Magnetic nanoparticle-supported tetrazole-functionalized palladium catalyst: synthesis, DFT study and application for Sonogashira and Heck cross-coupling reactions

Article abstract of DOI:10.1007/s11164-017-3018-0

Abstract: A novel magnetic catalyst-immobilized nano-palladium was efficiently prepared via a (2?+?3) cycloaddition pathway and characterized with different analyses, including FT-IR, FE-SEM, EDS, XRD, TEM and VSM methods. Also, topological and NBO analysis of a simple model of the catalyst with the M06/6-311?g(d,p)-LANL2DZ model chemistry have been introduced. In addition, the catalytic pursuit of this novel magnetic nanocatalyst was successfully evaluated in Sonogashira and Heck cross-coupling reactions. Furthermore, the catalyst could be easily separated by a conventional method using a permanent magnet and reused repeatedly in the model reaction without noteworthy loss of its catalytic activity.

Full text of DOI:10.1007/s11164-017-3018-0

Res Chem Intermed
DOI 10.1007/s11164-017-3018-0
Magnetic nanoparticle-supported tetrazole-
functionalized palladium catalyst: synthesis, DFT study
and application for Sonogashira and Heck cross-
coupling reactions
1
1
2
•
Ali Elhampour
Vahid Mohagheghi
•
Firouzeh Nemati
1
•
Hossein Taherpour Nahzomi
Received: 15 March 2017 / Accepted: 19 June 2017
Ó Springer Science+Business Media B.V. 2017
Abstract A novel magnetic catalyst-immobilized nano-palladium was efficiently pre-
pared via a (2 ? 3) cycloaddition pathway and characterized with different analyses,
including FT-IR, FE-SEM, EDS, XRD, TEM and VSM methods. Also, topological and
NBO analysis of a simple model of the catalyst with the M06/6-311 g(d,p)-LANL2DZ
model chemistry have been introduced. In addition, the catalytic pursuit of this novel
magnetic nanocatalyst was successfully evaluated in Sonogashira and Heck cross-cou-
pling reactions. Furthermore, the catalyst could be easily separated by a conventional
method using a permanent magnet and reused repeatedly in the model reaction without
noteworthy loss of its catalytic activity.
&
Firouzeh Nemati
fnemati@semnan.ac.ir
1
2
Department of Chemistry, Semnan University, Semnan 35131-19111, Iran
Department of Chemistry, Payame Noor University, Tehran, Iran
123

A. Elhampour et al.
Graphical Abstract
R2
(
11 entries)
G
DMF
o
X
1
20 C
NH
Pd
2
NH2
G
N
N
N
N
N
N
N
Pd
N
N
NH
2
N
N
N
Pd
N
DMF
Si
OOO O
Si
OO
o
N
90 C
NH2
N
N
Si
N
O
d
P
O
O
O
Si
N
O
O
R1
O
N
N
Si
2
H N
O
Pd
O
O
O
O
N
N
Si
N
N
2
H N
Pd
(17 entries)
G
Keywords Nano-magnetic catalyst Á Aminobenzonitrile Á
Azidopropyltriethoxysilane Á Tetrazole Á DFT study
Introduction
Palladium catalysis offers beneficial effects to synthetic chemists for the design of
new transformations and the making of numerous structural moieties. The
palladium-catalyzed cross-coupling of aryl halides, known as Sonogashira coupling
and Heck reaction, are the most powerful tools for the organization of C–C bonds.
These C–C cross-coupling reactions are among the most useful and most widely
studied synthetic transformations, as their products have great industrial potential
for the synthesis of fine chemicals such as pharmaceuticals and herbicides. Also,
they are widely used as versatile building blocks for many organic intermediates in
material sciences [1–4]. In the last two decades, they have been widely studied and
typically catalyzed by palladium catalysts [5, 6]. Recently, different palladium
catalysts including palladium–phosphine complexes, [7, 8] oxime-carbapalladacy-
cles [9, 10] and palladium–N-heterocyclic carbene complexes [11, 12] have been
designed and used in this transformation. Although homogeneous Pd catalysts
display high catalytic activity, the separation of them from the products is often
complicated. Furthermore, catalyst recovery in cases of using expensive noble metal
catalysts or environmental concerns may be desirable attributes. Hence, an
1
23

Magnetic nanoparticle-supported tetrazole functionalized…
interesting resolving of these problems could be heterogenization, since the
immobilization of the Pd catalyst onto the surfaces of solid supports is greatly in
demand. At present, a number of methods for the heterogenization of homogeneous
catalysts, including micro-porous polymers [13, 14], carbon materials [15, 16],
amorphous or mesoporous silica [17, 18] metal oxides [19, 20] and molecular sieves
[
21] have been reported, in which Pd nanoparticles or complexes are involved as
active species. Recently, in order for easy separation and recovery, a variety of
heterogeneous catalysts anchoring Pd nano-particles have been designed and
utilized as heterogeneous catalysts in different chemical reactions [22–27]. Indeed,
incorporation of magnetic NPs into the structure of a support material increases the
rate of the separation and lowers the cost of the process, which makes them ideal
candidates for practical use in catalytic processes [28]. Generally, magnetic particles
consisting of an iron oxide core and functional shell have gained much attention due
to their unique separable future. On the other hand, immobilization of metals onto
the surfaces of magnetic particles supported by linkages is tremendously attractive
in the development of retriable catalysts. Recently, the Cu(I)-catalyzed azide–
alkyne ‘‘click chemistry’’ reaction has received appreciable concentration because
of the effortlessness, gentle reaction conditions, quantitative conversions and high
fortitude of functional groups; this method has been shown to be one of the best
tools for covalent attachment between appropriately functionalized molecules and
solid supports via 1,2,3-triazole linkages [29, 30]. Of note, the importance of
tetrazole chemistry, especially in the fields of organometallic, medicinal materials
chemistry, has led to its widespread use by researchers [31–33]. These reactions
provide high regioselectivity to give 1H-tetrazoles with great atom efficiencies,
notable yields and low by-products. As a part of our program aimed at designing
supported metal catalysts [34–40], we here report an effective and widely applicable
approach for the synthesis of tetrazole-functionalized nanomagnetic Fe O -
3
4
supported palladium nano-particles and their application as a novel and
stable heterogeneous catalyst for the copper and phosphine-free Sonogashira and
Heck coupling reactions in excellent yields (Scheme 1).
Experimental
Materials and methods
Chemical reagents in high purity were purchased from Merck and Aldrich and were
used as recieved. The progress of the reaction was monitored by TLC on
R1
R1
X
R2
R2
H
Heck reaction
Sonogashira reaction
G
G
G
Scheme 1 The Sonogashira and Heck coupling reactions
123

