Copper(II) nanoparticles: an efficient and reusable catalyst in green oxidation of benzyl alcohols to benzaldehydes in water

Article abstract of DOI:10.1002/aoc.3509

Copper(II) nanoparticles immobilized onto 3-aminopropyltriethoxysilane (APTES) supported on mesoporous silica KIT-5 nanocomposite (AK) was prepared. The APTES group on KIT-5 was manipulated as a suitable coordinating agent for copper(II) and fully characterized using FT-IR, SEM, EDX, ICP, TGA, XRD and UV–visible methods. Moreover, a quantitative description for metal–ligand interactions in the APTES-KIT-5 mesoporous silica-supported copper(II) acetate complex was assessed via quantum chemistry computations. This novel transition metal supported heterogeneous surface was used as an effective catalyst in low loading for selective oxidation of differently substituted benzylic alcohols in water, using H₂O₂ as oxidant to give the corresponding carbonyl compounds in high to excellent yields. This heterogeneous nanocatalyst can be recovered and reused several times without any significant loss of activity. Copyright

Full text of DOI:10.1002/aoc.3509

Full paper
Received: 2 February 2016
Revised: 11 April 2016
Accepted: 14 April 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3509
Copper(II) nanoparticles: an efficient and
reusable catalyst in green oxidation of benzyl
‡
alcohols to benzaldehydes in water
a
b
b
Razieh Mirsafaei , Majid M. Heravi *, Tayebeh Hosseinnejad
c
and Shervin Ahmadi
Copper(II) nanoparticles immobilized onto 3-aminopropyltriethoxysilane (APTES) supported on mesoporous silica KIT-5
nanocomposite (AK) was prepared. The APTES group on KIT-5 was manipulated as a suitable coordinating agent for copper(II)
and fully characterized using FT-IR, SEM, EDX, ICP, TGA, XRD and UV–visible methods. Moreover, a quantitative description for
metal–ligand interactions in the APTES-KIT-5 mesoporous silica-supported copper(II) acetate complex was assessed via quantum
chemistry computations. This novel transition metal supported heterogeneous surface was used as an effective catalyst in low
loading for selective oxidation of differently substituted benzylic alcohols in water, using H O as oxidant to give the
2 2
corresponding carbonyl compounds in high to excellent yields. This heterogeneous nanocatalyst can be recovered and reused
several times without any significant loss of activity. Copyright © 2016 John Wiley & Sons, Ltd.
Keywords: copper(II) supported catalysts; oxidation in water; nanocatalyst; density functional theory; quantum theory of atoms in
molecules
homogeneous and heterogeneous catalysts have been reported
Introduction
[
13–15]
in the chemical literature,
to the well-known and well-established inherent advantages of het-
the latter of particular interest due
The most common technique applied for functionalization of
microporous and mesoporous materials is the covalent binding of
an appropriate metal complex to a solid support. This technique
can be applied using two strategies: (1) grafting and (2) tethering,
in which the spacer (‘tether’) is placed between the wall of the
[16,17]
erogeneous catalysis over homogeneous catalysis.
However, the selective oxidation of primary alcohols to the corre-
sponding aldehydes remains very difficult, because the oxidation of-
[18]
ten proceeds further to afford the corresponding carboxylic acids.
[
1,2]
material and the metal complex.
In this respect, various catalysts and methods are known for selective
oxidation of primary alcohols. Transition metals and their com-
pounds and complexes have been known and established for their
homogeneous and heterogeneous catalytic activity in oxidation. This
Mesoporous organosilica and silica are frequently decorated with
certain organic functional groups for the adsorption of metal ions
2
+
2+
2+
such as Pb , Cu and Hg in solution. Furthermore, mesoporous
carbon and polymer materials decorated with N-containing
activity is ascribed to their ability to adopt multiple oxidation states
[
3]
[19–26]
functional groups are often employed to adsorb CO
2
.
In compar-
and to form complexes.
The utilization of transition metals such
[19]
[20,21]
[22,23]
[23]
[24]
[25]
[26]
ison with calcium carbonates, zeolites, metal–organic frameworks
and activated carbon, N-included mesoporous carbon shows a
longer lifetime and high-temperature adsorption performance.
