Fe3O4@SiO2@Im[Cl]Mn(III)-complex as a highly efficient magnetically recoverable nanocatalyst for selective oxidation of alcohol to imine and oxime

Journal of Molecular Structure 1 186 (2019) 230e249

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Journal of Molecular Structure

journal homepage: h ttp://www.elsevier.com/locate/molstruc

Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex as a highly efﬁcient magnetically

recoverable nanocatalyst for selective oxidation of alcohol to imine

and oxime

,

Milad Kazemnejadi ^{a *}, Seyyedeh Ameneh Alavi ^a, Zinat Rezazadeh ^a,

Mohammad Ali Nasseri ^a, Ali Allahresani ^a, Mohsen Esmaeilpour ^b

^aDepartment of Chemistry, Faculty of Sciences, University of Birjand, Birjand, Iran

^bDepartment of Chemistry, College of Science, Shiraz University, Shiraz, 7194684795, Iran

a r t i c l e i n f o

a b s t r a c t

Article history:

An efﬁcient and environmentally friendly oxidation process for the one-pot preparation of oxime, imine

and carbonyl compounds through alcohol oxidation in the presence of H₂O₂and/or O₂have been

developed by a melamine-Mn(III) Schiff base complex supported on Fe₃O₄@SiO₂eCl nanoparticles,

named as Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite, at room temperature. Direct oxidation of

alcohol to carboxylic acid was performed using the catalyst in the presence of molecular O₂at room

temperature in a different approach. The oxidation products were obtained with excellent yields and

high TOFs. The properties of the catalyst were characterized by Fourier transform infrared spectroscopy

(FTIR), elemental analysis (C, H, N), X-ray diffraction (XRD), ﬁeld emission scanning electron microscopy

(FE-SEM), dynamic light scattering (DLS), energy dispersive X-ray analysis (EDX), X-ray photoelectron

spectroscopy (XPS), inductive coupled plasma (ICP), cyclic voltammetry (CV), nuclear magnetic reso-

nance (¹H & ¹³C NMR), vibration sample magnetometer (VSM), Brunauere EmmetteTeller (BET) and

differential pulse voltammetry (DPV) analyses. The mechanism of the oxidation processes was investi-

gated for the both H₂O₂and O₂oxidants. The role of the imidazolium moiety in the catalyst as a sec-

ondary functionality was investigated. Chemoselectivity behavior of the catalyst was studied by some

combinations. The catalyst could be recycled from the reaction mixture by a simple external magnet and

reused for several times without any considerable reactivity loss.

Received 14 January 2019

Received in revised form

28 February 2019

Accepted 4 March 2019

Available online 12 March 2019

Keywords:

Alcohol oxidation

Carboxylic acid

Imine

Magnetically recoverable

Mn Schiff base complex

1. Introduction

building blocks for the construction of pharmaceuticals and agro-

chemicals [5]. Different approaches have been developed for efﬁ-

Oxidation reactions are of particular importance in organic

synthesis and widely used in industrial processes in order to the

preparation of a variety of chemicals [1]. Among, oxidation of al-

cohols, as a cheapest starting materials, to aldehydes and carboxylic

acids is very valuable [2]. Carboxylic acids as valuable organic

chemicals, have countless applications in pharmaceuticals and ag-

rochemicals from both academic and industrial points of view [3].

One example is the process of metal-catalyzed carbonylation of

methanol to form acetic acid [4]. Aldehydes, also useful in-

termediates in organic synthesis that can be utilized as a precious

cient oxidation of alcohols to aldehydes and/or carboxylic acids,

which can be point to: lactase-mediator system [6], supported Pt/Bi

[7], supported Au [5], Pd catalyst [1], PdeBieTe/C (PBT/C) catalyst

[8], TEMPO/DAIB [TEMPO ¼ 2,2,6,6-tetramethylpiperidine-N-oxyl,

DAIB¼(diacetoxy)iodobenzene] [9], AgeNHC complex [10],

Fe(NO₃)₃/TEMPO [11], NHC-ruthenium catalyst [12], Ru PNP pincer

complexes [13] were some oxidation systems that have been re-

ported recently for oxidation of primary alcohol to carbonyl

compounds.

Also, the transformation of alcohol to carbonyls could be

conduct to direct preparation of imine and oxime in the presence of

an amine [14]. Due to their high reactivity, imines and oximes are

an important class of nitrogen-containing compounds, and are also

key intermediates in multi-component reactions, preparation of

asymmetric organocatalysis, cross-dehydrogenative couplings, etc.

* Corresponding author.

E-mail addresses: miladkazemnejadi@birjand.ac.ir, miladkazemnejad@yahoo.

com (M. Kazemnejadi).

https://doi.org/10.1016/j.molstruc.2019.03.008

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], TO^MRh(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 N₂as 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‑d₆as 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 s^ꢀ1. 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.

232

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

2.3. Preparation of Mn(III) Schiff base complex (4)

room temperature underwent N₂atmosphere (Scheme 1). The

product 4 was simply ﬁltered, washed with cooled ethanol and

diethyl ether, then dried at room temperature and isolated as a dark

brown powder (95% yield, isolated weight: 0.5 g) (Scheme 1).

Firstly, 2-hydroxy-5-chloromethyl benzaldehyde was synthe-

sized and puriﬁed according to the procedure described in Ref. [14]

(purple solid, 98% yield). 2-Hydroxy-5-chloromethyl benzaldehyde

(1) (2.0 mmol, 0.34 g) was dissolved in 20 mL of toluene and then

imidazole (10.0 mmol) was added to the mixture. The mixture was

reﬂuxed with stirring for 48 h under N₂atmosphere. After reﬂux-

ing, the reaction was stopped and cooled to room temperature. The

solvent was removed under reduced pressure and the product 2

was washed with dilute NaOH and deionized water then dried into

oven at 50 ^ꢁC (dark brown powder, 98% yield). Transformation of 2-

hydroxy-5-imidazole benzaldehyde to 3 was done according to the

procedure described in Ref. [36] (Pale yellow powder, 98% yield). 4-

2.4. Preparation of Fe₃O₄@SiO₂core-shell nanoparticles (7)

Core-shell Fe₃O₄@SiO₂nanospheres was prepared according to

previously described procedure (Scheme 2) [37].

2.5. Preparation of Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex

nanocomposite (9)

Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite was syn-

thesized according to the following procedure: Fe₃O₄@SiO₂eCl

(1 g) was added to the solution of Mn(III)-Im-Schiff base complex

(4, 1.0 mmol, 0.54 g) in 20 mL p-xylene and the resultant mixture

was stirred for 24 h under N₂atmosphere and reﬂux conditions.

((1H-imidazole-1-yl)methyl)-2-((allylimino)methyl)phenol

(3,

1.0 mmol, 0.24 g) was complexed to Mn(III) ions with dissolution to

15 mL of ethanol followed by addition of Mn(OAc)₃. 2H₂O

(0.5 mmol, 0.15 g). The resulting mixture was stirred for 12 h at

Scheme 1. The preparation of Mn(III) Schiff base complex 4.

Scheme 2. The preparation of Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex (9).

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

233

Then, the mixture was cooled to room temperature, ﬁltered by an

external magnet, and the product was washed with p-xylene to

remove any unreacted species and dried at 70 ^ꢁC for 6 h. Immobi-

lized Mn(III)-Im-Schiff base complex (8) along with a weighted

melamine (molar ratio 6: 1) were transferred to a 25 mL oven dried

round bottom ﬂask equipped with a magnet and a condenser. A

solution of trimethylamine (4.0 mmol) in absolute MeOH (5.0 mL)

and AlCl₃$2H₂O (4.0 mmol) were added to the mentioned mixture.