A. Elhampour et al.
commercial aluminum-backed plates of silica gel 60 F254. Magnetic Fe O @SiO –
3
4
2
T/Pd nanoparticles were characterized by Fourier transform infrared spectra (FT-IR)
-
in the range of 400–4000 cm (Shimadzu 8400 s spectrometer), X-ray diffraction
1
˚
XRD; Philips using Cu-Ka radiation of wavelength 1.54 A), scanning electron
(
microscopy analysis (SEM; Tescanvega II XMU Digital Scanning Microscope),
transmission electron microscopy (TEM) (CM30300Kv field emission transmission
electron microscope), and a vibrating sample magnetometer (VSM; Lakeshore7407)
at room temperature and elemental analysis of the catalyst was carried out on a
simultaneous ICP-Optical Emission spectrometer (Spectro Genesis, Germany).
Methods
Preparation of magnetic Fe O nano particles
3
4
The magnetic Fe O nano-particles were prepared in accordance with our earlier
3
4
report [35].
Synthesis of nano-Fe O @SiO core shell
4
3
2
This precursor was prepared in accordance with our earlier report [35]. MNPs (1 g)
were initially diluted via the sequential addition of water (20 mL), ethanol (60 mL),
and concentrated aqueous ammonia (2 mL, 25 wt%). The resulting dispersion was
then homogenized by ultrasonics. A solution of triethoxysilane (TEOS) (0.5 mL) in
ethanol (10 mL) was then added to the dispersion in a drop-wise manner under
continuous mechanical stirring. After vigorous stirring for 16 h, the product was
collected by an external magnetic and washed three times with ethanol. Finally, the
products were dried under vacuum at 70 °C for 5 h [41].
Preparation of 3-azidopropyltriethoxysilane
3
-Azidopropyltriethoxysilane was synthesized by procedures explained previously
42]. Briefly, 3-chloropropyltriethoxysilane (4.82 g, 20.0 mmol) was added to a
[
solution of sodium azide (1.95 g, 30.0 mmol) and cetyltrimethylammonium
bromide (1.45 g, 4.0 mmol) in dry acetonitrile (75 mL) under a nitrogen
atmosphere. The reaction mixture was stirred under reflux for 48 h. After that,
the solvent was removed under reduced pressure. The crude mixture was then
diluted in Et O (30 mL) and the suspension was filtered and washed with Et O
2
2
(2 9 10 mL). The collected solvent was removed and product 3 was obtained as a
pure and colorless liquid [4, 43].
Synthesis of azide-functionalized MNP
The functionalization of the azide group on the surface of Fe O @SiO was
3
4
2
performed as follows: 1.5 g of SiO -coated Fe O nanoparticles were dispersed in
2
3 4
dry toluene (75 mL) by sonication for 1 h. 3-Azidopropyltriethoxysilane (0.5 g,
.02 mmol) was then added, and the reaction mixture was refluxed for 24 h under
2
1
23

Magnetic nanoparticle-supported tetrazole functionalized…
nitrogen. After being cooled to ambient temperature, the azide-functionalized MNPs
4 were collected by a permanent magnet and washed thoroughly with toluene and
acetone, and then dried under vacuum overnight [4].
Synthesis of magnetic Fe O @SiO -T nano particles
3
4
2
The magnetic Fe O @SiO -T nano particles were synthesized according to
3
4
2
experimental results of our group. At first, 2-aminobenzonitrile (0.5 g) and azide-
functionalized magnetic nanoparticles 4 (1 g) were combined with CuI (17 mg,
0
5
.09 mmol) in DMF solvent (1:1, 10 mL) in air, and the mixture was stirred at
0 °C for 24 h. Then, the nanoparticles 5 were separated using a magnetic, washed
sequentially with Et O (3 9 20 mL), H O (3 9 20 mL), and then acetone
2
2
(
3 9 20 mL), and dried under vacuum overnight.
Loading Pd nanoparticles to obtain Fe O @SiO –T/Pd
3
4
2
For this purpose, doping of Pd nanoparticles onto Fe O @SiO -T (Fe O @SiO -T)
2
3
4
2
3
4
was performed according to the previous literature reports [43]. Therefore, 500 mg
of Fe O @SiO -T was dispersed in 20 mL ethanol solution under sonication for
3
4
2
0
hot water was added. To the resulting mixture, a solution of 1 mmol NaBH in
.5 h and, to this suspension, a solution of PdCl (12.5 mg, 0.07 mmol) in 10 mL
2
4
5 mL water was added and stirred for 12 h at ambient temperature. The solid
catalyst was separated using an external magnet, washed with EtOH (3 9 20 mL)
and dried under vacuum overnight.
General procedure for the Sonogashira coupling reaction
Aryl halide (1 mmol), terminal alkyne (1.2 mmol), triethylamine (2 mmol) and
Fe O @SiO –T/Pd (20 mg) were added to 3 mL DMF as solvent and the reaction
3
4
2
mixture was stirred at 90 °C for the appropriate time. After completion of the
reaction, the reaction mixture was cooled and diluted with hot ethanol (10 mL).
Then, the catalyst was separated by an external magnet from the cooled mixture,
washed with acetone, dried in an oven and re-used for a consecutive run under the
same reaction conditions. The filtrate was concentrated and the resulting residue
was purified by short column chromatography on silica gel to afford the desired
product in excellent yield [40].
General procedure for the Heck coupling reaction
Aryl halide (1 mmol), alkene (1.5 mmol), triethylamine (2 mmol) and Fe O @-
3
4
SiO –T/Pd (20 mg) were added to 3 mL DMF as solvent and the reaction mixture
2
was stirred at 120 °C for the appropriate time. After completion of the reaction, the
reaction mixture was cooled to room temperature and diluted with hot ethanol
(
10 mL). Then, the catalyst was separated by an external magnet from the cooled
mixture, washed with acetone, dried in the oven and re-used for a consecutive run
under the same reaction conditions. The filtrate was concentrated and the resulting
123