Mesoporous materials are also found to be suitable for adsorption
of molecules, chiefly due to their large pore size and high surface
as V, Cu,
Mo,
W, Ru, Pd and Mn along with an
appropriate oxidizing agent has recently attracted much attention
and is frequently employed as an alternative to conventional meth-
odologies which require stoichiometric amounts of non-ecofriendly
reagents and harsh reaction conditions. However, most of the proto-
cols involving the utilization of transition metals suffer from requiring
harsh reaction conditions such as hazardous or toxic solvents, high
temperature and one or more equivalents of environmentally un-
[
4]
area. They are also currently utilized as either heterogeneous
catalysts or supports. In heterogeneous catalysis, the quality of
the surface area of the catalyst is important since it determines
[
5,6]
[27,28]
the availability of catalytic sites for reactants.
Thus, mesoporous
friendly oxidizing agents.
silica-supported catalysts are well known as being highly effective,
due to their large area, and are more effective in organic heteroge-
[
7,8]
* Correspondence to: Majid M. Heravi, Department of Chemistry, Alzahra University,
Vanak, Tehran, Iran. E-mail: mmh1331@yahoo.com
neous catalysed reactions.
Among them, the nanocage silica
KIT-5 with unique properties discovered by Ryoo and co-workers
has been proven as a highly ordered cage-type mesoporous
material with cubic Fm3m close-packed symmetry, high surface
‡
Dedicated to my son, OMID, on the occasion of his 35th birthday.
[
9]
a Department of Chemistry, Yazd Branch, Islamic Azad University, Yazd, Iran
b Department of Chemistry, Alzahra University, Vanak, Tehran, Iran
c Iran Polymer and Petrochemical Institute, Tehran, Iran
area, large pores and a high specific pore volume.
Recently, the development of efficient catalytic systems for
oxidations in water or aqueous media has attracted much
[10–12]
attention.
To achieve selective oxidations, various
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

Mirsafaei R., Heravi M. M., Hosseinnejad T. and Ahmadi S.
Thus, an economic and environmental heterogeneous catalyst
working under mild reaction conditions, in green solvents and
using inexpensive and non-toxic oxidative reagents is still in much
Alpha spectrometer. Scanning electron microscopy (SEM; Philips,
XL30, SE detector) was used to observe the morphology of the
modified and unmodified mesoporous silicas. Energy-dispersive
X-ray spectroscopy (EDX; Genesis, with an SUTW detector equipped
with SEM equipment) was used in order to confirm the presence of
silica. Copper content was measured using inductively coupled
plasma (ICP) analysis with a Varian Vistapro analyser.
[29]
demand.
Undoubtedly oxidations in water are superior from known and
[
10–12]
established points of view.
Hydrogen peroxide is an attractive,
atom-economic and environmentally friendly oxidant as it is
inexpensive, non-toxic and commercially available and produces
[30,31]
only water as a by-product.
We are interested in oxidation reactions
tion of nanoparticles on suitable supports
[
32–35]
and immobiliza-
for taking advan-
In Situ nanocopper(ii) preparation
[
36–40]
[
41–45]
Preparation of cubic mesoporous KIT-5
tage of the superiority of heterogeneous catalysis.
Recently
we have reported the preparation of Cu(I) nanoparticles on modi-
fied KIT-5 as an efficient recyclable catalyst and its applications in
Three-dimensional large cage-type face-centred cubic Fm3m
mesoporous KIT-5 nanocages were synthesized successfully. The
large pore cage-type mesoporous silica, denoted KIT-5, was pre-
[
46]
click reactions in water. In continuation of our activities towards
green chemistry using nanoparticles, in this work we report the
pared using Pluronic F127 (EO106PO EO106) template as a
70
[45,47,48]
preparation, computational studies
and application of a
structure-directing agent and tetraethylorthosilicate (TEOS) as the
novel system of Cu(II) nanoparticles immobilized on nanocage-like
mesoporous KIT-5 as a support and a 3-aminopropyltriethoxysilane
silica precursor. In a typical synthesis, F127 (2.5 g, 0.198 mmol)
was dissolved in 120 g of distilled water and concentrated hydro-
chloric acid (5.25 g, 0.05 mol, 35 wt% HCl). To this mixture, TEOS
(12 g, 0.057 mol) was added. The mixture was stirred for 24 h at
45°C to produce the mesostructured product. Then, the reaction
mixture was heated at 95°C for 24 h under static conditions for
hydrothermal treatment. The solid product was then filtered,
washed with deionized water and dried at 100°C. Finally, the
samples were calcined at 550°C for 6 h to remove the template.
