous materials as support for immobilization of
enzymes resolves some processing diꢂculties of natural
enzymes and produces recoverable and stable heterogene-
ous biocatalysts [9, 11, 36].
In the present research work, we have used a dialdehyde
to form bis-Schiꢀ base with several chelating groups on
Fe O @MCM-41 nanocomposite. Our aim is to propose a
3
4
functionalized silica mesoporous nanomaterial with high
chelating potency to immobilize large amounts of palladium
and therefore improve catalytic activity along with low pal-
ladium leaching, also eꢀectively immobilize enzyme. Hence,
this hybrid nanocomposite, which have a combination of the
magnetic properties of iron oxide nanoparticles, the high
speciꢁc surface of mesoporous compounds and as well as
the properties of Schiꢀ base compounds, has been success-
fully used in Suzuki–Miyaura cross-coupling reactions and
α-amylase immobilization.
2 Experimental
2.1 Materials
All starting materials including tetraethylorthosilicate
(TEOS), hexadecyltrimethyl ammonium bromide (CTAB,
99%), N-(2-aminoethyl)-3-aminopropyltrimethoxysilane
(AEAPS), ammonia (25 wt%), Iron (III) chloride hexa-
hydrate (FeCl ·6H O), iron(II) chloride tetrahydrate
3
2
(FeCl ·4H O), ethanol and methanol, salicylaldehyde,
2
2
1,3,5-trioxane, sulphuric acid (98%), acetic acid (glacial),
N(4)-phenylthiosemicarbazide, DMF, aryl halides, toluene,
phenylboronic acid, potassium carbonate, n-hexane, CH CN
3
and palladium(II) acetate, were purchased from Merck
chemical company and used without further puriꢁcation.
5,5′-methylene-bis-salicylaldehyde was synthesized accord-
ing to the previous method using 1,3,5-trioxane as starting
material, in presence of catalytic amount of conc. H SO and
2
4
in glacial CH COOH as a solvent [37]. Deionized water was
3
obtained from Millipore pure water system.
2.2 Characterization Methods
FT-IR spectra were recorded by a Perkin Elmer spectro-
−
1
photometer model spectrum two within the 400–4000 cm
range using the KBr pellets. X-ray powder diꢀraction (XRD)
spectra of the catalyst were recorded with a PW 1730 X-ray
diꢀractometer model X Pert Pro using monochromatic Cu
Kα radiation at 40 kv and 30 mA. The particle size and
1
3

Magnetic Mesoporous Silica Nanocomposite Functionalized with Palladium Schif Base Complex:…
morphology of the catalyst was performed by a ꢁeld emis-
sion scanning electron microscope (FE-SEM) equipped
with EDX analyzer model Mira 3-XMU and with an accel-
erating voltage of 15 kv. High resolution transmission elec-
tron microscopy (HRTEM) images were carried out on a
HR-TEM FEI TEC9G20 (200 kV). The nitrogen adsorp-
tion–desorption isotherms were measured using a Belsorp
(100 mmol/L), as surfactant agent, was slowly added to
above solution and stirred for 1 h. Afterwards, 1.2 mL of
tetraethylorthosilicate (TEOS) was added dropwise to the
solution and the reaction mixture was stirred for 24 h. The
black precipitates were separated from the solution by an
external magnet, washed several times with ethanol/deion-
ized water mixture and then dried in oven at 60 °C for 24 h.
In the next step, CTAB was removed from the product by
calcining in air at 550 °C for 6 h to produce the mesoporous
material.
mini II instrument at liquid N temperature. Samples were
2
evacuated for 15 h at 120 °C prior to adsorption measure-
ments. The Brunauer–Emmett–Teller (BET) method was
used to calculate the speciꢁc surface area, and pore size
distribution and pore volume were estimated by applying
the Barrett–Joyner–Halenda (BJH) method. The saturated
magnetization of the synthesized nanoparticles was deter-
mined by means of vibrating sample magnetometer with
the magnetic ꢁeld range of between +12,000 and −12,000
Oe at room temperature. The metal amount of the catalyst
was measured by atomic absorption spectrometer model
Analytikjena-contrAA 700. Thermogravimetric analysis
2.5 Synthesis of Amine‑Functionalized
Fe O @MCM‑41 Nanocomposites (Fe O @
3
4
3 4
MCM‑41‑AEAPS)
0.5 g of mesostructured composite of Fe O @MCM-41 was
3
4
dispersed for 30 min in 35 mL of anhydrous toluene with an
ultrasonic bath. Then, AEAPS (4 mL) was added dropwise
and the mixture was reꢃuxed at 110 °C under N atmos-
2
(
TGA) of the prepared catalyst was performed on a STA
phere for 12 h. The reaction mixture was cooled and amino-
functionalized nanoparticles were collected by an external
magnet, washed with plenty of toluene and methanol, and
subsequently dried in an oven at 60 °C for 24 h.