A. Elhampour et al.
3 4 2
Scheme 2 Synthesis of magnetic Fe O @SiO –T/Pd nanoparticles
residue was purified by short column chromatography on silica gel to afford the
desired product in excellent yield [40].
Results and discussion
Synthesis of nano magnetic-Fe O @SiO –T/Pd particles
3
4
2
The route leading to nano-magnetic-Fe O @SiO –T/Pd is schematically repre-
2
3
4
sented in Scheme 2. The magnetized particles were synthesized from relatively
inexpensive available materials and identified using FT-IR, XRD, FE-SEM, TEM,
and VSM techniques. At first, the silica-coated Fe O nanoparticles (SiO @Fe O )
3
4
2
3 4
were prepared according to our reported method [35] and selected as a magnetic
nano-support. After that, the azide group was incorporated into the SiO @Fe O
3 4
2
with an easy procedure [39] and confirmed by observation of the N band at
3
-
080 cm in the FT-IR spectra (Fig. 1). Then, the prepared azide-functionalized
1
2
Fe O @SiO core–shell particles (Fe O @SiO -N ) were reacted with 2-aminoben-
3
4
2
3
4
2
3
zonitrile via the ‘‘(2 ? 3) cycloaddition’’ to give the tetrazole ring, which was
further metalated by PdCl2 [40]. The catalytic properties of the synthesized
Fe O @SiO –T/Pd(0) were screened in the Sonogashira and Heck coupling
3
4
2
reactions.
1
23

Magnetic nanoparticle-supported tetrazole functionalized…
Fig. 1 The FT-IR spectra of (a) Fe
@
3 4 3 4 2 3 3 4 2 3 4 2
O , (b) Fe O @SiO –N , (c) Fe O @SiO -T and (d) Fe O @SiO
T/Pd
Structural characterization of the magnetic-Fe O @SiO @T/Pd
3
4
2
nanoparticles
FT-IR spectra of magnetic Fe O nanoparticles, Fe O @SiO -N , Fe O @SiO @T
2
3
4
3
4
2
3
3
4
and Fe O @SiO @T/Pd(0) are depicated in Fig. 1a–d, respectively. In all of them,
3
4
2
-
1
a main absorption band around 550 cm is observed. This band can be attributed to
the intrinsic stretching vibrations of Fe–O in the sample. On the other hand; the
FTIR spectrum of magnetic Fe O nanoparticles (Fig. 1a) shows an other absorption
3
4
-
1
band at 3353 cm ascribed to the O–H vibration. In addition, the band at
1
1
Specifically, Fig. 1b shows a vigorous absorption band at 2080 cm
029 cm- related to the Si–O–Si stretching vibration of SiO shell can be seen.
2
-
1
(N3
vibrations), which upon the formation of the 1H-terazole linker during the
‘(2 ? 3) cycloaddition’’ reaction disappears. The other functional groups on the
surface of the magnetic nanoparticles were confirmed by the bands at 1520 and
‘
-
1
2
952 cm which were assigned to C=N and C–H stretching vibrations, respec-
-
1
tively. Moreover, the typical band observed about 1410 cm can be assigned to the
C=C vibration of the aryl ring. Thesed results confirmed that the magnetic
nanoparticles were successfully supported by the tetrazole linker.
The XRD patterns of the nano-magnetic catalyst are shown in Fig. 2. which
shows the diffraction peaks that correspond to the standard Fe O sample (JCPDS
file No. 19-0629, Fig. 2a and showed as a black line). The strong diffraction peaks
3
4
at 2h = 30.1°, 34.7°, 42.3°, 56.2°, 57.1° and 62.5° can be indexed to the (220),
(
311), (400), (422), (511) and (440) reflections, respectively. Moreever, the broad
peak in 2h = 20°–30° is compatible with an amorphous silica shell (Fig. 2b–d). The
characteristic peaks at 2h of 40°, 47.5°, 67.1° and 80.1° (shown as a red line) can be
attributed to the (111), (200), (220) and (311) facets of elemental palladium, which
confirmed the profitable preparation of crystalline magnetic Fe O @SiO –T/Pd
3
4
2
nanoparticles. Further, the average crystal size of the Fe O core and nano-
3
4
123