(APTES) group as a coordinating agent for Cu(II). Through modifica-
tion of the KIT-5 surface with moderate polarity (APTES), suitable for
adsorption of Cu(OAc)2 and reduction, Cu(II) particles of a few
nanometres in size were successfully confined inside the
nanocages of modified KIT-5, and employed as an active catalyst
in the successful selective oxidation of primary and secondary
alcohols in water. Furthermore, we have performed computational
assessments of the chelating behaviour in APTES-KIT-5 mesoporous
silica-supported copper(II) acetate complex to obtain a quantitative
description for metal–ligand interactions which also confirmed the
experimental characterization of the copper(II) nanocatalyst. It
should be stated that a computational investigation of the catalytic
role of APTES-KIT-5 silica-supported copper(II) nanocatalyst in
selective oxidation of substituted benzylic alcohols is underway
and another comprehensive computational report is planned.
Preparation of amine-functionalized mesoporous KIT-5
The AK nanocomposites were synthesized following the procedure
[
43]
given in our previous work.
Briefly, a suspension of APTES
(1.00 g) and calcined KIT-5 (1.00 g) in toluene (20 ml) was refluxed
with continuous stirring under inert atmosphere for 24 h. The resul-
tant mixture was filtered and washed with a 1:1 mixture of acetone
and chloroform to remove the un-reacted APTES, and then dried
under vacuum at 80°C. The un-reacted APTES was removed by
Soxhlet extraction using dry dichloromethane for 20 h and dried
at 60°C. The sample was denoted as AK ligand (shown in Scheme 1).
Experimental
Materials and solvents
All starting chemicals and reagents were commercially available
and were used without further purification. The reagents and sol-
vents were purchased from Merck, Fluka or Aldrich and were used
as received. APTES and copper acetate were purchased from Merck
and used without further purification. For drying tetrahydrofuran, it
was distilled off over sodium in the presence of benzophenone
until its colour changed to a deep blue. Silica gel KIT-5 (particle size,
Coordination of Cu(II) to AK ligand
In a round-bottom flask equipped with a condenser, Cu(OAc)
2
(1.00 g) was added to the modified support AK ligand (1.00 g) in
methanol (3 ml). The reaction mixture was refluxed for 6 h. The solid
was filtered and extracted in a Soxhlet extractor with methanol for
10 h, then it was dried at 60°C for 1 h to give Cu(II) supported on AK
(Cu(II)/AK composite) as shown in Scheme 2. ICP analysis demon-
strated that the copper content in the heterogeneous catalyst is
15.60% (w/w). Therefore, each gram of heterogeneous catalyst
contains 1.717 mmol of copper.
2
À1
10.38 nm; surface area, 1090 m g ; pore diameter, 2.62 nm) and
APTES/KIT-5 (AK) were prepared in accordance with our previously
[43]
published procedure.
Techniques
Melting points were determined with an X6 digital microscope
melting point apparatus. X-ray diffraction (XRD) patterns of the
materials were recorded with a Bruker D8 advanced powder X-ray
diffractometer using Cu Kα (λ = 1.54 Å) as the radiation source in
the range 2θ = 2–90°. Fourier transform infrared (FT-IR) spectra were
recorded in KBr discs with a Bruker FT-IR spectrophotometer.
Thermal gravimetric analysis (TGA) data were obtained with a
SetaramLabsys TG (STA) in the temperature range 10–600°C and
À1
heating rate of 5°C min under nitrogen atmosphere. UV–visible
solution spectra were recorded using a thermo-spectronic Helios
Scheme 1. Preparation of AK ligand.
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Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2016)

Green catalytic oxidation of benzyl alcohols to benzaldehydes
In the case of copper(II), LANL2DZ effective core potentials were
used together with the accompanying basis sets to describe the
[52]
valence electron density. The stationary points were determined
as minima through verifying the presence of all real frequencies. All
DFT computations were performed using the GAMESS suite of
[
53]
programs.
Results and discussion
Scheme 2. Coordination of Cu(II) to AK ligand.