The mixture was reﬂuxed in N₂atmosphere for 6 h. The product,

Fe₃O₄@SiO₂@Im[Cl]Mn(III) (9) was collected using an external

magnetic ﬁeld, washed with water (2ꢂ 10 mL) and EtOH

(2 ꢂ 10 mL) and dried into vacuum oven (6 h, 60 ^ꢁC). Scheme 2

shows the complete route for the preparation of 9.

removed and the catalyst 9 was ﬁlter off by an external magnet. The

resultant mixture was washed with diethyl ether to remove of

alcohol residues. The aqueous layer was acidiﬁed with HCl (3N),

then extracted with ethyl acetate. The organic layer was dried over

Mg₂SO₄, the solution was ﬁltered and the desire carboxylic acid was

obtained after remove of solvent under reduced pressure. All car-

boxylic acids were identiﬁed by comparison of their NMR spectra

with the literature data.

2.9. Reutilization of the catalyst

Reutilization of Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nano-

composite was examined using the oxidation of benzyl alcohol to

benzaldehyde. In this way, the catalyst was ﬁltered using a simple

external magnet after the experiment and reused for the next run.

The residue was analyzed by ICP to measure of Mn leached from the

catalyst to the solution. Also, the hot ﬁltration as well as three-

phase tests were employed for investigation of heterogeneous na-

ture of the catalyst.

2.6. General procedure for catalytic oxidation of alcohols catalyzed

by Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex in the presence of H₂O₂

Alcohol (10.0 mmol) and the catalyst 9 (40 mg, 0.5 mol % Mn)

were added to 1.5 mL H₂O₂37% (10 mL round bottom ﬂask). The

reaction was stirred at room temperature in air atmosphere for

sufﬁcient time. Progress of the reaction and product selectivity

were monitored by GC. After completion of the reaction, the cata-

lyst 9 was separated by an external magnet. The product was ﬁlter

off, washed with deionized water (3 ꢂ 5 mL), dried into oven

(50 ^ꢁC) and then isolated. Silica-gel plate chromatography utilized

for the case of benzaldehyde puriﬁcation.

3. Results and discussion

3.1. Characterization of catalyst

Elemental analyses for the samples were tabulated in Table 1.

The results were in good agreement with the theoretical calcula-

tions and conﬁrmed the preparation of 2-Hydroxy-5-chloromethyl

benzaldehyde (1), 2-hydroxy-5-imidazole benzaldehyde (2), 3 and

4. Also, the results were in accordance with the proposed structure

sketched in Scheme 2.

2.7. General procedure for one-pot preparation of imine and oxime

through oxidation of alcohols using Fe₃O₄@SiO₂@Im[Cl]Mn(III)

complex nanocomposite in the presence of H₂O₂

Fig. 1 shows the FTIR spectra of 1, 2, 3, 4, 5, 6, 7, 8 and 9 com-

pounds. The FTIR absorption spectra of 1, shows two characteristic

peaks at 1481 cm^ꢀ1and 725 cm^ꢀ1correspond to the methylene

groups (eCH₂e) in m-substitution and stretching vibration of CeCl

group respectively (Fig. 1a) [14]. The FTIR spectrum of 2 shows a

series of peaks at 2490-2780 cm^ꢀ1related to the imidazole moiety

in agreement with compounds bearing imidazole moieties re-

ported in the literature (Fig. 1b) [38].

In a typical run, alcohol (10.0 mmol) and the catalyst (40 mg,

0.5 mol % Mn) were added to 1.5 mL H₂O₂37% (10 mL round bottom

ﬂask). The reaction was stirred at room temperature in air atmo-

sphere for 10 min. Then NH₂OH$HCl and/or amine (10.0 mmol) was

added to the mixture. Progress of the reaction was monitored by

TLC. After completion of the reaction, 10 mL of hot ethanol was

added to the mixture, then the catalyst 9 was ﬁlter off by an

external magnet. The product was isolated by removal of the sol-

vent under reduced pressure followed by washing with deionized

water (3 ꢂ 5 mL). Products were puriﬁed by recrystallization in hot

ethanol and then isolated.

Moreover, elimination of CeCl bond at 725 cm^ꢀ1in the spec-

trum of 2 corroborates the imidazolium functionalization of 5-

chloromethylene salicylaldehyde. Stretching vibration related to

the NeH of imidazole moiety was appeared at 3413 cm^ꢀ1. Imine

formation of 3 was conﬁrmed by presence of imine bond at

1611 cm^ꢀ1(Fig. 1c) [27]. This absorption was shifted to 1563 cm^ꢀ1

due to Mn complex formation. This red shift along with MneO and

MneN stretching vibrations at 504 cm^ꢀ1and 615 cm^ꢀ1respec-

tively, conﬁrmed the successful coordination of Mn ions through N

and O in 4 (Fig.1d). Fe₃O₄NPs showed a single characteristic peak at

570 cm^ꢀ1related to FeeO bond [39] (Fig. 1e). The presence of three

main peaks at 779, 1072 and 3433 cm^ꢀ1correspond to stretching

vibrations of FeeO, SieOeSi (Symm.) and SieOeSi (Asymm.)

respectively, that strongly conﬁrm the coating of Fe₃O₄NPs by silica

shells (Fig.1f) [40]. Fig.1g demonstrated the vibrations attributed to

2.8. General procedure for the one-pot preparation of carboxylic

acid through oxidation of alcohols using Fe₃O₄@SiO₂@Im[Cl]Mn(III)-

complex nanocomposite in the presence of molecular O₂

In a 10 mL round bottom ﬂask equipped with O₂purging,

alcohol (10.0 mmol) and the catalyst (20 mg, 0.25 mol % Mn) were

added to 3 mL dry acetonitrile. Oxygen with ﬂow rate of 120 mL/

min was purged to the mixture during the reaction and the reaction

was stirred at room temperature. Reaction progress was monitored

by GC. After completion of the reaction, the purge system was

Table 1

Elemental analysis results and colours of the compounds.

Compounds

Appearance

Elemental analysis

%C

%H

%N

2-Hydroxy-5-chloromethyl benzaldehyde (1)

2-hydroxy-5-imidazole benzaldehyde (2)

4-((1H-imidazole-1-yl)methyl)-2-((allylimino)methyl)phenol (3)

Mn(III) Schiff base complex (4)

Purple powder

56.30 (56.82)^a

65.74 (65.34)

69.47 (69.69)

62.81 (62.80)

4.14 (4.25)

5.09 (4.98)

6.22 (6.27)

5.19 (5.27)

e

Dark brown powder

Pale yellow powder

Brown powder

13.97 (13.85)

17.64 (17.41)

15.00 (15.69)

a

Theoretically calculated.

234

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

Fig. 1. FTIR spectra of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, (f) 6, (g) 7, (h) 8, and (i) 9.