A. Elhampour et al.
3 4 3 4 2 3 3 4 2 3 4 2
Fig. 2 XRD pattern of (a) Fe O , (b) Fe O @SiO –N , (c) Fe O @SiO -T and (d) Fe O @SiO @T/Pd
palladium particles were measured using the Scherrer formula, being about 19 and
7
.8 nm, respectively.
The magnetic hysteresis loops of samples were conventionally studied by VSM.
The magnetization curve for magnetic Fe O nanoparticles, Fe O @SiO -T and
3
4
3
4
2
Fe O @SiO –T/Pd are shown in Fig. 3. The maximum saturation magnetization
3
4
2
(
and 25.8 emu/g, respectively. Clearly, the M value of the corresponding magnetic
M ) values of Fe O @SiO -T and Fe O @SiO –T/Pd were discovered to be 34.3
s 3 4 2 3 4 2
s
Fe O nanoparticles (61.1 emu/g) is significantly higher than that of the Fe O @-
3
4
3 4
SiO -T, which may be due to the contribution of the non-magnetic material.
2
Compared with Fe O @SiO -T, Fe O @SiO –T/Pd shows a slightly smaller M
s
3
4
2
3
4
2
value which is attributed to the immobilization of Pd nanoparticles on the surface of
the Fe O @SiO -T nanoparticles. Notably, having all these complex features, the
3
4
2
catalyst can be easily isolated from the reaction mixture which is actually a
suspension system using an external magnetic force.
The morphology of the nano-magnetic Fe O @SiO –T/Pd particle catalysts were
3
4
2
studied by FE-SEM, EDS and TEM. The morphologies of the Fe O , Fe O @SiO -N ,
3
4
3
4
2
3
Fe O @SiO -T and Fe O @SiO –T/Pd are shown in Fig. 4a–d, respectively. The FE-
4
3
4
2
3
2
SEM image of Fe O shows that almost spherical nanoparticles were obtained with an
3
4
average diameter of about 15–20 nm. Moreover, the EDS spectrum depicted in Fig. 4e
Fig. 3 The magnetization
curves of (a) Fe O ,
3 4
(
(
b) Fe
c) Fe
3
O
4
@SiO -T and
2
@SiO –T/Pd
2
3
O
4
1
23

Magnetic nanoparticle-supported tetrazole functionalized…
Fig. 4 The FEG-SEM–EDS analysis of
d Fe @SiO –T/Pd
3 4 3 4 2 3 3 4 2
a Fe O , b Fe O @SiO –N , c Fe O @SiO -T and
3
O
4
2
123

A. Elhampour et al.
indicates that the product is substantially composed of Fe O . Next, the Fe O
3 4
3
4
nanoparticles were subsequentlycoated witha layer ofSiO and azide groups. Ascan be
2
clearly seen, these layers formed via slight particles are closely shelled on the surface of
the Fe O nanoparticles. Figure 4c shows tetrazole-functionalization on the surface of
the Fe O nanoparticles. The overall morphology of the final Fe O @SiO -N products
3
4
3
4
3
4
2
3
has no significant variation except that the newly formed layers were uniformly coated
onto the magnetic core with a smooth surface. Finally, in order to prepare the
Fe O @SiO –T/Pd core shell structure, Pd (0) nanoparticles were adsorbed on
3
4
2
Fe O @SiO -T. The FE-SEM images of the final product are shown in Fig. 4d with an
2
3
4
average diameter of about 35–40 nm. As can be seen, the surfaces of the tetrazole
linkage are covered with medial quantities of small particles. Furthermore, the
corresponding EDS pattern (Fig. 4f) confirmed that the prepared core–shell contains six
elements(O, Fe, Si, C, N and Pd) and reveals the content of*2.0% Pd nanoparticles (by
mass) of the Fe O @SiO –T/Pd. Inductively coupled plasma results also indicate that
3
4
2
the loading capacity of the Pd on the support is 1.9 wt%.
3 4 2
Fig. 5 Elemental mapping data of Fe O @SiO –T/Pd
1
23

Magnetic nanoparticle-supported tetrazole functionalized…
3 4 2
Fig. 6 The TEM image of Fe O @SiO –T/Pd
Therefore, from element mapping of the magnetic Fe O @SiO –T/Pd nanocat-
3
4
2
alyst (Fig. 5), it can be clearly seen that the element sof Pd, C, N, Si, Fe, and O are
uniformly distributed on the surface of magnetite. As depicted in Scheme 2,
palladium nanoparticles are distributed on the surface of the magnetite which is
confirmed by element mapping images.
The TEM images of the nano- magnetic Fe O @SiO –T/Pd particles are shown
2
3
4
in Fig. 6. The dark nano-Fe O cores are enclosed by a gray silica shell. The
3
4
average size of the Fe O cores is about 15–20 nm (Fig. 6a), which coincides with
3
4
that determined using the Scherrer equation in the XRD pattern. In addition, some
Pd nanoparticles (about 7 nm) are attached to the support particles (about 35 nm)
and are denoted by the circles.
Computational
Computational details
In this part, we have presented a quantitative description for metal–ligand
interactions in the nano-Fe O @SiO -triazine-Pd complex by performing density
3
4
2
functional theory (DFT) [44–47] and quantum theory of atoms in molecules
QTAIM) computations and Natural Bond Orbital (NBO) Analysis [48–52]. In this
regard, effective model for Triazine-CH CH Si(OMe) ligand 1 and SiO -triazine-
(
2
2
3
2
PdCl complex 2 was designed considering that this model is a credible comprise
2
between accuracy and time-saving efficiency of the computational procedure. The
optimized structure of compounds 1 and 2 determined by the M06 method in
combination with the following basis sets, 6-311G(d,p) for C, H, O, N and Si atoms,
and in the case of Pd atom, the effective core potential (ECP), LANL2DZ, were
used together with the accompanying basis set to describe the valence electron
density [53]. It should be mentioned that the M06 functional has been introduced
recently as a hybrid meta-GGA (generalized gradient approximation) exchange–
correlation functional that was parametrized including both transition metals and
non-metals, and was recommended for application in organometallic and thermo-
chemistry, kinetic studies and noncovalent interactions [30, 53, 54] All DFT
computations were performed using the Gaussian 09 suite of programs [55].
123