KIT-5 mesoporous silica surfaces were modified with APTES as a
linking agent via a one-step chemical reaction. Ethoxy group in
the APTES structure was chemically bound with surface hydroxyl
groups of KIT-5 mesoporous silica (Scheme 1). APTES contains
one free amine moiety and was applied as a coordinating agent
for copper(II) in methanol. In order to introduce copper(II) into the
nanocages of KIT-5, APTES was employed to modify the surface of
KIT-5. Owing to the strong coordination capacity of amino groups,
Catalytic benzyl alcohol oxidation in water
Benzyl alcohol was selected as a model compound for the optimi-
zation of the oxidation process. A typical experimental procedure
was as follows. In a round-bottom flask, to a mixture of catalyst
(0.05 g) and benzyl alcohol (1 mmol), 2 ml of water plus hydrogen
peroxide (30%) aqueous solution (containing 3.0 mmol H O ) was
2
Cu(OAc) , even with a high loading, can be easily adsorbed by
2
2
added dropwise. This mixture was stirred at reflux condition for a
period of time. The progress of the reaction was monitored by
TLC (silica gel 60 F254) and/or GC (CP WAX 52, Varian CP 3800).
Evaporation of solvent gave the corresponding aldehyde. Upon
the completion of the reaction, the solid catalyst was recovered
by centrifugation, and washed repeatedly with ether and ethanol.
The recovered catalyst was dried under vacuum and was directly
used for the next reaction.
KIT-5 modified with amino groups.
FT-IR Spectroscopic analysis
Figure 1 shows the FT-IR spectra before and after the reaction of AK
2
ligand with Cu(OAc) . The major peaks for silica including the broad
À1
antisymmetric Si–O–Si stretching (1300 to 1000 cm ) and the
À1
symmetric stretching (820–740 cm ) are observed. In AK ligand
À1
spectrum, the 2935 cm peak can be assigned to C–H bond
2
stretching, while NH group shows two bands for primary amines:
Computational details
À1
À1
N–H stretching (3390 cm ) and N–H bending (1650 cm ). After
the reaction of AK ligand with Cu(II), the C–N stretching band shifts
Recently, we have assessed computationally metal–ligand
interactions in modified poly(styrene-co-maleic anhydride)
À1
À1
to lower frequency (1444 cm ), while N–H absorption shifts to
[
45]
[46]
palladium nanocatalyst,
sulfamic-aminated KIT-5 nanocatalysts
approaches. Armed with these experiences, we have concentrated
on presenting quantitative description for metal–ligand
copper (I)-aminated KIT-5
and N-
higher frequency (1604 cm ), which confirm the chemical
[
47]
via quantum chemistry
interactions of nitrogen lone pair electrons with vacant copper(II)
orbitals (Fig. 1).
a
interactions in the APTES-KIT-5 mesoporous silica-supported
copper(II) acetate complex by performing density functional theory
UV–visible spectroscopic analysis
[48]
(
DFT)
and quantum theory of atoms in molecules (QTAIM)
[49,50]
Figure 2 presents the solid-state UV–visible spectra of AK ligand and
copper(II) nanocatalyst. The absorptions around 223, 255 and
computations.
In this respect, we have designed an effective
model for AK ligand 1 and Cu(II)/AK complex 2 (as illustrated in
Scheme 3) considering that this model is a credible comprise
between accuracy and time-saving efficiency of computational
procedure. We first determined the optimized structure of
323 nm are attributed to n–π* and π–π* electron transfers for the
silica and amine groups. In the UV–visible spectrum of Cu(II)/AK
complex composite, a new absorption around 706 nm appears
which can be assigned to d–d transitions after loading of copper(II).
[51]
compounds 1 and 2 at M06/6-31G* level of theory.
It should
be mentioned that 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 inorganometallic thermochemis-
[
51]
try, kinetic studies and noncovalent interactions.
Scheme 3. Simple model for APTES-KIT-5 mesoporous silica-supported
copper(II) acetate complex.
Figure 1. FT-IR spectra of AK ligand and copper(II) nanocatalyst.
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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Mirsafaei R., Heravi M. M., Hosseinnejad T. and Ahmadi S.
Figure 2. Solid-state UV–visible spectra of (a) AK ligand and (b) copper(II)
nanocatalyst.
SEM Characterization
The morphology and the location of metallic species on the surface
of the catalyst were analysed using an SEM instrument equipped
with EDX capability. Figure 3 shows SEM images of AK support
before and after immobilization of copper(II). The porosity of AK,
shown in Fig. 3(b), has an average size of 21–49 nm. After
introduction of copper, clear changes in the morphologies of the
catalyst are observed. The SEM images of the Cu(II)/AK catalyst
Figure 4. EDX analysis of pure KIT-5 (top) and Cu(II)/AK complex (bottom).