CeH Str., eCH₂e Bend. and CeCl Str. at ~2920, 1458, 633 cm^ꢀ1

,

2Ɵ ¼ 15e25^ꢁrepresent the coated amorphous silica on Fe₃O₄NPs

(Fig. 2b and c). XRD pattern of Fe₃O₄@SiO₂@Mn(III) complex (8) as

well as Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex nanocomposite is

respectively, that strongly conﬁrmed the preparation of Fe₃O₄@-

SiO₂eCl (Fig. 1g). Incorporation of complex 4 on Fe₃O₄@SiO₂eCl

was proved by the presence of C]N, SieOeSi, FeeO, C]C corre-

spond to the wavenumbers at 1560 cm^ꢀ1, 1018 cm^ꢀ1, 405 cm^ꢀ1and

1414 cm^ꢀ1. Fig. 1i shows the spectrum of Fe₃O₄@SiO₂@Im[Cl]

Mn(III)-complex nanocomposite (9) and clearly conﬁrming the

intercalation of melamine between the 8 compounds and conse-

quently preparation of the nanocomposite 9.

Loading amount of Mn on the catalyst was measured by atomic

absorption spectroscopy (AAS) as well as ICP instruments due to

more ensure attainment. The results for the two analyses were

close to each other with few difference (0.008). The experiments

indicated that 1.25 mmol of Mn metal per gram of the catalyst

(1.25 mmol g^ꢀ1) was loaded on the catalyst framework 9. Also, the

analyses give the percentage of the heavy metals as: 45.74 %w, 2.22

%w, 6.80 %w for Fe, Si, Mn respectively.

contain silica as well as Fe₃O₄peaks in their corresponding 2q with

amorphous appearance (Fig. 2d and e). The compound 8 represents

an amorphous peak at 2

incorporation of Mn(III)-Schiff base complex on the Fe₃O₄@SiO₂

core-shell NPs [41].

q

¼ 25^ꢁthat may be attributed to the

The broadening of the peaks shows that Fe₃O₄@SiO₂NPs func-

tionalized with amorphous melamine-Mn(III) Schiff base complex

and revealed the successfully preparation of the nanocomposite 9.

The broad peak at 2

q

¼ 22^ꢁwas assigned to amorphous melamine-

Mn(III) Schiff base complex (Fig. 2e). This broad peak was trans-

ferred to lower angles than the corresponding amorphous peak for

compound 8. This shift is due to the synergetic effect of Mn(III)

Schiff base complex as well as melamine that is incorporated into

the catalyst framework [42]. Beside of other analyses, size of

XRD spectra for the synthesized nanoparticles were shown in

Fig. 2. As presented in Fig. 2, Fe₃O₄nanoparticles have highly

crystalline cubic inverse spinel structure in agreement well with

the standard Fe₃O₄(cubic phase) XRD spectrum (PDF#88-0866).

The characteristic diffraction peaks at 2Ɵ ¼ 30.1, 35.4, 43.1, 53.4, 57

and 62.6, which were marked respectively by their indices (220),

(311), (400), (422), (511) and (440) (reference JCPDS card no. 19-

629) (Fig. 2aed) demonstrated the inverse spinel structure of Fe₃O₄

NPs and were also observed for Fe₃O₄@SiO₂, Fe₃O₄@SiO₂eCl and

Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex nanocomposite [37]. This,

manifested that the surface modiﬁcation of the Fe₃O₄NPs does not

lead to their phase change or structure. The XRD pattern of

Fe₃O₄@SiO₂and Fe₃O₄@SiO₂eCl shows an obvious diffusion peak at

nanoparticles was calculated using Scherrer's equation: D ¼ K

cos ), D is the average

, where K is a constant (K ¼ 0.9 for Cu-K

diameter in ^ꢁA,

is the broadening of the diffraction line measured

at half of its maximum intensity in radians, is the wavelength of

the X-rays and is the Bragg diffraction angle. The crystalline size of

g/b

q

a

b

g

q

the Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex nanoparticles was calcu-

lated to be around 53 nm using the equation.

XPS spectrum survey of the Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex

nanocomposite was shown in Fig. 3a and revealed the surface

chemical compositions as well as valence state of the elements.

Fig. 3a shows the overall survey spectrum of Fe₃O₄@SiO₂@Im[Cl]

Mn(III) complex nanocomposite in which the peaks corresponding

to Mn 3p (48.5 eV), Mn 3s (84.2 eV), Si 2p (105.3 eV), Si 2s

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

235

XPS spectrum was used to investigate the oxidation state, and the

binding energy of the core electron of the Mn metal (Fig. 3b). The

Mn2p spectrum presented two peaks with binding energies of

653.6 eV and 642.5 eV related to Mn2p_1/2and Mn2p_3/2, respec-

tively. Magnitude of peak splitting is 11.1 eV, which conﬁrmed that

Mn ions are þ3 in valence state [27].

The elements in the catalyst was probed by EDX and given in

Fig. 3c. The results were completely in agreement with the XPS

analysis of the catalyst and conﬁrmed successfully immobilization

of the Mn(III) Schiff base complex on Fe₃O₄@SiO₂eCl nanoparticles.

CV and DPV proﬁles of the Mn(III) Schiff base complex 4 and

Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite 9 versus SCE

at scan rate of 100 mVs^ꢀ1are exhibited near symmetric redox cy-

cles between 0 and 1.6V (Fig. 4A, B). As shown in Fig. 4A, the CV

experiments are characterized by two reversible redox couples and

determined the electro activity and stability of the complexes. The

two oxidation processes at ~0.63 V and 1.42 V were assigned to

oxidation of Mn(II) to Mn(III) and Mn(III) to Mn(IV) for the Mn(III)

Schiff base complex (4). As evident from Fig. 4A-b, two reduction

peaks were also appeared at 0.30 V and 1.34 V can be attributed to

reduction of Mn(IV) to Mn(III) and Mn(III) to Mn(II) respectively.

Same redox behavior was observed for Fe₃O₄@SiO₂@Im[Cl]Mn(III)-

complex nanocomposite. The anodic and cathodic peaks current of

the catalyst was found to be 0.72 V, 1.37 V (E_ac) and 0.43 V, 1.30 V

(E_pc). This behavior exhibits that the catalyst provide faster electron

transfer kinetics and larger electroactive surface area compared to

Mn(III) Schiff base complex 4 and was in agreement with the

literature [43]. Differential pulse voltammetry (DPV) that is a

technique much more sensitive than cyclic voltammetry [44,45],

shows a more precise the redox potential of the redox process and

provided further support for the reversible redox behavior of the

Mn complexes 4 and 9 (Fig. 4B). DPV clearly conﬁrms the reversible

couples for Mn(III) Schiff base complex 4 and Fe₃O₄@SiO₂@Im[Cl]

Mn(III)-complex nanocomposite 9 from cyclic voltammetry. The

quasi-reversible couples seen in the CV appears here with a current

similar to the reversible couples, but with a smoothly and sharp-

ened peak (Fig. 4B).

VSM analyses for Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂@Im[Cl]

Mn(III) complex nanocomposite are shown in Fig. 4C. These NPs

exhibited high permeability in magnetization and good magnetic

responsiveness, which their magnetization was sufﬁcient in order

to separation with an external magnetic ﬁeld. The magnetization

and demagnetization curves are coincident and did not any hys-

teresis phenomenon was found. As shown in Fig. 4C, the remanent

magnetization is equal to zero for all NPs. Magnetic measurements

show saturation magnetization values of 70.0, 39.8 and 35.9 emu/g

for Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex

nanocomposite, respectively (Fig. 4C-a,b,c). The results demon-

strated that the magnetization of Fe₃O₄decreased considerably due

to coated silica-shell and Schiff base Mn complex on its surface

(Fig. 4C-b,c). Nevertheless, Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex

nanocomposite exhibited the superparamagnetic characteristic

and a high magnetization value, which can be readily separated

from the mixture by a simple external magnet.