1
8
500 thermal analyzer under N atmosphere from 25 to
2
00 °C and heating rate of 10 °C/min. X-ray photoelectron
spectroscopy (XPS) analysis was conducted by an 8025-Bes
Tec instrument. UV–Visible diꢀuse reꢃectance spectrum
(
UV-DRS) of catalyst was recorded by Avaspec 2048 Thech
2.6 Synthesis of Aldehyde‑Functionalized Fe O @
3
4
spectrophotometer.
MCM‑41 Nanocomposites (Fe O @MCM‑41‑ald)
3 4
2
.3 Synthesis of Fe O Superparamagnetic
The amino-functionalized magnetic nanoparticles (1 g) were
sonicated for 30 min in 20 mL of ethanol. Then, a solution
of 5,5′-methylene bis(salicylaldehyde) (4 mmol) in EtOH
was added to it and the mixture was reꢃuxed for 24 h under a
nitrogen gas. The product was separated by an external mag-
net, washed with ethanol and ꢁnally dried in oven overnight.
3
4
Nanoparticles
Magnetic nanoparticles of Fe O were prepared by co-pre-
3
4
3
+
2+
cipitating of Fe and Fe ions in water solution with a
molar ratio of 2:1. In this way, 1.35 g FeCl ·6H O and 0.5 g
3
2
FeCl ·4H O were dissolved in 50 mL of deionized water
2
2
under N with vigorous shaking at 85 °C. The solution was
2.7 Synthesis of Schiꢀ Base‑Functionalized Fe O @
2
3 4
stirred for 1 h. Thereafter, 7 mL of ammonia solution was
dropped into the mixed solution to adjust the pH value to
MCM‑41 Nanocomposites (Fe O @MCM‑41‑SB)
3 4
1
0. The addition of the ammonia solution changed the color
The aldehyde-functionalized Fe O @MCM-41 nanocom-
3 4
of solution from orange to black immediately. The reaction
mixture was stirred at the same temperature for another 1 h.
After completion of the reaction, the formed magnetite pre-
cipitates were separated from the reaction medium using an
external magnet, washed several times with deionized water
and dried under vacuum at 60 °C.
posites (1 g) were added to round-bottom ꢃask comprising
absolute ethanol (20 mL) and were homogeneously dis-
persed. After 30 min of ultrasonication, N(4)-phenylthio-
semicarbazide (4 mmol) in EtOH was added into the mixture
and reꢃuxed for 24 h. At the end of the reaction, Schiꢀ base
functionalized mesoporous materials were collected using
an external magnet, washed with ethanol, and dried in oven
at 60 °C.
2
.4 Synthesis of Fe O @MCM‑41 Nanocomposites
3 4
A mixture of Fe O MNPs (1.0 g), deionized water (60 mL)
2.8 Synthesis of Fe O @MCM‑41‑SB‑Pd
3
4
3
4
and ethanol (75 mL) were dispersed by ultrasonic vibration
for 30 min. Then 5 mL of ammonia aqueous solution was
added to the mixture during 10 min and allowed to stir for
Nanocomposites
500 mg of Fe O @MCM-41-SB was dispersed in metha-
3
4
1
h at room temperature under an N atmosphere. To form
nol (50 mL) and a solution of Pd(OAc) (1 mmol, 0.224 g)
2
2
the framework of mesoporous, 20 mL of CTAB solution
in acetone (10 mL) was added dropwise. This mixture was
1
3

A. Ahmadi et al.
Fig. 1 The route for synthesis of the functionalized magnetic mesoporous silica nanocomposite
reꢃuxed under N atmosphere for 24 h. The resulting cata-
phenylboronic acid (1.2 mmol) and K CO (1.2 mmol) were
2 3
2
lyst was magnetically separated, washed with acetone and
added to the reaction ꢃask and the mixture was reꢃuxed at
120 °C. The progress of the reaction was checked by TLC
methanol and ꢁnally dried in oven at 60 °C for 10 h.
(
n-hexane/EtOAc). After completion of the reaction, the
2
.9 General Procedure for the Suzuki–Miyaura
reaction mixture was cooled to the room temperature. The
catalyst was separated by an external magnet and washed
with ethyl acetate. The resultant mixture was extracted with
n-hexane to isolate the product and then the crude product
was recrystalized from n-hexane.