(Figs 3(c) and (d)) clearly show the Cu(II) nanoparticles are
XRD Results
homogeneously immobilized on the modified surface. As shown
in Fig. 3(d), the copper(II) catalyst is nanometric, being 32–40 nm
in size.
Comparisons of EDX analysis of pure KIT-5 with that of Cu(II)/AK
complex (after extracting with methanol in a Soxhlet extractor)
confirm the addition of copper(II) to the catalyst, which suggests
the formation of metal complexes with the anchored ligand
Figure 5 displays the wide-angle XRD pattern of Cu(II)/AK
nanocatalyst (2θ = 2–90°). The crystallite size of particles was
determined using Bragg’s equation (nλ = 2dsin θ), found to be
approx. 45 nm for Cu(II)/AK catalyst (2θ ≈ 11°). Figure 5 shows that
all reflection peaks could be indexed to the pure cubic crystal phase
of nanocrystalline Cu(II). Sharp peaks of copper (2θ = 11.0°, 12.0°,
14.8° and 23.7°) indicate the crystalline character of the product.
(Fig. 4).
No specific peaks are observed for impurities. This pattern is in
reliable agreement with the reported pattern for copper acetate
(Ref. 00-027-1126) and also confirms the formation of Cu(II)
nanoparticles on APTES/KIT-5. This behaviour can be mainly
attributed to the fact that symmetry has been destroyed by the
hybridization in the ordered mesoporous silica loaded with guest
matter.
To determine the copper content, the supported copper was
treated with concentrated HCl and HNO
ICP-OES analysis. The copper content was evaluated to be 15.60%
w/w).
3
(1:1) followed by
(
Figure 3. SEM images of (a) pure KIT-5 and (b) AK and (c, d) copper(II)
immobilized on AK.
Figure 5. XRD pattern of copper(II) nanocatalyst.
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Appl. Organometal. Chem. (2016)

Green catalytic oxidation of benzyl alcohols to benzaldehydes
Thermal properties
The thermal stability of pure AK and Cu(II)/AK catalyst was initially
investigated using TGA (under nitrogen flow), confirming the
incorporation of copper acetate groups in AK. As illustrated in Fig. 6,
the catalyst shows a slight loss of mass (about 5%) below 110°C,
probably due to the loss of a small quantity of adsorbed water. If
the decomposition is performed at 127°C, the yield of decomposi-
tion is only 4%. At higher temperatures, the decomposition yield
increases continuously. Decomposition of copper acetate on AK
support starts with a yield of 6.58% at 200°C and 30% at 280°C. With
increasing temperature, the decomposition of the aminopropyl
[
47]
group in AK occurs at about 350°C.
The TGA curve of the copper(II) catalyst at higher temperatures
demonstrates a more marked mass loss in comparison to that
observed for AK. This can be chiefly due to the gradual desorption
and thermal decomposition of the copper groups. It should be
stated that the copper groups decompose at lower temperatures
in comparison with AK. The total percentage of the mass loss of
the catalyst is determined to be around 38%, and 7% for AK. The
mass loss for the copper(II) catalyst also attests to the presence of
copper groups inside the pores. This catalyst is stable up to 200°C
and can be remarked as a catalyst of choice in various organic
reactions in the temperature range 80–140°C. However, AK is
thermally stable even up to 400°C, and therefore it is frequently
applied as a thermally stable catalyst support in several organic
transformations.
Computational assessment
On the basis of our obtained DFT and QTAIM computational results,
we can present a quantitative description for metal–ligand
interactions and also interpret some experimental elucidations in
the characterization of the APTES-KIT-5 mesoporous silica-
supported copper(II) acetate complex from structural and
electronic viewpoints.
Figure 7. Atomic numbering and optimized structures obtained at M06/6-
1G* level of theory for AK ligand 1 and Cu(II)/AK complex 2. C–N bond
length (and bond order) calculated at 6-311G** level of theory.
3
In Fig. 7, we presented the optimized geometries of AK ligand 1
and Cu(II)/AK complex 2 calculated at M06/6-31G* level of theory
with the atomic numbering. In Fig. 7, we also present M06/6-
in AK ligand through complexation. In the following, the structural
stability of Cu(II)/AK complex 2 is analysed based on the various
electronic indicators.