The morphology and structure of the Fe₃O₄@SiO₂@Im[Cl]

Mn(III)-complex nanocomposite were also observed using TEM

(Fig. 4D). The TEM image reveals that the catalyst having well

shaped spherical or ellipsoidal particles. The diameter of the

nanoparticles is found to be approximately 60 nm. According to FE-

SEM images, the synthesized Fe₃O₄NPs and Fe₃O₄@SiO₂are nearly

spherical in shape and well dispersed; although in some areas

bigger structures with non-spherical morphology are observed

(Fig. 5a and b). The Fe₃O₄and Fe₃O₄@SiO₂nanoparticles have an

average diameter of 10e12 nm and 20e22 nm respectively (Fig. 5a

and b), consistence with their DLS analyses (Fig. 5d and e). Fig. 5c

Fig. 2. XRD patterns of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, (c) Fe₃O₄@SiO₂eCl, (d) Fe₃O₄@-

SiO₂@Mn(III) complex 8, and (e) Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex nanocomposite

(9).

(149.8 eV), Cl 2p (199.2 eV), C 1s (284.9 eV), N 1s (401.5 eV), O 1s

(532.2 eV), Mn 2p_3/2(642.5 eV), Mn 2p_1/2(653.6 eV), Fe 2p

(712.1 eV) are clearly observed. The presence of elements such as

Mn, C, Si and N indicating that Fe₃O₄core was completely covered

by SiO₂shell and Mn(III) Schiff base complex. The high resolution

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M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

Fig. 3. (a) Overall survey, (b) high resolution XPS spectra, (c) EDX analysis of Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex nanocomposite and (d) it's the corresponding FE-SEM image shows

where the EDX analysis is taken from.

showing FE-SEM image of Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex

nanocomposite with near-spherical morphology. After successive

coating with Schiff base Mn complex, these composites still

maintain their spherical shape with an average diameter of ~59 nm

(Fig. 5d). The size distribution of these NPs were centered at a value

of 12, 20 and 59 nm for Fe₃O₄, Fe₃O₄@SiO₂and Fe₃O₄@SiO₂@Im[Cl]

Mn(III)-complex nanocomposite, respectively (Fig. 5def), and were

in agreement with the particle sizes exhibited in SEM images.

Speciﬁc surface area and porosity of the samples were investi-

gated by N₂adsorptionedesorption isotherm analysis. The speciﬁc

surface area values, the average pore diameter (according to BJH

method) and total pore volume are tabulated in Table 2. According

to the BET isotherm, the active surface area of Fe₃O₄, Fe₃O₄@SiO₂,

Fe₃O₄@SiO₂eCl, Fe₃O₄@SiO₂eMn(III)-Schiff base complex (8) and

Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite 9 was deter-

mined as 480, 455, 400, 386 and 375 m²/g, respectively (Table 2).

3.2.1. Effect of solvent

Did not use any solvent for alcohol oxidation to aldehyde/ke-

tone. We investigated solvent effect on oxidation of benzyl alcohol

to benzoic acid. As shown in Fig. 6a, solvent had a promising role for

this oxidation case. According to the radical proposed mechanism

that will be discussed later, when the solvent polarity is increased

causes to solvation of the polar transition state for the propagation

reaction and the reaction conversion is increased. Due to polar ef-

fect of electron-withdrawing functional groups (nitro, cyano, and

carboxyl) in solvents such as CH₃CN, NO₂CH₃, acetic acid, they

would be inert to peroxy radical attack and provide high conversion

and selectivity than solvents such as DEE and THF. These solvents

provide complications from co-oxidation and delay the oxidation

[46e48].

3.2.2. Effect of oxidant

The oxidation of benzyl alcohol was investigated with different

oxidants (Fig. 6b). All of oxidants cause to benzyl alcohol oxidation

to benzaldehyde except O₂and NaOCl. As we discussed in

following, molecular oxygen, as an oxygen source, support a radical

mechanism. High selectivity and conversion was achieved by H₂O₂.

Next, H₂O₂/urea provides same conversion with a lower selectivity.

On the other hand, molecular oxygen and NaOCl proceeded the

oxidation to benzoic acid. NaOCl provides lower efﬁciencies; it may

3.2. Optimization studies

Initially, oxidation of benzyl alcohol was chosen as model re-

action in order to ﬁnding optimum conditions interfered in the

reaction. These parameters were solvent, oxidant and catalyst

amount.

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

237

Fig. 4. (A) Cyclic, and (B) differential pulse voltammograms of (a) Mn(III) Schiff base complex (4) and (b) Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite (9) in 0.1 mol L^ꢀ1BR

buffer solution (pH 7.0) at 100 mVs^ꢀ1scan rate. (C) Magnetization curves at 300 K for (a) Fe₃O₄, (b) Fe₃O₄@SiO₂and (c) Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite. (D) TEM

image of Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite.

be due to radical side reactions concerning with NaOCl [27]. The

model reaction was performed in various concentration of H₂O₂as

an oxidant. The results indicated that there is no effect of the

concentration of the oxidant on the model reaction (yESI, Table S1).

3.3. Catalytic oxidation of alcohols to imine and/or oxime and one-

pot preparation of carboxylic acid from alcohol using

Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite

The utility of the Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex for

oxidation of alcohols to aldehydes and to carboxylic acids, oxida-

tion of alcohols to imine and to oxime in the presence of amine or

NH₂OH$HCl was tested with a wide range of substrates (Scheme 3,

Tables 3 and 4). Various benzylic and aliphatic alcohols, were

subjected to the reaction conditions. In most cases, a high yield of

the oxidation products was achieved. The scope is included electron

rich and deﬁcient alcohols derivatives, primary and secondary al-

cohols, aromatic and aliphatic alcohols, most of which underwent

efﬁcient oxidation in excellent yield (>90%) (Scheme 3, Tables 3 and

4).

Direct preparation of imines or oximes was afforded by a variety

of primary benzylic alcohols in the presence of amines or hydroxyl

amine hydrochloride catalyzed by Fe₃O₄@SiO₂@Im[Cl]Mn(III)-

complex nanocomposite in H₂O₂at room temperature (Scheme 3

and Tables 3 and 4). High to excellent yields and selectivities was

obtained for all of the compounds. The system demonstrated well

performance for the preparation of salen-type ligands (Table 3, 12c,

12d, 12h) in the presence of a diamine and two equivalent of

3.2.3. Effect of mol% of catalyst

The effect of catalyst amount on the oxidation of benzyl alcohol

to benzaldehyde and benzoic acid are presented in Fig. 6c. The

reaction was performed with different amounts of 9 in the presence

of O₂(for oxidation to benzoic acid) or H₂O₂(for oxidation to

benzaldehyde) at room temperature. The observations were clearly

proved the effectiveness of the catalyst for oxidation of the com-

pounds. It was observed that the conversion increases with an in-

crease in the amount of the catalyst to 0.25 for oxidation to benzoic

acid in the presence of O₂and 0.5 mol% for oxidation to benzalde-

hyde in the presence of H₂O₂. Larger quantities of manganese did

not show considerably inﬂuence under the chosen conditions and

did not provide more efﬁciency for the reactions. Selectivities are

independent to catalyst amount and they remain constant during

variation of the catalyst mol% (98% for benzoic acid and 99% for

benzaldehyde).

