M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249
231
[15], which are used for organic synthesis, pharmaceuticals and
agricultural chemicals [16]. So, synthesis of imines or oximes with
different functionality, potential versatility and wide scope of ap-
plications are essentially ever-appealing matter in chemistry. On
the other hand, low price, green nature, availability and variety of
alcohols (than aldehydes) have been made the one-pot oxidation
tandem coupling of alcohol and amine into imine, to one of the
most popular methods, rather than the traditional method, i.e.
condensation reaction of aldehydes or ketones with amines [10].
Several new methods have been developed in recent years either
by a homogeneous or heterogeneous catalyst in this area such as:
ruthenium PNP pincer complex [17], Co PNP complex [18], CeO2
[16], Au/HAP (Gold Supported on Hydroxyapatite) [2], TOMRh(CO)2
[19], CuClO4$4H2O [20], Pd-catalyst/TEMPO [21], Pt@TiO2 [15], CuI/
2,20-bipyridineTEMPO [21], PdAlO(OH) [22], CeO2-5 [23], CuI/
Reaction mechanisms were deeply investigated. Also, chemo-
selectivity behavior of the system was thoroughly studied in this
article.
2. Experimental
2.1. Materials and apparatus
All chemicals were obtained from Sigma and Fluca suppliers and
used without further purification. All the solvents were distilled
and dried before use. All other reagents are of analytical grade. 4-
Benzyloxybenzyl alcohol, polymer-bound was purchased from
Sigma with 100e200 mesh, extent of labeling: 1.0e1.5 mmol/g OH
loading and 1% cross-linked with divinylbenzene. Reaction progress
to be monitored by thin layer chromatography (TLC) on silica gel or
gas chromatography (GC) using a Shimadzu-14B gas chromatog-
raphy equipped with HP-1 capillary column and N2 as carrier gas.
Anisole was used as internal standard. Purification of imines and
oximes was achieved by recrystallization from ethanol. FTIR spectra
were obtained using a JASCO FT/IR 4600 spectrophotometer using
KBr pellet. The 1H NMR (250 MHz) and 13CNMR (62.9 MHz) spectra
were recorded on a Bruker Avance DPX-250 spectrometer in CDCl3
or DMSO‑d6 as a solvent and TMS as an internal standard. Elec-
trochemical measurements (CV: cyclic voltammetry, DPV: differ-
ential pulse voltammetry) for the Mn catalyst were performed on a
CHI 1210A electrochemical workstation (CH Instrument, China)
with a three-electrode system consisting of a calomel electrode
(SCE) as the reference electrode, a platinum wire electrode and a
modified glassy carbon [35] were used as the auxiliary and working
electrode, respectively. The CVs were recorded in the potential
range from ꢀ1.0e1.6 V after 200 s accumulation under stirring with
a scan rate of 100 mV sꢀ1. The cell temperature was maintained at
25.0 0.1 ꢁC by means of a HAAKE D8 recirculating bath. Elemental
analyses were performed on Perkin Elmer-2004 instrument. XPS
studies were conducted using an XR3E2 (VG Microtech) twin anode
ligand [24], Ru/AC [25], [Ru(
graphene oxide [26].
h m-Cl)Cl]2 [3] and MnO2/
6-p-cymene)(
The main impediments for the reported oxidative trans-
formation of alcohols to carbonyls, carboxylic acids and imines are
the use of expensive, non-economic, stoichiometric toxic oxida-
tions with expensive transition metals that may generate a notable
amount of (salt) waste along with toxicity problems. Also, most of
them are limited to activated (benzylic) alcohols and leads to the
formation of undesirable products. To now, many attempts have
been regularly investigated to introduction of new cleaner and
ecofriendly catalytic oxidation approaches by using less toxic metal
catalysts and molecular oxygen or air as oxidant [22e25]. However,
contrary to various progresses, many of them still suffer from harsh
and tedious reaction conditions, high costs, long reaction times,
operational complexity, functional group incompatibility, produc-
tion of un-processable and metal-containing wastes causing sig-
nificant environmental concerns, poor atom efficiency, and
instability. Moreover, selectivity for most of them could not be
controlled, because oxidation of an alcohol leads to different
oxidation products [27]. Hence, there is a continuing demand to
develop a more sustainable and environmentally benign and se-
lective oxidation processes that could be prevail the mentioned
drawbacks.
X-ray source with Al K
a
¼ 1486.6 eV. Transmission electron mi-
croscopy (TEM) was performed on a Philips EM208 microscope and
was operated at 100 kV. Field emission scanning electron micro-
scopy (FE-SEM) images were obtained on HITACHI S-4160. EDX
spectroscopy was performed using field emission scanning electron
microscope (FE-SEM, JEOL 7600F), equipped with a spectrometer of
energy dispersion of X-ray from Oxford instruments. Size distri-
bution of the nanoparticles were measured by dynamic light scat-
Potential of the Mn-based catalysts for oxidation and epoxida-
tion of organic compounds have been widely studied and known
for decades. Manganese metal can adopt a wide variety of oxidation
states in the range 2 þ to 5þ [28]. This behavior made Mn catalysts
as
a pivotal redox catalyst for oxygen transfer approaches.
Furthermore, Mn complexes have lower toxicity and low cost
compare to the other transition metal complex systems, which has
attracted a lot of attention for oxidation purposes. Various reports
have been appeared on Mn-base catalysts which are able to cata-
lyze the oxidation of alcohols with high efficiency [28e32]. How-
ever, very few reports for Mn-catalyzed cross coupling of alcohols
with amines or hydroxyl amine are available [14,33,34].
tering (DLS) analysis on
a HORIBA-LB550 instrument. ICP
experiments were accomplished using VARIAN VISTA-PRO CCD
simultaneous ICP-OES instrument. The surface area, pore volume,
and pore diameter of the obtained NPs were determined by N2
physisorption at ꢀ196 ꢁC with surface area and pore size analyzer
(Micromeritics ASAP 2000 instrument) using the BET method.
In this work we successfully functionalized the Fe3O4@SiO2
core-shell NPs with melamine- Mn(III) Schiff base complex as a
novel heterogeneous magnetically recoverable nanocatalyst for
general, efficient, environmentally benign and selective trans-
formation of primary and secondary alcohols to the corresponding
carbonyl compounds in the presence of H2O2. Also, direct trans-
formation of alcohol to carboxylic acid have been accomplished
through a different route by O2. To extent of versatility of the pre-
sent method, one pot oxidation tandem coupling of alcohol and
amine was performed by this approach with high yields. The
presence of imidazolium part in the catalyst provide an additional
functionality that could be accomplish the oxidation in absence of
any metal site. In this point of view, it plays a synergetic effect,
which studied in this paper.
2.2. Preparation of the working electrode Fe3O4@SiO2@Im[Cl]
Mn(III)-complex nanocomposite/GCE electrode for CV analyses
In first, the bare GCE was polished to give a smooth surface with
0.3 and 0.05 mm g-alumina. Then, the treated electrode was cleaned
by deionized water into an ultrasonic cleaner for 5 min followed by
further cleaning with nitric acid: ethanol (1:1 v/v) solution. Finally,
the electrode was washed with deionized water. Fe3O4@SiO2@Im
[Cl]Mn(III)-complex nanocomposite (1.0 mg) was added into
deionized water (1.0 mL) and the mixture was ultra-sonicated for
2.5 h at room temperature to obtain a homogenous mixture. The
Fe3O4@SiO2@Im[Cl]Mn(III)-complex nanocomposite/GCE electrode
was prepared by dropping 5
mL of the mixture on the pretreated
GCE and then dried in a desiccator.