Cross‑Coupling Reaction Using Fe O @
3
4
MCM‑41‑SB‑Pd as Catalyst
2
mg of the nano-catalyst (Fe O @MCM-41-SB-Pd)
3 4
was dispersed in DMF (5 mL) using sonication for
3
0 min at room temperature. Then, aryl halide (1 mmol),
1
3

Magnetic Mesoporous Silica Nanocomposite Functionalized with Palladium Schif Base Complex:…
2
.10 Enzyme Immobilization
Figure 2 shows the FT-IR spectra of all synthesized
nanocomposites which are in agreement with the proposed
structure shown in Fig. 1. In Fig. 2a, the bands appearing
2
mg of α-amylase enzyme was dissolved in 1 mL of phos-
−
1
phate buꢀer saline (PBS) and then 20 mg of each sample,
Fe O @MCM-41, Fe O @MCM-41-NH , Fe O @MCM-
at 3406 and 1626 cm were attributed to the vibrational
modes of water molecules adsorbed to the surface. The
3
4
3
4
2
3
4
−
1
4
1-ald and Fe O @MCM-41-SB-Pd was added to this solu-
sharp peak at about 585 cm that is observable in all spec-
tra, was assigned to stretching vibrations of Fe–O bonds in
tetrahedral sites of iron oxide [38]. The strong absorption
3
4
tion. The mixture was incubated with continuous shaking
(
75 rpm) at 37 °C for 24 h. Then the mesoporous nano-
−
1
composites loaded with α-amylase enzyme were separated
using high-speed centrifuging with 10,000 rpm for 5 min
and washed three times with PBS. In the following, these
nanocomposites were dispersed in phosphate buꢀer saline
bands at 1075, 805 and 453 cm in Figs. 2b–g are cor-
responding to the asymmetric and symmetric stretching
of Si–O–Si framework vibrations and Si–O–Si or O–Si–O
bending modes, respectively [39–41]. In the IR spectrum of
Fe O @ MCM-41-CTAB (Fig. 2b), the strong bands at 2919
(
500 μL) and the paper disks were saturated with these solu-
3
4
−1
tions. Saturated paper disks were placed on plates containing
starch 10%. Also, a disc saturated with the third wash solu-
tion was considered as a control for each compound. Finally,
plates were incubated at 37 °C for 24 h. Then the area of
inhibition around each disc was measured after addition of
iodine solution.
and 2850 cm were related to the C–H stretching vibrations
of CTAB. These bands were completely eliminated at the
spectrum of Fe O @ MCM-41, which conꢁrms the com-
3
4
plete removal the template. In the FT-IR spectra of Fe O @
3
4
MCM-41-AEAPS, the presence of the anchored n-pro-
pylamine groups are conꢁrmed by new bands at 2936 and
−
1
2
882 cm assigned to the C–H stretching vibrations, and
−1
the band at 1630 cm related to NH/NH bending vibration.
2
3
Results and Discussion
In the IR spectrum of Fe O @MCM-41-ald and Fe O @
3
4
3
4
−
1
MCM-41-SB, a sharp band at 1635 cm is appeared which
3.1 Preparation and Characterization
attributed to the stretching vibration of the imine group
(
C=N) and conꢁrms successful formation of Schiꢀ base. In
Figure 1 illustrates the synthesis pathway of the Fe O @
the FT-IR spectrum of Fe O @MCM-41-SB-Pd, the C=N
3 4
3
4
MCM-41-SB-Pd nanocomposite. First, Fe O magnetic
stretching vibration slightly shifts to lower frequencies and
3
4
−
1
nanoparticles were synthesized by a conventional co-pre-
cipitation method. Subsequently, to increase the stabil-
ity and activity of the magnetic nanoparticles, silica was
coated on the surface of Fe O by basic hydrolysis of TEOS
appears at 1628 cm . This shift conꢁrms the coordination
of the imine nitrogen atom to palladium and reveals that the
catalyst has been successfully synthesized.
3
4
in the presence of CTAB. After the removal of CTAB,
Fe O @MCM-41 nanoparticles were surface-modiꢁed with
3
4
N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPS).
In this reaction, –NH groups were introduced on the sup-
2
port by treatment of the methoxy groups of AEAPS with the
active silanol groups on Fe O @MCM-41. Schiꢀ base was
3
4
covalently anchored to the surface of magnetic nanoparti-
cles in two steps using condensation of the carbonyl groups
of the dialdehyde with terminal amine of AEAPS and then
with the amine group of the N(4)-phenylthiosemicarbazide.
Finally, incorporation of palladium onto the nanoparticles
was carried by reaction of the Pd(OAc) with functionalized
2
Fe O @MCM-41. The concentration of palladium (0/II) was
3
4
9
.5 wt% (0.89 mmol/g), which was determined by AAS. It is
noteworthy that due to presence of several good coordination
sites on the bis-Schiꢀ base ligand including imine, amine,
thiol and hydroxyl groups, high amount of palladium was
loaded on the catalyst. The synthesized new heterogeneous
catalyst was characterized by FT-IR, TGA, XRD, FE-SEM,
Fig. 2 FT-IR spectra of synthesized nanocomposites a Fe O , b
3
4
HRTEM, EDX, VSM, XPS and DRS techniques and N
2
Fe O @ MCM-41-CTAB, c Fe O @ MCM-41, d Fe O @ MCM-
3
4
3
4
3
4
adsorption–desorption isotherms.