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M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

Fig. 5. (a, d) FE-SEM images and DLS results of Fe₃O₄, (b, e) Fe₃O₄@SiO₂, and (c, f) Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite.

Table 2

Surface characteristics of Fe₃O₄, Fe₃O₄@SiO₂Fe₃O₄@SiO₂eCl, Fe₃O₄@SiO₂eMn(III)-Schiff base complex (8) and Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex nanocomposite (9).

Entry

Sample

Speciﬁc surface area (m²/g)

Pore volume (cm³/g)

Average pore radius (nm)

1

2

3

4

5

Fe₃O₄

Fe₃O₄@SiO₂

Fe₃O₄@SiO₂eCl

Fe₃O₄@SiO₂eMn Sch. Complex

Catalyst 9

480

455

400

386

375

0.803

0.785

0.755

0.734

0.721

1.254

1.787

1.802

1.852

1.993

alcohol, and bis-imine compounds (Table 3, 12i, 12l) with excellent

yields. An interesting result was the preparation of the heavy-bulky

tetra-imines (Table 3, 12j, 12k) by 3,3⁰-diaminobenzidine with high

yields using the present method. However, the products with some

unreacted amino groups were also observed for two entries 12j,

12k.

Oxime as well as imine derivatives were prepared using the Mn-

catalyzed oxidation system in H₂O₂in quantitative yields (Table 4).

Both, electron rich and electron deﬁcient aromatic rings operated

well with this method.

The possibility of the formation of oxime and/or aldehyde were

examined in the presence of molecular O₂catalyzed by Fe₃O₄@-

SiO₂@Im[Cl]Mn(III)-complex. As will be discussed at “mechanism

studies” section, O₂promotes a radical mechanism causes alcohol

oxidation to proceeds to benzoic acid. The oxidation reaction of

benzyl alcohol in CH₃CN catalyzed by Fe₃O₄@SiO₂@Im[Cl]Mn(III)-

complex gives 13% imine product (1-diphenylmethanimine) for 5 h.

The major product was benzoic acid. Surprisingly, the experiment

gives 76% imine in the presence of hydroquinone as a radical

scavenger. An explanation for this phenomenon may be related to

the more radical formation rate than the formation of an amine. In

addition, in accordance with the mechanism shown in Scheme 5,

the acid (H^þ) produced during the reaction can also be contribute to

the imine hydrolysis and is responsible for the low efﬁciency

observed for the imine product (Scheme 5).

The efﬁciency of the oxidation to aldehyde is depends on the

nature of the substituents on the aromatic ring. Donor-substituted

benzylic alcohols gave the corresponding alcohols in good to

excellent yields, whereas acceptor-substituted provided lower ef-

ﬁciencies (Table 5, 15c, 15e, 15h, 15i, 15m). The best results were

obtained with alcohols with eN(Me)₂15g and eOMe 15d sub-

stituents. Efﬁciency was dropped for oxidation of 1,4-

benzenedimethanol 15h. It is believed that the formation of the

ﬁrst carbonyl group on the ring causes the ring to be deactivated

and therefore, slow down the oxidation for the second group on the

ring. The same phenomenon can also be responsible for its low

selectivity.

Mn-catalyzed oxidation of primary benzylic alcohol to carbox-

ylic acid in the presence of O₂didn't show any remarkable substi-

tute effect. All substrates were efﬁciently transformed to acid with

high to excellent yields (Table 5, 14a-m). Also, the method shows an

excellent selectivity whether for acid or aldehyde and there is not

any considerable oxidation by-product (Table 5, yESI, Tables. S2, S3).

These results illustrate that the direct oxidation of alcohol to

acid pass another route, where the substitute plays a very poor role

in efﬁciency. Aliphatic alcohols reacted poorly (17a-d, 18a-d), but

good efﬁciency was achieved for cinnamaldehyde 18d and cin-

namic acid 17d (Table 6). Secondary alcohols represented lower

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

239

Fig. 6. Effect of (a) solvent, (b) oxidant, and (c) catalyst amount on the oxidation of

benzyl alcohol to benzaldehyde, and

benzyl alcohol to benzoic acid catalyzed by

Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite 9. Alcohol (10.0 mmol), catalyst (20 mg, 0.25 mol % Mn), CH₃CN (3 mL), O₂purging (120 mL/min), room temperature, 8 h Alcohol

(10.0 mmol), catalyst (40 mg, 0.5 mol % Mn), H₂O₂37% (1.5 mL), room temperature, air atmosphere, 4 h * (1: 1, 1.5 mL), ** Cumene hydroperoxide.

additional substituent in secondary alcohol cause to steric effect

and consequently causes to lack of suitable interaction between the

substrate and the catalyst (as a macromolecule) and requires a

longer reaction time relative to primary alcohol. On the other hand,

according to the proposed mechanism in the next section, sec-

ondary alcohol provides more stable carbocation and thus acts in

the opposite direction and prevent excessive loss of efﬁciency due

to the effects of steric hindrance. Oxidation of some secondary

alcohol to ketone was shown in Table 7.

In order to demonstrate the effect of melamine in the catalyst 9,

the reaction of benzyl alcohol with O₂as well as H₂O₂was carried

out in the presence of 20 mg and 40 mg of compound 8 respec-

tively, under identical reaction conditions used for 9 (Table 5, entry

14a, 15a). Oxidation of alcohol in the presence of molecular oxygen

afforded 88% benzoic acid conversion for 8 h. However, the yield of

the reaction reached to 89% after 12 h. No change in selectivity was

observed. This experiment was also applied for oxidation reaction

of benzyl alcohol in the presence of H₂O₂catalyzed by 8. The re-

action afforded 90% benzaldehyde conversion for 4 h. However, a

small drop in selectivity was observed in this case (94%). The results

indicated the nanocomposite 9 is obviously superior to the com-

pound 8. Intercalation of melamine to this system provides a

robust-rigid network structure with an enhance in selectivity and

efﬁciency.

Scheme 3. Synthesis of aldehyde, ketone, oxime and imine from alcohol, by the Mn

catalyst 9.

3.4. Mechanism studies

A typical mechanism for direct preparation of oxime and imine

from alcohol catalyzed by Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex in

the presence of hydrogen peroxide have been shown in Scheme 4.

The mechanism could be generalized for other oxidants except

efﬁciency than their corresponding primary type. Nevertheless,

moderate efﬁciency was obtained for aromatic substrates. It seems

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M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

Table 3

a

Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex catalyzed one-pot preparation of imine by oxidation of alcohol in H₂O₂

.

a

Alcohol (base amount: 10.0 mmol) used in stoichiometric amount according to amine functional groups. Amine: 10.0 mmol (except 12i: 5.0 mmol). H2O2: 12a, 12b, 12e,

12f, 12g, 1.5 mL; 12c, 12d, 12l: 3.0 mL; 12j, 12k: 6.0 mL.

molecular oxygen. Various mechanisms for Mn(III)-catalyzed

oxidation of alcohol by peroxide have been reported in literature

[27,49e52]. However, in accordance with our observations (effect

of oxidant and substituent) a carbocation mechanism is plausible

for this transformation. Previously, Saffar-Teluri reported a mech-

anism for alcohol oxidation by Mn(III)-salophen, where in ﬁrst,

manganese salophen transferred to the manganese-oxo interme-

diate in the presence of NaIO₄[51]. This transformation was shown

for Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex in the presence of H₂O₂in

Scheme 4. One OAc^ꢀligand may be release from the catalyst due to

absence of Mn(V) in the medium that was conﬁrmed by XPS

analysis of the mixture, in which did not found any Mn(V) species in

the mixture (Fig. 7a) [27]. The reaction of oxo-compound (Scheme

4-b) with alcohol lead to (c) and (d) intermediates. We observed

that the reaction conversion noticeably drops with aliphatic alco-

hols. To elucidation of this matter, in a model reaction, butanal in a

same condition was oxidized and only 49% conversion was afforded

for butanal. The second reason was the effect of oxidant. Carbonyl

compounds can be formed by nucleophile attack of the oxygen of

L₃Mn(IV) (d) to hydroxyl proton of alcohol, leading to the reduction

of L₃Mn(IV) to L₃Mn(III). In this step a water molecule was elimi-

nated and the catalyst was released for the next catalytic cycle.