A. Elhampour et al.
Fig. 7 Atom labels and
optimized structures obtained at
the M06/6-311 g(d,p)/
LANL2DZ level of theory for 1
and 2 bond length (and bond
order) calculated at the M06/6-
311 g(d,p)/LANL2DZ level of
theory
Computational assessment
Topological analysis
The quantitative description of metal–ligand interactions of the SiO -triazine-Pd
2
complex 2 was presented based on the results using DFT and QTAIM outcomes. In
this case, structures 1 and 2 are considered as simple models for the triazine-
CH CH Si(OMe) ligand and SiO -triazine-Pd complex, respectively. Modeling the
2
2
3
2
surface in the form of a cage-like cluster of Si–O–Si sequences is extracted from the
reported literature [56–66]. In this study, the 6-edge cage-like cluster of Si–O–Si
sequences has been considered as a model of silica gel. In Fig. 7, we represent the
optimized geometries of ligand 1 and complex 2 which have been calculated by the
M06 method along with some atom labels. In Fig. 7, we also present M06-
1
23

Magnetic nanoparticle-supported tetrazole functionalized…
calculated C12–N20, N20–H21 and C2–H4 bond lengths (and also bond orders in
parentheses) in each of 1 and 2.
Comparing the structural features of 1 and 2 indicates a geometrical deformation
of 1 through complexation, for example, the dihedral angle N25–C9–C10–C12
ranges from 1.68 in ligand 1 to 32.28 in complex 2, which means that a deviation
from the co-planarity of the two rings amount to 308. Then, the structural stability of
the 2 is analyzed based on the various electronic indicators.
To characterize the variation of bond orders via complexation, the bond orders of
some key bonds in 1 with their corresponding bonds in the complex 2 are shown in
Table 1. They demonstrate that the bond order of C2–H4, C12–N20 and N20–H21
bonds decreases in 2 with respect to 1 through complexation. A slightly bonding
character observed between the electron lone pair of the atom N25 with the atom
H21 is the result of the near co-planarity of the two rings in the ligand 1.
In the next step, the the nature of the metal–ligand interactions in complex 2 was
analyzed using the Multiwfn program package [66] via the QTAIM method [47, 48].
2
b
The QTAIM calculated values of electron density, q , its Laplacian, rq ,
b
electronic kinetic energy density, G , electronic potential energy density, V , total
b
b
electronic energy density, H , and |V |/G ratio at some selected bond critical points
b
b
b
(
BCPs) for ligand 1 and complex 2 are given in Table 2 [49, 55, 67].
The following can be stated from the results collected in Table 2: (1) the
calculated values of q decrease through complexation at C2–H4, N20–H21 and
b
C12–N20 BCPs, which verifies the donation of shared electrons to the metal centers,
(2) the covalent character of the chemical bonds of C2–H4, N20–H21 and C12–N20
BCPs in 1 and 2 were confirmed by the large values of electron density together
2
b
with the negative values of rq and H , and (3) the small values of electron density
b
2
b
with the positive values of rq and H at N25–H21 in 1 and Pd30–H4, Pd30–
b
H21and Pd30–N25 BCPs in 2, demonstrate that the metal–ligand interactions do not
have covalent characters. However, the calculated values of 1 \ |V |/G \ 2,
confirms the presence of partially covalent-electrostatic interaction for N25–H21 in
b
b
1
and Pd30–H4, Pd30–H21and Pd30–N25 BCPs in 2, respectively [55].
Natural bond orbital (NBO) analysis
The energies of delocalization of electron density among the filled Lewis type NBOs
and the empty non-Lewis NBOs that leads to transfer of occupancy from the localized
Table 1 The calculated values
Bonded atoms
Ligand 1
Complex 2
of some selected bond orders in
ligand 1 and complex 2 obtained
at the M06/6-311 g(d,p)/
C2–H4
0.947
1.153
0.896
0.072
0.910
1.143
0.878
LANL2DZ level of theory
C12–N20
N20–H21
N25–H21
Pd30–H4
Pd30–N25
Pd30–H21
0.079
0.335
0.079
123