4
1-AEAPS, e Fe O @MCM-41-ald f Fe O @MCM-41-SB and g
3 4 3 4
Fe O @MCM-41-SB-Pd
3
4
1
3

A. Ahmadi et al.
Fig. 3 FE-SEM images of the magnetic nanoparticles: a Fe O b Fe O @ MCM-41 c Fe O @MCM-41-SB-Pd, d, e TEM images of Fe O @
3
4
3
4
3
4
3
4
MCM-41-SB-Pd
The morphology and size of Fe O , Fe O @ MCM-41
3
4
3
4
and Fe O @MCM-41-SB-Pd nanoparticles were studied by
3
4
ꢁ
eld-emission scanning electron microscopy (FE-SEM) and
transmission electron microscopy (TEM) analysis (Fig. 3).
FE-SEM and TEM images show that the prepared com-
posites are about uniform spherical nanoparticles with a
diameter in the range of 12–30 nm. TEM images conꢁrm a
core–shell structure of the catalyst.
The chemical composition of the Fe O @MCM-41-SB-
3
4
Pd nanocatalyst was determined by energy-dispersive X-ray
spectrum (EDX) analysis. The results have been shown in
Fig. 4. This analysis exhibits that the Fe O @MCM-41-SB-
3
4
Pd contains Fe, Si, Pd, N, C, S and O elements, which con-
ꢁ
rms the immobilization of Schiꢀ base complex on silica
coated Fe O nanoparticles. The presence and distribution
3
4
Fig. 4 The elemental composition of Fe O @MCM-41-SB-Pd deter-
3
4
of elements were as well as determined by EDX mapping
mined by EDX analysis
(
Fig. 5).
1
3

Magnetic Mesoporous Silica Nanocomposite Functionalized with Palladium Schif Base Complex:…
Fig. 5 Elemental mapping of Fe O @MCM-41-SB-Pd
3
4
The wide angle XRD (WAXRD) patterns of bare Fe O
cubic spinel structure [42–44]. All diꢀraction peaks of pure
Fe O were also observed in the XRD patterns of Fe O @
3
4
nanoparticles, Fe O @ MCM-41, Fe O @MCM-41-ald,
3
4
3
4
3
4
3
4
Fe O @MCM-41-SB-Pd are given in Fig. 6. In the XRD
MCM-41, Fe O @MCM-41-ald and Fe O @MCM-41-SB-
3
4
3 4 3 4
pattern of bare Fe O nanoparticles, the characteristic
Pd, suggesting that the magnetic core has been preserved
during the surface modiꢁcation. In the XRD pattern of the
Fe O @MCM-41-SB-Pd, the appearance of the new peaks
3
4
peaks observed at 2θ = 30.50°, 35.87°, 43.51, 53.88°,
5
7.43°, 63.00° and 74.55° are assigned to the crystalline
3
4
planes (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0)
and (5 3 3) of magnetite, respectively, which agrees with
the standard Fe O XRD spectrum of highly crystalline
at 2θ=40.40°, 46.59° and 68.11° corresponding to (1 1 1),
(2 0 0) and (2 2 0) planes, respectively, is related to the fcc
palladium [45–49]. Therefore, Pd nanoparticles were formed
3
4
1
3

A. Ahmadi et al.
anchoring of palladium complex provides an evidence that
functionalization occurs also inside the mesopore channels
[
58].
The nitrogen adsorption–desorption isotherms of Fe O @
3
4
MCM-41 and Fe O @MCM-41-SB-Pd are shown in Fig. 8.
3
4
BET and BJH data of Fe O @ MCM-41 in comparison to
3
4
Fe O @MCM-41-SB-Pd are shown in Table 1. According
3
4
to the IUPAC classiꢁcation, these materials show a typical
type IV isotherm, which is a characteristic of mesoporous
materials. The results display the BET surface area, pore
2
volume and pore size for Fe O @ MCM-41 is 481.13 m /g,
3
4
3
0
.505 cm /g and 3.22 nm, respectively. After the grafting
2
of palladium complex, these value change to 28.16 m /g,
3
0
.162 cm /g and 10.09 nm, respectively, consistent with the
presence of complexes in the pore channels [30, 57, 59].
The magnetic properties of the synthesized nanomate-
rials were investigated at room temperature in the applied
magnetic ꢁeld sweeping from −12,000 to 12,000 Oe using
vibrating sample magnetometry (VSM), as shown in Fig. 9.
The saturation magnetization values for Fe O , Fe O @
Fig. 6 Wide angle X-ray diꢀraction patterns of bare Fe O nano-
3
4
4
particles, Fe O @ MCM-41, Fe O @MCM-41-ald, Fe O @MCM-
3
4
3
4
3
4
1-SB-Pd and reused Fe O @MCM-41-SB-Pd
3 4
3
4
3
4
MCM-41, Fe O @MCM-41-SB and Fe O @MCM-41-SB-
3
4
3
4
Pd were 80.5, 55.8, 51.3 and 36.9 emu/g, respectively. As it
obvious in Fig. 9, after addition of the SiO and then func-
2
tionalization with palladium Schiꢀ base complex, the satura-
tion magnetization of the Fe O decreased considerably from
3
4
8
0.5 to 36.9 emu/g. Nevertheless, the synthesized catalyst
can still be separated from the reaction medium easily by
applying an external magnetic ꢁeld.