3
11G** calculated C–N bond length (and also bond order in
The bond orders of some key bonds in AK ligand 1 with their
corresponding bonds in Cu(II)/AK complex 2 are listed in Table 1
to characterize the variation of bond orders via complexation. The
data in Table 1 demonstrate that the bond order of C–N bond
decreases from 1.02 in AK ligand to 0.98 in Cu(II)/AK complex (while
there occurs an increase in bond order of N–H bonds) due to the
donation of shared electrons from this chemical bond to copper(II)
cation through complexation, which is in agreement with our FT-IR
spectroscopic observations.
parentheses) in each of AK ligand 1 and Cu(II)/AK complex 2.
Comparative survey of the optimized structures of AK ligand 1
and Cu(II)/AK complex 2 demonstrates a geometrical deformation
In the next step, we focused on topological analysis of electron
[
49,50]
density via the QTAIM method
to interpret the nature of
Table 1. Calculated values of some selected bond orders in AK ligand
and Cu(II)/AK complex obtained at M06/6-311G** level of theory (note
that numbering of atoms is in accordance with Fig. 7)
AK ligand
Cu(II)/AK complex
N1–C1
N1–H1
N1–H2
Cu–N1
1.023
0.946
0.943
0.897
0.984
0.991
0.419
Figure 6. TGA curves of AK and Cu(II) catalyst.
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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Mirsafaei R., Heravi M. M., Hosseinnejad T. and Ahmadi S.
metal–ligand interactions in Cu(II)/AK complex 2. In this respect, we
the electron density at BCPs usually corresponds to the strength of
**
analysed M06/6-311G calculated wave function of electron
b
the bond between two atoms. Values of ρ < 0.1 au are indicative of
[54]
density using the AIM2000 program package. QTAIM molecular
graphs of AK ligand 1 and Cu(II)/AK complex 2 including all bond
and ring critical points and their associated bond paths are
displayed in Fig. 8.
an electrostatic interaction; it is usually correlated with a relatively
2
[50,55]
small and positive value of ∇ ρ
b
.
By contrast, for a covalent
2
interaction, usually ρ > 0.1 au and ∇ ρ is usually negative with
Moreover, a good authentic indicator
b
b
[
50,55]
b
the same order as ρ .
Table 2 gives QTAIM calculated values of electron density, ρ
b
, its
for classifying interatomic interactions is the total electronic energy
2
Laplacian, ∇ ρ , electronic kinetic energy density, G , electronic
density that is defined as H = G + V at BCPs. For electrostatic
b
b
b
b
b
potential energy density, V , total electronic energy density, H ,
interactions, H has a positive value and for covalent interactions
b
b
b
and |V |/G ratio at some selected bond critical points (BCPs) for
it is negative.
b
b
AK ligand 1 and Cu(II)/AK complex 2. It is important to mention that
On the basis of the results reported in Table 2, the following can
be stated: (i) through complexation, calculated values of ρb
decrease at C–N BCP and increase at N–H BCP, which confirms
the donation of shared electrons to the metal centre and is in
agreement with variations in the stretching frequency of the FT-IR
spectrum; (ii) the large values of electron density together with
2
the negative values of ∇ ρ and H at C–N and N–H BCPs indicate
b
b
the covalent character of C–N and N–H chemical bonds in AK ligand
1
and Cu(II)/AK complex 2; and (iii) the small values of electron
2
density with the positive values of ∇ ρ and Hb at Cu–N BCP
demonstrate that the metal–ligand interactions do not have
b
covalent character. For Cu–N BCP, calculated value of 1 < |V |/G
b
b-
2
confirms the presence of partially covalent–electrostatic
interaction. Furthermore, a more stringent analysis of QTAIM
molecular graphs demonstrates some new ring critical points
which can be attributed to the interaction of copper(II) and acetate
groups with oxygen and hydrogen atoms of AK ligand and leads to
an electronic stabilization effect in the complexation procedure.
Investigation of catalytic activity for oxidation of alcohols
In order to investigate the effect of solvent on the oxidation
reaction, various solvents such as acetonitrile, toluene, acetone,
n-hexane and water and solvent-free condition were chosen for
oxidation of benzyl alcohol (Table 3). The results show that the best
solvent for this reaction is water. For solvent-free condition, the
yield of benzaldehyde is low.
The effect of catalyst amount in the oxidation of benzyl alcohol
was investigated (Table 4). The results show that with increasing
amount of catalyst to 0.05 g, in fixed time, the yield of reaction
increases. Increasing in the amount of catalyst to 0.20 g does not
have any considerable effect on the reactivity.