A. Elhampour et al.
Table 2 The obtained mathematical properties of some selected BCPs in the ligand 1 and the complex 2
via QTAIM analysis of the M06/6-311 g(d, p)/LANL2DZ calculated wave function of electron density
2
b
Bonded atoms
of BCPs
q
b
rq
G
b
V
b
H
b
b b
|V |/G
Ligand 1
C2–H4
0.2659
0.0239
0.3339
0.3100
-0.8376
0.0917
0.0463
0.0201
0.0551
0.1900
-0.3020
-0.01713
-0.51926
-0.6001
-0.2557
0.0028
6.5227
0.8607
9.4301
3.1584
N25–H21
N20–H21
C12–N20
Complex 2
C2–H4
-1.6377
-0.88021
-0.4645
-0.4100
0.2618
0.3272
0.3069
0.0169
0.0220
0.0752
-0.8127
-1.5300
-0.8863
0.0475
0.0450
0.0576
0.1742
0.0112
0.0157
0.0982
-0.2933
-0.4977
-0.5700
-0.0105
-0.0153
-0.1148
-0.2482
-0.4401
-0.3958
-0.0007
0.0004
6.5178
8.6406
3.2721
0.9375
0.9745
1.1690
N20–H21
C12–N20
Pd30–H4
Pd30–H21
Pd30–N25
0.0643
0.3261
-0.0167
NBOs of the idealized Lewis structure into the empty non-Lewis orbitals can be
estimated by second-order perturbation theory [52]. Based on the M06/6-311 g(d,p)/
LANL2DZ optimized geometries and electronic structure characteristics of the ligand
1
and complex 2, the NBO analysis of donor–acceptor interactions (Table 3) shows
that the stabilization energies associated with the electron delocalization from
nonbonding N25 lone-pair orbitals n to rN20–H21* anti-bonding orbitals are
N25(1)
-1
around 1.27 kcalmol , which is in line with the calculated bond order amounting to
.072 reported in Table 1 due to the presence of a weak interaction in the ligand 1.
According to the NBO analysis, the nonbonding N20 and N25 lone-pairs nN20(1)
0
,
nN25(1) and also rN20–H21 and rC2–H4 bonding orbital occupancies decrease in 2
compared to 1, whereas the rN20–H21* and rC2–H4* antibonding orbitals occupancies
increase to the values 0.0371 and 0.0224, respectively.
Inspection of the NBO data reported in Table 9 for complex 2 reveals that a very
strong interaction prevails inbetween the N25 lone-pair and the unoccupied lpPd30(6)*
orbital along with much weaker interaction inbetween rN20–H21 and rC2–H4 bonding
orbitals and the unoccupied lpPd30(6)* orbital resulting for the latter orbital in an
occupancy of around 0.0887. The corresponding stabilization energy is 23.63
-1
kcalmol . Also, the electron donation from the nonbonding orbital lpPd30(5) to
rN20–H21* and rC2–H4* antibonding orbitals provide noticeable stabilization energies.
Catalytic activity of Fe O @SiO –T/Pd(0)
4
3
2
Sonogashira reaction
Having prepared and fully characterized Fe O @SiO –T/Pd, its activity as a
heterogeneous magnetically catalyst was then assessed in the Sonogashira and Heck
3
4
2
1
23

Magnetic nanoparticle-supported tetrazole functionalized…
Table 3 NBO occupancies and
Bonded atoms
Ligand 1
Complex 2
stabilization energies
-1
(kcalmol ) characterizing the
ligand 1 and the complex 2
obtained in the M06/6-
Occupancies
n
N20(1)
1.8215
1.9382
1.9866
0.0125
1.9747
0.0101
1.8114
1.8722
1.9830
0.0371
1.9718
0.0224
1.9348
0.0887
0.0087
311 g(d,p)/LANL2DZ level of
theory
nN25(1)
r
r
r
r
N20–H21
N20–H21
C2–H4
*
C2–H4
*
lpPd30(5)
lpPd30(6)
lpPd30(7)
*
*
2
Stabilization energies (E )
n
N25(1) ? rN20–H21
*
1.27
n
N25(1) ? lpPd30(6)
*
22.58
0.66
0.39
3.82
1.00
r
r
N20–H21 ? lpPd30(6)
*
*
C2–H4 ?lpPd30(6)
*
lpPd30(5) ? rN20–H21
lpPd30(5) ? rC2–H4
*
cross-coupling reactions. To find the optimized reaction conditions, the Sonogashira
coupling reaction of iodobenzene and phenylacetylene as the model reaction was
selected which gave the corresponding 1,2-diphenylethyne. First, we examined the
Sonogashira coupling reaction of iodobenzene (1 mmol) with phenylacetylene
(
1.2 mmol) using Fe O @SiO –T/Pd (20 mg) as the catalyst in DMF at 100 Æ C for
3 4 2
2
/5 h (Table 4). The different amounts of Fe O @SiO –T/Pd as catalyst and bases
3 4 2
like K CO (85%), NaOH (60%), Et N (96%), Cs CO (90%), in DMF at 100 °C
2
3
3
2
3
were screened for the model reaction. According to the results, the reaction was
more favorable using Et N as the base, so this manner was applied for the coupling
3
reaction. The effect of temperature on the activity of nano-magnetic Fe O @SiO –
3
4
2
T/Pd(0) particles was also examined. When the temperature was decreased from 100
to 25 °C, the yield of the corresponding product decreased from 97% to 45%
(
Table 4, entry 13). Additionally, with the goal of the importance of green
chemistry, the model reaction was also conducted in solvent-free conditions
Table 4, entry 8). As can be seen in Table 4, without the catalyst, the products were
obtained in low yield. When the reaction was conducted in DMF, the corresponding
(
product was isolated in high yield. Other polar solvents such as H O and THF were
2
less effective, and toluene was an ineffective solvent (Table 4, entries 5–7). Hence,
among the tested solvents, DMF was selected as the perfect one. The best result was
achieved when 1.0 mmol of iodobenzene and 1.2 mmol of phenylacetylene were
reacted in the presence of 20 mg catalyst at 100 °C in DMF as the solvent and Et N
3
as the base (Table 4, entry 1).
Considering the optimal reaction conditions, we investigated the area and
limitations of the reaction with a range of iodobenzenes and phenylacetylenes. We
123