The thermogravimetric analysis (TGA) of Fe O @MCM-
3
4
4
1, Fe O @MCM-41-SB and Fe O @MCM-41-SB-Pd are
3 4 3 4
shown in Fig. 10. Fe O @MCM-41 shows only a small
3
4
weight loss due to the removal of physically adsorbed water.
Fe O @MCM-41-SB and Fe O @MCM-41-SB-Pd nano-
3
4
3
4
composites also show a ꢁrst weight loss below 200 °C due to
the removal of physically adsorbed water and solvent inside
the pores channels [30, 60]. Then, the main weight loss in
the temperature range of 200–800 °C corresponds to the
thermal degradation of immobilized organic materials on
Fe O @MCM-41 surface.
Fig. 7 Low angle X-ray diꢀraction patterns of Fe O @ MCM-41 and
3
4
Fe O @MCM-41-SB-Pd
3
4
3
4
in the absence of any additional reducing agent [50]. Fig-
ure 7 shows the low-angle XRD patterns of the Fe O @
The oxidation state of palladium in catalyst was deter-
mined by XPS (Fig. 11). Figure 11a shows C, N, O, S, Fe
and Pd peaks and Fig. 11b gives the high-resolution Pd
3d XPS signals. The characteristic peaks at 335.36 and
341.04 eV can be indexed to the Pd (0) state and the other
set of peaks at 337.32 and 343.18 eV are related to the Pd
(II) oxidation state [61, 62]. The integration areas of these
peaks indicate that Pd (0) is the major phase on the catalyst.
The optical properties of Fe O @MCM-41-SB-Pd have
3
4
MCM-41 and Fe O @MCM-41-SB-Pd. A single intense
3
4
peak at 2θ=2.13 and two weak peaks at 2θ=3.54 and 4.35
are appeared in the XRD pattern of Fe O @ MCM-41 which
3
4
are assigned to reꢃections from the (1 0 0), (1 1 0) and (2
0
0) planes, respectively. These peaks are the characteristic
of the ordered hexagonal mesoporous structure of MCM-41
type MSN (mesoporous silica nanoparticles) [30, 51–56].
Fe O @MCM-41-SB-Pd gives only one broad Bragg peak
3
4
been studied through UV–Visible DRS spectrum in the
range of 200–800 nm (Fig. 12). A broad absorbance band
at range of 500–800 nm may be related to the charge-trans-
fer transitions from nitrogen or sulfur lone pair to vacant
d-orbitals of the metal [63]. This strong absorbance in the
3
4
at low angle corresponding to (1 0 0) plane. This broadening
and disappearing of XRD peaks are due to functionaliza-
tion that decreases the regularity of hexagonal mesostructure
[
30, 57]. The decrease in the intensity of the (1 0 0) peak in
1
3

Magnetic Mesoporous Silica Nanocomposite Functionalized with Palladium Schif Base Complex:…
Fig. 8 N adsorption–desorption isotherms of a Fe O @ MCM-41 and b Fe O @MCM-41-SB-Pd
2
3
4
3
4
Table 1 BET and BJH data of Fe O @ MCM-41 and Fe O @MCM-
3
4
3
4
4
1-SB-Pd
Sample
BET surface Pore
Pore size (nm)
2
area (m /g) volume
3
(
cm /g)
Fe O @ MCM-41
481.13
28.16
0.505
0.162
3.22
10.09
3
4
Fe O @MCM-41-SB-Pd
3
4
Fig. 10 The TGA curve of the a Fe O @MCM-41 b Fe O @MCM-
3
4
3
4
4
1-SB and c Fe O @MCM-41-SB-Pd
3
4
(
C=N), respectively, and also spin-allowed d–d transition of
palladium [63–66].
3.2 Catalytic Reaction
Catalytic performance of Fe O @MCM-41-SB-Pd nano-
3
4
Fig. 9 Magnetization curves at 300 K for Fe O , Fe O @MCM-41,
3
4
3
4
catalyst was evaluated through the Suzuki–Miyaura cou-
pling reaction. In order to optimize the reaction conditions,
the coupling of bromobenzene with phenylboronic acid has
been chosen as a model reaction and various parameters
including catalyst and base dose, solvent and temperature
were optimized. The reactions were monitored using TLC
and the yields were reported after isolation of product.