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

241

Table 4

Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex catalyzed one pot preparation of oxime by oxidation of alcohol in H₂O₂.

Scheme 4. The proposed mechanism for direct preparation of oxime and imine from alcohol catalyzed by Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex nanocomposite in the presence of

H₂O₂.

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M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

The proposed mechanism for the Fe₃O₄@SiO₂@Mn(III) complex-

catalyzed oxidation of alcohol by O₂is presented in Scheme 5. Se-

lective transformation of alcohol to carboxylic acid by O₂persuaded

us to investigate the reaction mechanism in the presence of mo-

lecular oxygen. In contrast to the mechanism proposed for

Fe₃O₄@SiO₂@Mn(III) complex-catalyzed oxidation of alcohol by

oxidants such as H₂O₂, NaOCl, KHSO₅, BuOOH and NaIO₄, a radical

mechanism was postulated according to our observations: (i) The

reaction did not show any progress under N₂or Ar atmosphere, (ii)

in situ XPS investigation on the reaction mixture demonstrated that

Mn(II), Mn(III) and Mn(IV) species are present in the mixture

(Fig. 7b). Mn(II) has a satellite feature (~647 eV), which is not pre-

sent for either Mn(III) or Mn(IV) [27], (iii) the reaction progress

dropped immediately after addition of hydroquinone or nar-

ingenin, as a radical scavenger (Fig. 7c), after 5 h of the reaction (GC

analysis). Also, in one another experiment, radical scavenger (hy-

droquinone or naringenin) was added in the beginning of the re-

action. It was observed that alcohol oxidation proceeds to

aldehyde/ketone and didn't see any acid product (GC analysis).

Furthermore, it was proved that aldehyde plays a role of co-

reductant in the epoxidation reactions [56] and a radical mecha-

nism was acceptable in literature [57e59], which could be

extended to this reaction. Theses evidences revealed that the re-

action path for oxidation of alcohol to aldehyde (Scheme 5, cycle A)

didn't involve any radical, and more oxidation of aldehyde to acid

go through from a radical path (Scheme 5, cycle B) that is consis-

tence with the literature [60,61]. For Mn-catalyzed aerobic oxida-

tion of alcohols, a redox mechanism was assumed [62]. As depicted

in Scheme 5, the Mn(IV) species oxidized alcohol to aldehyde. The

Mn(IV) was reduced to Mn(III), which was re-oxidized to Mn(IV) by

molecular oxygen (Scheme 5, cycle A). In the circle B, the initiation

starts with the transformation of aldehyde to the corresponding

acyl radical (RCO_) catalyzed by the L₄Mn(III) metal complex. Then,

this radical reacts with O₂producing an acylperoxy radical which

can generate another acyl radical by reacting with second aldehyde

(propagation stage), where upon it is converted to the RCO₃H

(peroxyacid). A manganese-oxo species [L₄Mn]O], as reactive

oxidation species, is assumed, which is formed after reaction be-

tween the RCO₃H (peroxyacid) and the metal complex. The cycle is

terminating in this step with formation of carboxylic acid, the

product. Finally, manganese-oxo species converted to L₄Mn(III) by

reacting with an aldehyde molecule and formed another carboxylic

acid product (Scheme 5). This conversion could also be afforded by

two equivalents of proton, where one molecule of water is formed.

The cycle continues by reformation of L₄Mn(III).

Scheme 5. The proposed mechanism for direct preparation of carboxylic acid from

alcohol catalyzed by Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite in the pres-

ence of molecular oxygen.

Carbonyl compound can be converted to oxime or imine in the

presence of hydroxyl amine hydrochloride or an imine (Scheme 4).

Due to the nature of carbocation intermediate, a dry non-proton

environment was considered necessary for this transformation to

adsorb the by-product water [53]. However, the Cozzi et al. in a

nucleophilic substitution of alcohols in water, expressed that water

on the surface phase can form hydrogen bonds with the hydroxyl

group of alcohols, and the alcohols were activated and converted to

carbocation [54,55]. So, it can be concluded that presence of water

in H₂O₂37% helps to carbocation formation and provides highest

efﬁciencies than others. Other non-proton environments also pro-

vided sufﬁcient stability for the preparation of carbocations.

In order to manifest of effect of imidazole counter ion into

oxidation, we prepared a homologue of 9 that is free coordinated

metal form (10, Table 8), then applied it for oxidation of benzyl

alcohol in the presence of an oxidant. The reaction was performed

with previously premium conditions obtained in the presence of 9.

The experiment clearly demonstrated synergetic effect of the

counter ion for oxidation of benzyl alcohol. The results were

tabulated in Table 8. The reaction did not show any progress in the

presence of molecular oxygen; demonstrating the inﬂuence of Mn

complex to oxidation as shown in Scheme 4. Oxidation occurs with

other oxidants, but with lower efﬁciencies than when using catalyst

9 for oxidation; this matter proving the synergetic effect of the

imidazolium counter ion that helps oxidation.

The mechanism was completely consistence with our observa-

tions in CV, where the coordinated Mn undergo reversible oxida-

tion in the solution, which was assigned to Mn^II/Mn^III/Mn^IVredox

couplings.

3.5. Chemoselectivity

As shown in the scheme of Table 8, it was suggested that an

exchange ion led to the formation of imidazolium-oxidant ion. The

highest and the lowest conversion was belonging to NaIO₄and

KHSO₅with 62 and 42% conversion, respectively. It seems the most

stable anion (better distribution of negative charge) gives the most

Chemoselectivity behavior of the Fe₃O₄@SiO₂@Im[Cl]Mn(III)-

complex nanocomposite was studied by oxidation of various

combinations of alcohols, in the presence of the catalyst under

optimized conditions as shown in Table 9. The catalyst was

completely selective operated toward oxidation of primary than

secondary alcohols (Table 9, entry 1). Also, high selectivity was

shown for aromatic or conjugated alcohol in the mixture of benzyl

alcohol/butyl alcohol (Table 9, entry 2) and cinnamyl alcohol/allyl

alcohol (Table 9, entry 3). It was shown that there is a serious

competition between oxidation of benzyl alcohol and cinnamyl

alcohol (Table 9, entry 4), which spread of products can be seen in

this case. This could be attributed to the facile carbocation forma-

tion for them as demonstrated in the proposed mechanism

conversion.