A. Elhampour et al.
1
23

Magnetic nanoparticle-supported tetrazole functionalized…
Table 5 Fe
3
O
4
@SiO
2
–T/Pd catalyzed Sonogashira reactions
Terminal
Product
alkyne
Time
(h)
Yieldb
(%)
Mp°C
Entry
Aryl halide
(References)
5
7–59
1
2.5
2
97
[
68]
107–109
2
95
[69]
6
8–69
3
4
5
2.5
2.5
4
90
93
86
[
68]
5
9–60
[
68]
Yellow oil [68]
8
2–84
6
3.5
87
[
69]
6
9–71
7
8
9
4
89
87
[
70]
126–129
3.5
2.5
[
71]
1
24
95
91
[
70]
1
1
0
1
4
Yellow oil [72]
9
6–97
3
3
94
92
[72]
–7 97
1
2
[
72]
123

A. Elhampour et al.
Table 5 continued
Terminal
alkyne
Time
(h)
Yieldb
(%)
Mp°C
Entry
Aryl halide
Product
(References)
5
7–59
1
1
3
4
5
72
[
68]
107–109
5
88
83
[
69]
6
8–69
1
5
7.5
[
68]
1
1
6
7
6.5
10
85
55
Yellow oil [72]
5
7–59
[
68]
Reaction condition: aryl halide(1.0 mmol), terminal alkyne (1.2 mmol), Fe
20 mg), Et
N (2.0 mmol) in DMF (3.0 mL) at 110 °C
Isolated yield
3 4 2
O @SiO –T/Pd catalyst
(
b
3
were pleased to find that all the reactions afforded the desired coupling products in
excellent yields within 2.5–5 h. Substituted phenylacetylenes and aliphatic alkynes
were also good substrates and coupled with iodobenzene to give the corresponding
products in good to excellent yields (Table 5, entries 1–4 and 7–12). Moreever,
bromobenzene and chlorobenzene also worked well for the magnetic heterogeneous
Sonogashira coupling reactions (Table 5, entries 13–17). The reason is that the bond
reactivity order is C–I [ C–Br [ C–Cl, and, when the reaction was carried out with
iodobenzene, the expected products were obtained in high yields.
Heck reaction
The applicability of the catalyst Fe O @SiO –T/Pd for the Heck reaction was
3
4
2
further explored between iodobenzene with methyl acrylate in DMF as a solvent at
2
5 °C. The model reaction for the optimization of various parameters and the results
are summarized in Table 6. The most suitable solvent was found to be DMF,
because it gave higher conversion as well as yield of the desired product. In view of
the fact that the Heck coupling is strongly influenced by the nature of the base, we
have applied several bases (Table 6, entries 1, 7–9) among which Et N gave
3
maximum conversion and selectivity in 94% yield (Table 6, entry 1). When the
reaction temperature was increased to 120 °C, the reactants were completely
consumed during 2 h, with the formation of compounds with good yields (Table 6,
1
23

Magnetic nanoparticle-supported tetrazole functionalized…
Table 6 Optimization of reaction conditions for Heck cross coupling reaction of iodobenzene and
methylacrylate
a
Entry Loading of catalyst (mg) Condition (solvent/
temperature °C)
Base
Time
(h)
Yield
(%)
1
2
3
4
5
6
7
8
9
1
1
1
20
10
30
20
20
20
20
20
20
20
20
20
DMF/120
DMF/120
DMF/120
Et
Et
Et
Et
Et
Et
3
3
3
3
3
3
N
N
N
N
N
N
2
2
94
72
2
93
H
2
O/Reflux
3
50
DMSO/120
EtOH/Reflux
DMF/120
DMF/120
DMF/120
DMF/150
DMF/110
DMF/r.t
2
86
3
62
K
2
CO
NaOH
Cs CO
3
2
82
2
69
2
3
2
75
0
1
2
Et
Et
Et
3
N
N
N
2
92
3
3
2
83
10
Trace
Reaction condition: 3 mL solvent, 1 mmol iodobenzene, 1.5 mmol methyl acrylate, and 2 mmol base
a
Isolated yield
entries 1, 10–12). Increasing the temperature to 150 °C had no effect on the yield
and reaction time. Hence, the temperature of 120 °C was selected for the optimized
condition. The loading of the catalyst was also investigated and it was found that
2
0 mg of Fe O @SiO –T/Pd catalyst enhances the yield (Table 6, entry 1) as
3 4 2
compared to 30 mg and 10 mg (Table 6, entries 2, 3). Especially, the best result was
achieved when 1.0 mmol ioiodobenzene and 1.5 mmol of methyl acrylate were
reacted in the presence of 20 mg catalyst (1.9 mol% Pd) at 120 °C in DMF as the
solvent and Et N as the base (Table 6, entry 1).
3
After optimization of the reaction conditions, the catalytic activity for a variety of
iodobenzenes with various acrylates was examined. In the substrate study, various
substituted aryl halides have been successfully reacted with different acrylates. The
presence of substituents on the aromatic ring of iodides has a significant effect on
the yield of the desired products. Substituents such as –NO , –Cl of aryl halides
2
gave good responses as compared to –OMe or –Me substituents (Table 7, entries
2
–5). However, in spite of the presence of a strong C–Br bond, substituted aryl
bromide afforded good to excellent yields of corresponding products but needed
longer reaction times (Table 7, Entries 10, 11).
The supremacy of the present protocol could be established by comparing these
obtained results with previously reported methods. The results are summarized in
Tables 8 and 9, which show that these methods have their own advantages, and that
the newly synthesized catalyst is efficient and has good fabricating conditions
123