Fe O @MCM-41-SB and Fe O @MCM-41-SB-Pd
3
4
3
4
visible light region may be also due to the surface plasmon
resonance (SPR) of Pd nanoparticles [64]. The absorptions
at lower wavelength range is due to π→π* and n→π* tran-
sition of aromatic phenyl moieties (C=C) and imine systems
1
3

A. Ahmadi et al.
Fig. 11 a XPS spectrum of Fe O @MCM-41-SB-Pd catalyst and b high-resolution XPS Pd 3d spectrum
3
4
Table 3 Inꢃuence of diꢀerent solvents and temperature on Suzuki
reaction
Entry
Solvent
Temperature (°C) Yield (%)a
1
2
3
4
5
6
7
H O
EtOH
100
80
78
120
100
75
Trace
30
40
98
60
35
Trace
2
H O–EtOH
2
DMF
DMF
DMF
DMF
25
Reaction carried out with 1 mmol of bromobenzene, 1.2 mmol of
phenylboronic acid, 2 mg of catalyst, 1.2 mmol of K CO and 5 mL
2
3
of solvent
Isolated yield
a
Fig. 12 UV–Visible DRS spectrum of Fe O @MCM-41-SB-Pd
3
4
Table 4 Inꢃuence of amount
Yield (%)a
Entry Amount
of base
of base (K CO ) on Suzuki
2
3
reaction
Table 2 Inꢃuence of amount of catalyst (Fe O @MCM-41-SB-Pd)
(mmol)
3
4
on Suzuki reaction
1
2
3
4
1
80
98
98
98
Entry
Amount of catalyst Pd content
Yield (%)a
1.2
1.5
2
(mg)
(mol%)
1
2
3
4
1
2
3
5
0.089
0.18
0.27
0.45
70
98
98
98
Reaction carried out with
1
mmol of bromobenzene,
1.2 mmol of phenylboronic
acid, 2 mg of catalyst and 5 mL
of DMF at 120 °C
Reaction carried out with 1 mmol of bromobenzene, 1.2 mmol of
phenylboronic acid, 2 mmol of K CO and 5 mL of DMF at 120 °C
2
3
a
Isolated yield
a
Isolated yield
The results are summarized in Tables 2, 3, and 4. Table 2
illustrates the eꢀect of catalyst molar ratio on the reaction
time of bromobenzene as a model substrate at 120 °C in
DMF. The optimum amount of the catalyst was found to
be 2 mg (0.18 mol% of Pd) in current conditions (Table 2,
entry 2). According to the results shown in Table 3, biphe-
nyl product was detected in trace-40% yield when the reac-
tion was carried out in water, EtOH and H O-EtOH (1:1)
2
(Table 3, entries 1–3). When N,N-dimethylformamide
(DMF) was used as solvent, an excellent yield (98%) was
1
3

Magnetic Mesoporous Silica Nanocomposite Functionalized with Palladium Schif Base Complex:…
Table 5 Suzuki reaction catalyzed by various catalysts
TOF (Table 6, entry 7) and the reaction of 3-bromotoluene
−
1
Yield (%)d
proceeds with high yield of 95% in 10 min and 31.05 h
Entry
Catalyst
Time (h)
TOF. Also aryl iodides react much faster than aryl bromides
with higher TOF (Table 6, entries 9–11). Chlorobenzene
was also successfully coupled with phenylboronic acid in
the presence of this catalyst with high yield of 98% in 0.5 h
1
2
3
4
5
6
Nonea
24
24
24
24
4
0
0
0
0
40
98
a
Fe O @MCM-41
3
4
a
Fe O @MCM-41-SB
3
4
a
−
1
Fe O
3
4
and 10.88 h TOF (Table 6, entry 12).
b
Pd(OAc)2
The catalytic performance of the Fe O @MCM-41-SB-
3
4
c
Fe O @MCM-41-SB-Pd
0.17
3
4
Pd is compared with some Pd-based heterogeneous catalysts
reported in the literature for the model reaction (Table 7)
Reaction carried out with 1 mmol of bromobenzene, 1.2 mmol of
a
b
[
46, 49, 67–71]. The results show that the high loading of
phenylboronic acid and 5 mL of DMF at 120 °C. 2 mg, 0.4 mg,
c
d
2
mg of catalyst (9.5 wt% Pd), Isolated yield
palladium on Fe O @MCM-41-SB eꢀectively makes per-
3 4
forming the reaction with only a small amount of the catalyst
and a higher reaction yield within shorter time. Furthermore,
the catalyst is easily recovered using an external magnetic
ꢁeld.
detected (Table 3, entry 4). As the result of the tables, after
a series of optimization experiments, the best outcome was
achieved with 1.2 mmol of K CO (Table 4) in the presence
In order to investigate the reusability of the magnetic het-
erogeneous palladium catalyst, a set of experiments were
performed under optimized conditions for the reaction of
bromobenzene with phenylboronic acid. After each run,
the reaction mixture was cooled at room temperature and
the catalyst was extracted by applying an external mag-
net, washed several times with water and ethyl acetate and
ꢁnally dried under vacuum for the next run. As shown in
Fig. 13, the Fe O @MCM-41-SB-Pd catalyst is recycled
2
3
of 2 mg of catalyst (0.18 mol% Pd) and in DMF as solvent at
1
20 °C. To investigate the eꢀective component of the cata-
lyst, a series of control experiments were conducted. The
results were summarized in Table 5. In the absence of any
catalyst, or in the presence of only Fe O , Fe O @MCM-41
3
4
3
4
or Fe O @MCM-41-SB as a control, hardly any reaction
3
4
occurred (Table 5, entries 1–4). The results indicated that
the Suzuki reaction could not proceed in the absence of Pd.