Figure 8. Complete molecular graphs (MGs) of AK ligand 1 and Cu(II)/AK
complex 2 obtained using QTAIM analysis of M06/6-311G** electron
density functions. Bond critical points: red circles; ring critical points:
yellow circles; bond paths: pink lines.
After the investigation of the effects of different parameters, the
best conditions were chosen and various alcohols were oxidized in
the presence of a catalytic amount of Cu(II)/AK complex and H O2
2
as an oxidant in refluxing water. However, this reaction was
Table 2. Mathematical properties of some selected BCPs in AK ligand 1
and Cu(II)/AK complex 2. The properties have been obtained via QTAIM
analysis of the M06/6-311G** calculated wave function of electron
density (note that numbering of atoms is in accordance with Fig. 8)
a
Table 3. Effect of solvent in oxidation of benzyl alcohol
b
Solvent
B.p. (°C lit.)
Time (min)
Yield (%)
2
∇ ρ
ρ
b
b
G
b
V
b
H
b
b b
|V |/G
Acetonitrile
Solvent free
Toluene
81–82
—
65
40
55
42
70
73
80
90
AK ligand
BCP(N1–C1) 0.2664 À0.6944 0.1193 À0.4123 À0.2930 3.4559
BCP(N1–H1) 0.3357 À1.4768 0.0632 À0.4957 À0.4325 7.8435
BCP(N1–H2) 0.3343 À1.4592 0.0630 À0.4910 À0.4280 7.7936
Cu(II)/AK complex
110–111
56
85
Acetone
240
35
n-Hexane
Water
69
100
20
BCP(N1–C1) 0.2507 À0.5928 0.0987 À0.3457 À0.2470 3.5025
BCP(N1–H1) 0.3433 À1.5612 0.0636 À0.5175 À0.4539 8.1367
BCP(N1–H2) 0.3361 À1.6016 0.0579 À0.5163 À0.4584 8.9170
a
All reactions carried out in reflux condition in the presence of benzyl
alcohol (1 mmol), 0.05 g of copper nanocatalyst and 3 mmol of H
and 5 ml of solvent.
b
2 2
O
BCP(Cu–N1) 0.0784
0.2732 0.1030 À0.1377 À0.0347 1.3368
Time of maximum conversion.
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2016)

Green catalytic oxidation of benzyl alcohols to benzaldehydes
a
a
Table 4. Effect of catalyst amount in oxidation of benzyl alcohol
Table 6. Catalyst reusability of Cu(II)/AK complex
Entry
Catalyst amount (g)
Yield (%)
Selectivity (%)
Entry
Time (min)
Yield (%)
Selectivity (%)
1
2
3
4
5
—
5
70
90
91
90
100
100
100
100
100
1
2
3
4
5
6
21
21
21
21
21
21
90
90
85
77
75
70
100
100
100
100
100
100
0.03
0.05
0.10
0.20
a
Reaction conditions: benzyl alcohol =1 mmol, solvent = water, amount
= 3 mmol, time = 21 min at reflux condition.
a
2 2
of H O
Reaction conditions: alcohols =1 mmol, amount of catalyst =0.05 g,
= 3 mmol, solvent = water. All reactions were carried out in
2 2
H O
reflux condition.
performed with H O without catalyst in water and during 24 h
2
2
[
56]
produces a trace of aldehydes and so does not work.
Table 5 summarizes the results of the oxidation of various
benzylic alcohols using Cu(II)/AK complex as catalyst. The reaction
yields are considerably dependent on substituents (–Cl, –OCH3,
Table 7. Comparison of efficiency of various catalysts with that of Cu
(
II)/AK complex in the oxidation of benzyl alcohol to benzaldehyde
a
No.
Catalyst
Conditions Time (min) Yield (%)
Ref.