A. Elhampour et al.
Table 7 Fe
3
O
4
@SiO
2
–T/Pd catalyzed Heck reaction of aryl halides with alkenes
Time
Mp°C
b
Entry
Aryl halide
Alkene
Product
Yield (%)
(
h)
(References)
1
2
94
Yellow oil [73]
2
3
2
3
96
90
161 [74]
52–54 [73]
4
3
2
89
91–91.5 [74]
5
6
88
94
71-73 [73]
2.5
3
Colorless oil [73]
7
90
Yellow oil [73]
8
9
2
3
87
90
134–136 [75]
Yellow oil [73]
1
1
0
1
3.5
7.5
88
71
Yellow oil [73]
Yellow oil [73]
3 4 2 3
Reaction conditions: aryl halide (1 mmol), alkene (1.5 mmol), Fe O @SiO –T/Pd catalyst (20 mg), Et N
(
b
2.0 mmol) in DMF (3.0 mL) at 120 °C
Isolated yields
1
23

Magnetic nanoparticle-supported tetrazole functionalized…
3 4 2
Table 8 Comparison of catalytic activity of Fe O @SiO –T/Pd with various reported catalysts for Heck
reactions of iodobenzene and methylacrylate
Entry Catalyst
Condition (base,
solvent, temperature)
Time Yield References
(h)
(%)
1
2
3
4
5
6
7
8
Pd/CuO NPs (3.0 mol%)
Pd NPs/CNFs (0.001 g)
Et
Et
Et
3
3
3
3
N, DMF, 110 °C
N, DMF, 120 °C
N, DMF, 120 °C
N, DMF, 120 °C
20
6
98
98
95
90
99
93
97
93
[76]
[77]
Fe
3
O
4
@SiO
2
–T/Pd (20 mg)
3
This work
[78]
Pd/Nf-G (0.3 mol% or 0.32 mg of Pd) Et
Pd/CNCs (50 mg 3 wt%)
Pd/La (1 mmol%)
2
K
2
CO
3
, DMF, 40 °C
3
[79]
2
O
3
NaOAc, DMA, 120 °C 10
Et
[80]
Pd/MnBDC (0.12 mol% or 13 mg)
3
N, DMA, 90 °C
N, DMF, 130 °C
9
[81]
Cellulose supported poly(hydroxamic
acid) Pd(II) (0.03 mol%)
Et
3
5
[82]
3 4 2
Table 9 Comparison of catalytic activity of Fe O @SiO –T/Pd with various reported catalysts for
Sonogashira reactions of iodobenzene and phenylacetylene
Entry Catalyst
Condition (base, solvent,
temperature)
Time Yield
(h)
References
(%)
1
2
3
PdNPs/DNA (8 mg, 0.5 mol%)
Pd@PANI (0.005 mol%)
Pd@bisindole@SiO @Fe
4 mg 0.18 mol%)
Nano Pd@Fe (1 mol% Pd)
Pd–CuFe @SiO (10 mg,
.3 mol% Pd)
Cs
2
CO
3
, MeOH, 65 °C
24
48
24
85
86
94
[83]
[84]
[85]
Et
3
N, MeCN, 80 °C
2
3
O
4
DABCO, DMA, 90 °C
(
4
5
3
O
4
DMF, piperidine, 110 °C
DABCO, DMA, 50 °C
24
24
88
80
[86]
[87]
2
O
4
2
0
0
Pd -Mont (0.07 mol%)
6
7
8
Et
KOAc, DMSO, 110 °C
Et
N, DMF, 120 °C
3
N, CH
3
CN, 82 °C
3
90
94
97
[88]
CS/MMT/Pd (0.3 mol%)
5
[70]
Fe
3
O
4
@SiO
2
–T/Pd (20 mg)
3
2.5
This work
including the ability to undergo simple separation, the reusability of the catalyst,
and short reaction times for the synthesis of the target molecules.
Recycling and leaching of the catalyst
The reusability of catalysts is an important parameter for practical applications.
The recyclability of the Fe O @SiO –T/Pd catalyst was checked for both
3
4
2
reactions and the results are depicted in Fig. 8. After the first cycle, the catalyst
was removed by using an external magnet and extensively washed (see
‘‘Experimental’’). Then, the catalyst was dried under air before performing the
123

A. Elhampour et al.
Fig. 8 Reusability of the Fe
3
O
4
@SiO –T/Pd catalyst in Sonogashira and Heck cross-coupling reactions
2
reusability test. As seen in Fig. 8, the catalyst could be used for six sequential
runes with excellent isolated yields.
Conclusion
In summary, an active, stable and recyclable Fe O @SiO –T/Pd catalyst was
2
3
4
prepared using a conceptually and practically simple approach from inexpensive
and available materials. The prepared catalyst was fully identified by FT-IR, FE-
SEM, EDS, XRD, TEM and VSM analysis and applied in the Sonogashira cross-
coupling reaction in DMF at 90 °C and the Mizoroki–Heck cross-coupling reaction
in DMF at 120 °C using Et N as a base. It was found that the catalyst can be
3
removed by a simple method using an external magnet and reused six times with
only a moderate decrease in product yield. Another useful merit of the catalyst is
that it facilitates the recovery and reuseability of palladium. It can also reduce the
palladium contamination of the isolated product, eliminate the ligand or copper and
ensure moisture stability of the catalyst, which are much superior to commercial Pd
catalysts and thus make it attractive from the viewpoint of green chemistry. In
addition, the QTAIM and NBO analysis provided information about the electrostatic
interactions together with the stabilization energies resulting from the complexation.
Acknowledgments The authors gratefully acknowledge Semnan University Science and Technology
Park, Research Council and Vice-chancellor for Education and Postgraduate student for financial support
of this work.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of interest.
1
23

Magnetic nanoparticle-supported tetrazole functionalized…
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