3
4
In the presence of only Pd(OAc) as a catalyst, low yield was
for ꢁve times without signiꢁcant decreasing in the catalytic
activity. FT-IR spectroscopy also conꢁrmed that the struc-
ture of the catalyst after 5 cycles has not changed (Fig. 14).
Furthermore, XRD pattern of recovered catalyst (Fig. 6)
demonstrates that the catalyst can be recycled several times
without any considerable change in the structure. The meas-
urement of palladium by atomic absorption before (9.5 wt%)
and after (9.2 wt%) catalytic reaction also showed that pal-
ladium leaching was negligible, probably due to excellent
binding of the amine, imine, thiol and hydroxyl groups [72].
This result proves a strong interaction between the support
material and palladium. These analyses clearly demonstrate
that the catalyst is stable, recoverable and the crystalline
structure of it did not change after several runs.
2
obtained at a longer time (Table 5, entry 5). While using the
supported catalyst, Fe O @MCM-41-SB-Pd, yield is up to
3
4
9
8% (Table 5, entry 6). Therefore, supporting of palladium
on functionalized mesoporous silica promoted the Suzuki
reaction and made the catalyst achieve excellent catalytic
performance.
After the optimization of the reaction conditions, the
reaction of the various aryl halides, even aryl chloride, con-
taining electron-donating and electron-withdrawing groups
with phenylboronic acid were carried out under optimized
conditions and the results were summarized in Table 6. The
results show that the catalyst system is very eꢂcient and the
biphenyl derivatives were obtained in a short time with high
yields. Suzuki–Miyaura reactions of activated aryl bromides
including 4-bromotolune, 4-bromoanisole and 4-bromophe-
nol proceeded with high yields of 98%, 85% and 80%, in
short reaction time and high turnover frequencies (TOF) of
3.3 Enzyme Immobilization
The results of α-amylase enzyme immobilization on the
Fe O @MCM-41 and functionalized Fe O @MCM-41
−
1
−1
−1
3
2.02 h , 9.44 h , and 13.47 h , respectively, (Table 6,
3
4
3
4
entries 2–4). The Suzuki reaction of deactivated aryl bro-
mides with diꢀerent substituents also gave high yields but in
longer reaction times and lower TOF than activated aryl bro-
mides (Table 6, entries 5 and 6). The reactions of sterically
hindered ortho and meta-position halides with phenylbo-
ronic acid also produced the desired biaryls under the opti-
mized reaction conditions (Table 6, entries 7 and 8). Based
on these data, 2-bromotoluene which is sterically hindered
nanocomposites are shown in Fig. 15 and listed in Table 8.
The results show that the α-amylase was immobilized on all
composites, but the functionalized composites more success-
fully load the enzyme. Immobilization of enzymes on a sup-
port occurs by covalent or noncovalent interactions through
binding to a support, encapsulation and cross-linking
[73–75]. Hydroxyl groups on Fe O @MCM-41 may interact
3
4
with OH groups of α-amylase by hydrogen bonding. Func-
−
1
could be coupled in good yield of 75% in 1 h and 4.02 h
tionalized nanocomposites have amine, imine, carbonyl or
1
3

A. Ahmadi et al.
Table 6 Suzuki–Miyaura reaction of aryl halides with phenylboronic acid catalysed by Fe O @MCM-41-SB-Pd
3
4
Fe O @MCM-41-SB-Pd
3
4
X
+
B(OH)2
G
o
K CO , DMF, 120 C
G
2
3
Entry
1
Aryl halide
Product
Time (min)
10
Yield (%)a
98
TONb TOF (h-1)c
Br
5.44
5.44
4.72
4.44
5.00
32.02
32.02
9.44
H C
Br
3
10
30
20
40
98
85
80
90
2
3
4
5
H C
3
H CO
3
H CO
Br
3
HO
HO
Br
Br
13.47
7.46
OHC
O
OHC
O
H CC
Br
6
7
3
60
60
90
75
5.00
4.02
5.00
4.02
H CC
3
Br
CH3
CH3
Br
8
9
10
95
5.28
31.05
H C
H C
3
3
I
5
15
5
98
85
98
98
5.44
4.72
5.44
5.44
68.06
18.89
68.06
10.88
H CO
3
10
11
12
H CO
3
I
O N
2
O N
2
I
Cl
30
Reaction conditions: aryl halides (1.0 mmol), phenylboronic acid (1.2 mmol), K CO (1.2 mmol), Fe O @MCM-41-SB-Pd (2 mg, 0.18 mol%
2
3
3
4
Pd) and DMF (5 mL) at 120 °C
a
Isolated yield
b
TON, turnover number, moles of aryl halides converted per mole of Pd
c
TOF, turnover frequencies, TON/time of reaction
1
3

Magnetic Mesoporous Silica Nanocomposite Functionalized with Palladium Schif Base Complex:…
Table 7 Catalytic performance of diꢀerent Pd-based catalysts in the coupling of bromobenzene and phenyl boronic acid
o
Entry