–
OH, –NO2 and –NH ) and on their positions. o-Hydroxy and
2
p-hydroxy and o-amino electron-donating substituents are
converted to the corresponding aldehydes in excellent yields
1
2
5 mol% TEMPO
0.25 mol%
H
H
2
O/60°C
O/60°C
720
720
98
55
56
56
2
(entries 3–6). On the other hand, the greatest slowing effect is
Cu(OAc)
2
detected with strong electron-withdrawing nitro substituents
3
4
5
6
1 mol% Pd on
SBA-15
H
2
O/reflux
480
13
>99
99
57
34
(
(
entries 8–10), whereas, halogen substituents cause modest yields
entry 2). Similarly, the oxidation of secondary benzylic alcohol
H14[NaP W
5 30
Acetic
acid/reflux
CH Cl /reflux
also leads to the corresponding ketone in satisfactory yield (entry
2). All of the corresponding aldehydes are obtained with
O
110]/SiO
2
1
Caffeinilium
2
2
45
95
58
excellent selectivity, and over-oxidation into carboxylic acids is
not observed.
chlorochromate
Cu(II)/AK complex H
0.05 g)
2
O/reflux
21
90
This work
(
a
Yield refers to isolated and pure product.
Table 5. Cu(OAc)
to corresponding aldehydes
2
supported on AK as catalyst in oxidation of alcohols
a
Catalyst recycling
One of the major reasons for supporting a homogeneous metal
complex on silica is to improve the activity and reusability of the
catalyst. To examine the reusability of the silica-gel-based complex
catalyst, the catalyst was separated by filtration after the reaction,
washed, dried under vacuum and then reused in a second run.
The yield and structure of the final products were determined
and compared with that of being used in the original run. As
evident from Table 6, this catalyst could be used for the oxidation
of alcohols to aldehyde up to six times without loss of selectivity.
To show the advantages of the Cu(II)/AK complex as a heteroge-
neous nanocatalyst in this reaction, our results and reaction
conditions for the oxidation of benzyl alcohol to benzaldehyde
c
d,e
Entry
R
Time(min)
Yield (%)
1
2
H
21
23
50
5
90
75
85
97
98
97
83
48
61
56
96
70
o-Cl
b
3
m-OH
o-OH
p-OH
4
5
6
7
8
9
5
o-NH
m-OMe
2
7
22
23
35
22
5
(Table 5, entry 1) were compared with previously reported
o-NO
p-NO
2
homogeneous and heterogeneous catalysis of this reaction
2
[34,56–58]
(Table 7)
. The results show that our catalyst is quite compa-
10
11
12
m-NO
2
rable with other catalysts regarding the yields and reaction times.
Hydroquinone
Diphenylmethanol
60
Conclusions
a
Reaction conditions: alcohols =1 mmol, amount of catalyst =0.05 g,
= 3 mmol, solvent = water. All reactions carried out in reflux
2 2
H O
Via modification of the mesoporous cage-like material KIT-5
followed by adsorption of Cu(OAc) and reduction with methanol,
2
condition, except entries 5, 9, 11, which were performed at room
temperature.
b
copper(II) nanoparticles with a uniform size distribution were
successfully confined in the nanocages of KIT-5, leading to a new
solid catalyst for the selective oxidation of alcohols. The solid
catalyst was characterized using FT-IR spectra, SEM–EDX and
solid-state UV–visible spectra. FT-IR and UV–visible specrta show
Acetone was used as a solvent.
c
Time of maximum conversion (determined by TLC and/or GC).
d
Isolated yield.
e
All reaction give corresponding aldehyde in 100% selectivity.
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

Mirsafaei R., Heravi M. M., Hosseinnejad T. and Ahmadi S.
[
20] Y. P. Paula Cruz, I. del Hierro, M. Fajardo, Micropor. Mesopor. Mater.
the structure of Cu(II)/AK complex, confirming that the ligand is
attached to the silica surface. UV–visible and EDX analysis also
demonstrate the existence of copper(II) on the resulting catalyst.
Moreover, we have employed quantum chemistry computational
methods to interpret and confirm the experimental characteriza-
tion of the nanocatalyst from computational viewpoints.
This new nanocatalyst was applied as a highly efficient catalyst
for the selective and environmentally friendly oxidation of benzylic
alcohols to corresponding aldehydes with hydrogen peroxide in
green media. The main advantages of this catalytic system are ease
of preparation, needing mild reaction conditions, easy work-up
procedure, low cost of the catalyst and finally it can be recycled
and reused several times without loss of activity. This catalyst
showed a high activity for the selective oxidation alcohols to the
corresponding aldehydes and ketones.
2
016, 220, 136.
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The MMH is grateful for partial financial support from the Iran
National Science Foundation (INSF). The authors are thankful to
the Alzahra University, Iran Polymer and Petrochemical Institute
for providing the SEM-EDX facility and Islamic Azad Yazd University
for providing FT-IR analyses.
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