The

order

of

conversion

was:

NaIO₄> KHSO₅> NaClO₄ꢃ CHP > TBHP > H₂O₂(Table 8). An inter-

esting note was their selectivity. As shown in Table 8, benzaldehyde

selectivity was completely lost for all of oxidants, which clearly

demonstrating the role of the coordinated Mn on benzaldehyde

selectivity. In accordance with proposed mechanism depicted in

Scheme 4, Mn complex controlled the oxidation process and pro-

vides high selectivity toward benzaldehyde.

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

243

Table 5

Oxidation of alcohols to carbonyl compounds catalyzed by Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite^a.

a

Y: Yield, S: Selectivity.

(Scheme 4). In another experiment, we performed the oxidation

process in the mixture of benzyl alcohol, cinnamyl alcohol and 1-

phenylethanol (Table 9, entry 6), in which same results as ob-

tained with the previous combinations. It worth note that alcohol

oxidation selectively occurs without interference of amine, that

was valuable for one pot preparation of imine by alcohol oxidation.

Complete results for the catalyst selectivity was tabulated in

Table 9.

stability of the magnetic catalyst in the oxidation reactions was

studied. In this way, the catalyst 9 was recovered by applying an

external magnetic ﬁeld and reused for at least seven times without

observation of noticeable loss in its catalytic activity and selectivity

(Fig. 8). Also, amount of Mn leached into the solution for benzyl

alcohol oxidation to aldehyde and/or acid after each cycle was

measured by ICP. The amount of Mn loading after 1st cycle reached

to 0.8% and 0.3% for oxidation to benzaldehyde and benzoic acid

respectively, and after 7 consecutive recycling they were 4.3% and

3.0% (Fig. 8). Moreover, ICP analysis of the catalyst for each heavy

metal demonstrated an insigniﬁcant change in their weight per-

centage than the corresponding fresh values: Fe 45.77, Si 2.23, Mn

6.6 w%. These results demonstrated insigniﬁcant changes in the

percentages of the heavy metals and conﬁrm the durability of the

3.6. Catalyst reuse and stability

Reusability of a heterogeneous catalyst is one of the most

important properties of metal catalysts from the green chemistry

point of view, that should be investigated [63]. Reusability and

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Table 6

Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite catalyzed preparation aldehyde and carboxylic acid by oxidation of alcohol through two different approaches^a.

a

Y: Yield, S: Selectivity.

Table 7

Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite-catalyzed oxidation of secondary alcohol to ketone in H₂O₂.

catalyst during recycling.

test was performed by reaction of 4-benzyloxybenzyl alcohol-

polymer-bound, aniline and H₂O₂in the presence of Fe₃O₄@-

SiO₂@Im[Cl]Mn(III) complex (Scheme 6). As shown in Scheme 6,

the imine product did not form in this conditions, which in the end

of the reaction the catalyst was separated, and the result of hy-

drolysis treatment of the resin was 4-(hydroxymethyl)phenol. The

obtained results strongly suggest that the catalyst is truly hetero-

geneous, which the one-pot preparation of imine did not occurs in

the liquid phase.

Finally, in order to ensure about structure of the recovered

catalyst, we studied it after 7th run with some analyses and

compared them with the corresponding fresh catalyst. FTIR ana-

lyses corroborated that structure of the catalyst remains completely

intact (Fig. 9a). The recovered catalyst was responded to the

external magnetic ﬁeld and readily collected from the reaction

mixture as same as the fresh catalyst. But, a slight promotion of

magnetization was seen for the recovered than the fresh catalyst

(Fig. 9b). However, this behavior may be reasonable, because of

3.6.1. Hot ﬁltration test

In order to conﬁrm the heterogeneous nature of the catalyst and

to determine the extent of Mn leached after the reactions, we used

the hot ﬁltration test [64]. For this purpose, we studied the oxida-

tion of benzyl alcohol in the presence of Fe₃O₄@SiO₂@Im[Cl]Mn(III)

complex nanocomposite under optimized conditions. The catalyst

was magnetically removed in situ after 50% conversion (GC) and the

reactants were allowed to undergo further reaction. These results

indicated that after removal of the heterogeneous catalyst, the free

catalyst residue was weakly active and conﬁrms that the catalyst

acts heterogeneously in the reaction and only slight leaching was

take place during the reaction.

3.6.2. Three phase test

Furthermore, a three-phase test was applied for more insight

into heterogeneous nature of the nanocatalyst (Scheme 6) [64]. The

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

245

Fig. 7. (a) XPS spectra for in situ recovered catalyst 9 after 140 min of reaction progress in the oxidation reaction of benzyl alcohol to benzaldehyde in the presence of H₂O₂. (b) XPS

spectra for in situ recovered catalyst 9 after 4 h of reaction progress in the oxidation reaction of benzyl alcohol to benzoic acid in the presence of O₂. (c) Comparison between

reactions with a hydroquinone (Blue curve) or naringenin (black curve) as two radical scavengers and a control reaction without adding a radical scavenger (green curve).

Table 8

Ligand oxidation (Mn free) of benzyl alcohol in the presence of an oxidant^a.

Peroxide

H₂O₂

TBHP

NaIO₄

KHSO₅

NaClO₄

CHP

Conversion (%)^b

Selectivity (%)^b

42

68

45

75

62

50

54

79

50

55

48

78

a

Reaction Conditions: Alcohol (10.0 mmol), 10 (40 mg), oxidant (1.5 mL), room temperature, air atmosphere, 4 h.

GC.

b

leaching happening of the recovered catalyst that cause to the

in shape. Furthermore, the hydrodynamic diameter of the catalyst

was studied by DLS technique.

catalyst more affected by magnetic ﬁeld than before. EDX spectrum

of the recovered catalyst revealed the presence the elements that

conﬁrming the stability of the catalyst during the reactions. One

interesting point was intensity reduction of Cl peak. This was

completely consistence with ion exchange of imidazolium Cl^ꢀion

with oxidant causes the peak intensity decreases, proved its con-

sumption (Fig. 9c). The XPS analysis revealed that Mn species

present in the catalyst remains intact after 7th run and were in

oxidation state as same as the fresh catalyst. It was also in agree-

ment with the proposed catalytic cycles in the mechanism studies.

No other species with other oxidation states were found in the

catalyst (Fig. 9d). The SEM image of the recovered catalyst after 7th

run shows that the nanoparticles are still approximately spherical

As shown in Fig. 9e, a narrow size distribution Fe₃O₄@SiO₂@Im

[Cl]Mn(III) complex was obtained with a mean diameter of around

66 nm. Generally, the size of catalysts and leaching of metal species

will be increased after each cycle and so, the increase of the catalyst

size causes to decreases in the yield. The TEM image of the recov-

ered catalyst exhibited the morphology and homogeneity of the

particles (Fig. 9f).

Finally, the obtained results by the present catalytic methodol-

ogy were compared with some reported catalysts for direct prep-

aration of N-benzylideneaniline from benzyl alcohol and aniline.

Table 10 shows the efﬁciency and advantages of the present

method in terms of time, yield and TOF. The catalyst produced the

246

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

Table 9

desired product in much lower time and amount of manganese

(based on mol%). More importantly, the heterogeneous catalyst was

prepared from a readily available inexpensive materials and could

be easily separated from the mixture, recycled and reused for

several times (Fig. 8) with minimal of metal contamination.

Chemoselectivity behavior of the Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nano-

composite in oxidation of various types of alcohols^a,b

.