Catalyst
Solvent
Temp. ( C)
Time (h)
Yield (%)
References
1
2
3
4
5
6
7
8
SBA-15/AO/Pd(II)a
Anchored Pd(II)–Schiꢀ base complex
H O:EtOH
50
50
120
75
60
80
70
120
1
98
100
80
92.22
98
97.23
96
[46]
[49]
[61]
[62]
[63]
[64]
[65]
Present work
2
b
H O
0.5
12
6
2
6
1
0.16
2
c
[PdCl(PC(CH ) CP) (CH CN)]BF
DMF
2
5
3
4
d
Fe O @C-Pd-550
Pd@imine-SiO
EtOH
3
4
e
i
H O: PrOH
2
2
f
i
Fe O @SiO2@mSiO –Pd(0)
PrOH
3
4
2
g
MNP@SPGMA@AP@Pd
DMF:H O
2
h
Fe O @MCM-41-SB-Pd
DMF
98
3
4
a
Pd immobilized on amidoxime-functionalized Mesoporous SBA-15 (0.010 g cat., 0.002 mol% Pd)
Nanosilica (derived from rice husk)-anchored Pd(II)–Schiꢀ base complex (10 mg)
Silver(I) and palladium(II) complexes of pentamethylene-functionalized bis-imidazolium dication ligands (0.1 mol% Pd complex)
Core–shell magnetic composite catalyst (10 mg)
b
c
d
e
f
Silica supported palladium Schiꢀ base catalyst (0.02 g cat., 0.08 mol% Pd)
Immobilized palladium on the amine-modiꢁed core–shell magnetic mesoporous Fe O @SiO @mSiO microspheres (0.075 mol% cat.)
3
4
2
2
g
h
Palladium loaded magnetic nanocomposite based on natural polysaccharide salep modiꢁed with poly (amino pyridine) (0.1 mol% cat.)
Magnetic mesoporous silica nanocomposite functionalized with palladium Schiꢀ base complex (2 mg cat., 0.18 mol% Pd)
Fig. 13 Reusability of catalyst in Suzuki reaction of bromobenzene
with phenylboronic acid
thiocarbonyl groups that can interact with functional groups
of amylase by hydrogen bonding or covalent bond forma-
tion. Free probable coordination sites on the palladium may
also be suitable for binding of Fe O @MCM-41-SB-Pd with
Fig. 14 Comparison of the IR spectrum of Fe O @MCM-41-SB-Pd
3
4
composite a before catalytic reaction and b after recycling ꢁve times
3
4
enzyme. In view of the importance of biocatalysts, these
modiꢁed magnetic mesoporous silica nanoparticles can be
good candidates for enzyme immobilization.
4 Conclusion
Magnetic mesoporous silica nanocomposites were synthe-
sized and functionalized with an ONS donor Schiꢀ Base
and then loaded with relatively large amounts of palladium
(
9.5 wt%). Nanocomposites were characterized by various
methods. Then, Fe O @MCM-41-SB-Pd was success-
3
4
fully used for the Suzuki–Miyaura coupling reaction and
1
3

A. Ahmadi et al.
1
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02:3615
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a Fe O @MCM-41, b Fe O @MCM-41-NH , c Fe O @MCM-41-
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2
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Fe O @
Fe O @
Fe O @
Fe O @
3 4
3
4
3
4
3
4
MCM-41
MCM-
MCM-41-ald MCM-
2
3. Manjunatha K, Koley TS, Kandathil V, Dateer RB, Balakrishna
G, Sasidhar B, Patil SA, Patil SA (2018) Appl Organomet Chem
4
1-NH
41-SB-Pd
2
3
2:e4266
Eꢀect zone 16
20
26
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Catal Commun 60:70
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showed excellent catalytic performance for the synthesis
of substituted biaryls. The recovery of the catalyst is easily
performed using an external magnetic ꢁeld and also can be
reused several times without considerable loss of its catalytic
activities. Synthesized mesoporous nanocomposites were
also investigated for immobilization of amylase enzyme and
the good results were achieved. According to our results,
high chelating ability of imine, amine, thiol and hydroxyl
groups of bis-Schiꢀ base ligand can load large amounts of
palladium, restrict palladium leaching in the catalysis pro-
cess and also eꢀectively immobilize amylase enzyme.
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Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional aꢂliations.
Aꢀliations
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1
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2,3
Ameneh Ahmadi · Tahereh Sedaghat · Roya Azadi · Hossein Motamedi
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3
Department of Chemistry, Faculty of Science, Shahid
Chamran University of Ahvaz, Ahvaz, Iran
Biotechnology and Biological Science Research Center,
Shahid Chamran University of Ahvaz, Ahvaz, Iran
2
Department of Biology, Faculty of Science, Shahid Chamran
University of Ahvaz, Ahvaz, Iran
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3