Entry

Sample

Selectivity (%)^b

Conversion (%)^c

BAd

AP

BtAd

CAd

AAd

1

2

3

4

BzA þ PE

99

96

e

0

e

4

e

94

e

-

94

87

83

65

83

65

94

BzA þ BtA

AA þ CA

e

0

e

4. Conclusion

0

BzA þ CA

68

e

32

94

e

We have introduced a general methodology for oxidation of

alcohol to imine, oxime and carbonyl compounds by a magnetically

recoverable Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite as

an efﬁcient catalyst in the presence of H₂O₂and/or O₂as an oxidant.

A broad scope of primary and secondary alcohols was efﬁciently

and selectively converted into the corresponding carbonyls, car-

boxylic acids, imines and oximes under several versatile reaction

conditions. Imidazolium counter ion can participate to oxidation

and provide a synergetic effect. But, in the metal-free homologue of

the catalyst, the selectivity is lost. Mn transition metal controls well

selectivity for the oxidation product. Mechanism studies suggest

that Mn-catalyzed oxidation of alcohol in the presence of molecular

5

PE þ CA

6

BzA þ CA þ PE

BzA þ BzAm

68

99

7^d

a

Deﬁnition: BzA: Benzyl alcohol; PE: 1-phenylethanol; BtA: Butyl alcohol; AA:

Allyl alcohol; CA: Cinnamyl alcohol; BAd: Benzaldehyde; AP: Acetophenone; BtAd:

Butanal; CAd: Cinnamaldehyde; AAd: Acryl aldehyde BzAm: benzylamine.^bReac-

tion conditions: 10.0 mmol from each alcohol, catalyst 9 (0.5 mol%), H₂O₂37%

(1.5 mL), 5 h, room temperature.

b

GC analysis.

For the major product.

c

d

Didn't found any amine oxidation products.

Fig. 8. Recycle ability of the catalyst 9 for oxidation of benzyl alcohol to benzaldehyde and benzoic acid through two different approaches.

Scheme 6. The three phase test of 4-benzyloxybenzyl alcohol-polymer bound and aniline in the presence of 9.

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

247

Fig. 9. (a) FTIR, (b) VSM, (c) EDX, (d) XPS, (e) FE-SEM/DLS (inset), and (f) TEM analyses of the fresh (green) and the recovered catalyst after 7th run (dark red) for the oxidation

reaction of benzyl alcohol to benzaldehyde catalyzed by Fe₃O₄@SiO₂@Im[Cl]Mn(III) complex. Reaction conditions: benzyl alcohol (1.0 mmol), Cat. (0.5 mol%), H₂O₂37% (1.5 mL), r.t,

4 h.

Table 10

Comparison of Fe₃O₄@SiO₂@Im[Cl]Mn(III)-complex nanocomposite with different reported methodologies used for direct synthesis of N-benzylideneaniline from benzyl

alcohol and aniline.

Entry

Catalyst/Oxidant

[O]

Conditions

Time (h)

Yield (%)^a

TOF (h^ꢀ1

)

Ref.

1

2

3

4

5

6

7

8

9

Cu(ClO₄)₂$H₂O (5 mol%)

Pd(OAc)₂(1 mol%)

Au/TiO₂(0.02 mol%)

AgeNHC complex (0.1 mol%)

MnCo₂O₄-500

O₂(Balloon)

air

O₂(atm.)

air

KOH, Toluene, 70 ^ꢁ

Neat, r.t.

C

19

72

24

12

36

12

20

12

4.5

97^b

75^c

7

1.02

1.04

14.5

76.6

e

[65]

[66]

[67]

[68]

[69]

[70]

[71]

[22]

KOCH₃(10 mol%), r.t.

BnMe₃NOH (1.1 eq), r.t.

Toluene, 80 ^ꢁC,

92^b

96^c

75

O₂

Fe(NO₃)₃$9H₂O (10 mol%)

TEMPO (10 mol%)/air

Air

Toluene, 80 ^ꢁC, KOH

0.20

8.25

2.25

e

Ru(II) (

h

⁶-p-cymene) complex (1.0 mol%)

Toluene, Cs₂CO₃, 110 ^ꢁ

C

99

Pd/AlO(OH) (2 mol%)

Pt(0.3)@TiO₂

catalyst 9 (0.5 mol%)

O

₂(atm.)

Heptane, 90 ^ꢁ

N_2,r.t., h >300 nm

Solvent free, r.t.

C

90^b

98^c

93

e

n

[15]

This work

11

H₂O₂37%

41.3

a

Isolated yield.

Determined by ¹H NMR.

GC yield.

b

c

248

M. Kazemnejadi et al. / Journal of Molecular Structure 1186 (2019) 230e249

imines from alcohols and amines with TiO₂loading Pt nanoparticles under UV

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O₂leads to carboxylic acid through a radical path; instead, a car-

bocation mechanism was plausible for H₂O₂oxidant. The properties

of the catalyst were studied by FTIR, CHN, XRD, FE-SEM, DLS, EDX,

XPS, ICP, CV, ¹H & ¹³C NMR, VSM, BETand DPV analyses. The catalyst

has an appropriate magnetism, so that it can be readily separated

from the mixture by an external magnet. The recovered catalyst

was utilized for at least 7 consecutive runs with slight reactivity

loss. The spectral investigations on the recovered catalyst revealed

its stability during the processes. The catalyst was completely se-

lective toward secondary alcohol, aliphatic alcohol and amine,

which oxidation of aromatic alcohol could be readily accomplishing

without any interference of them. Low cost, safe, heterogeneous,

recoverable, no waste products, nano dimension, high surface area,

thermal stability, chemoselectivity, good performance (mild reac-

tion conditions and high TOFs), low metal leaching, spherical in

shape, well redox behavior and versatile with a broad scope of

substrates are some of the properties and advantages of the present

catalyst, which make this methodology as a suitable, general and

efﬁcient method for oxidation purposes.

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nanoparticles catalyzed synthesis of imines from aerobic oxidation of alcohols

with amines, Catal. Lett. 147 (2017) 20e28.

Acknowledgments

The authors are grateful to the University of Birjand, Iran for

ﬁnancial support.

[26] S. Cheng, X. Ma, Y. Hu, B. Li, MnO₂/graphene oxide: a highly efﬁ cient catalyst

for imine synthesis from alcohols and amines, Appl. Organomet. Chem. 31

(2017) e3659.

[27] M. Kazemnejadi, M. Nikookar, M. Mohammadi, A. Shakeri, M. Esmaeilpour,

Melamine-Schiff base/manganese complex with denritic structure: an ef ﬁ -

cient catalyst for oxidation of alcohols and one-pot synthesis of nitriles,

J. Colloid Interface Sci. 527 (2018) 298e314.

[28] O.V. Nesterova, D.S. Nesterov, A. Krogul-Sobczak, M.F.C.G. da Silva,

A.J. Pombeiro, Synthesis, crystal structures and catalytic activity of Cu (II) and

Mn (III) Schiff base complexes: in ﬂ uence of additives on the oxidation catal-

ysis of cyclohexane and 1-phenylehanol, J. Mol. Catal. A Chem. 426 (2017)

506e515.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

https://doi.org/10.1016/j.molstruc.2019.03.008.

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Article Doi

Fe3O4@SiO2@Im[Cl]Mn(III)-complex as a highly efficient magnetically recoverable nanocatalyst for selective oxidation of alcohol to imine and oxime

DOI: 10.1016/j.molstruc.2019.03.008

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Full text of DOI:10.1016/j.molstruc.2019.03.008

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