Synthesis, characterization and immobilization of a new cobalt(II) complex on modified magnetic nanoparticles as catalyst for epoxidation of alkenes and oxidation of activated alkanes

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Synthesis, characterization and immobilization of a new cobalt (II)

complex on modified magnetic nanoparticles as catalyst for

epoxidation of alkenes and oxidation of activated alkanes

Zeinab Asgharpour^a, Faezeh Farzaneh ^a*, Alireza Abbasi^b

The [Co(2ꢀhydroxy1naphtaldehyde)₂(DMF)₂] complex 1 prepared and characterized by Xꢀray crystallography, successfully

supported on modified Fe₃O₄nanoparticles using tetraethylorthosilicate (TEOS) and (3ꢀaminopropyl)trimethoxysilane

(APTMS) and designated as Fe₃O₄@SiO₂@APTMS@complex nanocatalyst. FTꢀIR, elemental analysis, XRD, VSM and

TEM studies have been used to characterize the nanocatalyst. The catalytic activity of Fe₃O₄@SiO₂@APTMS@complex

was evaluated by the epoxidation of norbornene, cyclooctene, styrene, αꢀmethyl styrene, transꢀstilbene and limonene, with

50–100% conversions and 83–100% selectivities. Furtheremore, it was found that Fe₃O₄@SiO₂@APTMS@complex

successfully catalyzes the oxidation of activated secondary alkanes such as fluorene, adamantane, diphenyl methane, and

ethylbenzene with 40–100% conversions and 80ꢀ100% selectivities toward the corresponding products. Based on the

obtained results, the heterogeneity and reusability of the catalyst seems promising.

Introduction

On the other hand, the coordination chemistry of ligands derived

from catechols and oꢀbenzoquinones has been developed, primarily,

over the past twenty five years with interesting and surprising

results.^21ꢀ23Initial interest in these complexes was associated with

the redox activity of the quinone ligands and the potential for

forming compounds that may exist in a number of electronic states

due to the combined electrochemical activity of the cobalt ion and

one or more quinone ligands.²³

The catalytic oxidation of alkanes and alkenes has been a subject

of growing interest in the production of chemicals and fine

chemicals. New active and selective epoxidation catalysts have been

developed due to the key role of epoxides as starting materials for a

wide variety of products. On the other hand, whereas alkanes are the

most abundant and least expensive chemicals for the production of

valuable products, methods of selective oxidations are either rare or

inefficient.^1ꢀ8Transition metal complexes have successfully been

used to catalyze the oxidation of hydrocarbons under moderate

reaction conditions,^2,9ꢀ10amongst which different cobalt complexes

are of particular significance.¹¹Interestingly, cobalt compounds not

only are used as active catalysts for oxidation and epoxidation of

hydrocarbons with low cost and less environmentally damaging, but

also are used by nature as active catalysts in a variety of

metalloenzymes.¹²Cobalt salts and cobalt containing coordination

complexes are wellꢀknown catalysts for the selective oxidation of

alkanes and selective epoxidation of alkenes and quite a number of

studies have been conducted. This includes cobalt(III)

Homogeneous catalysis has been practiced for a very long time in

largeꢀscale oxidation reactions. However, these homogeneous

catalysts face the problem of separation from the reaction mixture

and reuse; they very often decompose during the catalytic reactions.

A homogeneous complex catalyst can be recovered and reused if it is

heterogenized on an insoluble support.^24ꢀ25The dispersion and

attachment of catalyticallyꢀactive metallic nanoparticles or

organometallic complexes onto polymeric or inorganic solid support

materials such as silica,²⁶inorganic layers,²⁷microporous and

mesoporous such as zeolites, MCMꢀ41 and SBA15,^28ꢀ30and

magnetic nanoparticles^31ꢀ32via nonꢀcovalent or covalent interactions

can generally produce effective heterogeneous catalysts for

numerous reactions including oxidation of alkanes and alkenes.^33ꢀ36

acetylacetonate

complexes/O₂/isobutyrylaldehyde,¹⁴cobalt (II) calix[4] pyrrole

complexe/2ꢀethylbutyraldehyde/O₂,¹⁵

Schiff base cobalt(II)

complexes/aliphatic aldehydes or βꢀketoesters,¹⁶polymer supported

cobalt(II) complex/O₂, (Nꢀhydroxylꢀ

H₂O₂,¹⁷

phthalimide)/Co(OAc)₂/O₂¹⁸and cobalt(II) phthalocyanine.^19ꢀ20

(acac)/O₂,¹³

cobalt(II)ꢀsalen In this study, a new cobalt complex with 2ꢀhydroxynaphtaldehyde

was prepared and characterized followed by immobilization on

modified iron oxide nanoparticles as heterogeneous catalyst for

oxidation reactions of alkanes and alkenes.

^aDepartment of Chemistry, Faculty of Physics

& Chemistry, Alzahra University,

Results and discussion

P.O.Box 1993891176, Vanak, Tehran, Iran ^bSchool of Chemistry, College of Science,

University of Tehran, P.O. Box 14155 6455, Tehran, Iran,

^*Corresponding author. Tel.: +98ꢀ21ꢀ88258977; fax: +98ꢀ21ꢀ+98ꢀ21ꢀ88041344;

Eꢀmail address: faezeh_farzaneh@yahoo.com, Farzaneh@alzahra.ac.ir

[Co(2ꢀhydroxy1ꢀnaphtaldehyde)₂(DMF)₂] complex 1 was prepared

from reaction of 2ꢀhydroxy1ꢀnaphtaldehyde and CoCl₂.6H₂O in the

presence of melamine and sodium acetate in ethanol and water under

reflux condition.

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Description of crystal structure of complex

The crystal structure of complex 1 with atomꢀnumbering is shown

in Fig. 1. Basic crystal data, diffraction description and details of the

structure reﬁnement are given in Table S1. Selected bond distances

and angels are presented in Table 1.

The crystal structure of 1 was satisfactorily described in the space

group P^ꢀ1with Co(II) ion in a distorted octahedral 6ꢀcoordinated

geometry. Bond angles in the octahedron vary from 88.97(9)^oto

91.03(9)^ofor cis and 180^ofor transꢀsubstituted, indicating the small

distortion degree in the octahedron.

Table 1 Selected bond lengths (Å) and angles (°) for complex 1

Bond lengths (Å)

Co(1)ꢀO(2)ⁱ

Co(1)ꢀO(2)

Co(1)ꢀO(3)ⁱ

Co(1)ꢀO(3)

Co(1)ꢀO(1)ⁱ

Co(1)ꢀO(1)

1.989 (2)

2.059 (2)

2.191 (2)

Bond angles (°)

O(2)ⁱꢀCo(1)ꢀO(2)

O(2)ⁱꢀCo(1)ꢀO(3)ⁱ

O(2)ꢀCo(1)ꢀO(3)ⁱ

O(2)ⁱꢀCo(1)ꢀO(3)

O(2)ꢀCo(1)ꢀO(3)

O(3)ⁱꢀCo(1)ꢀO(3)

O(2)ⁱꢀCo(1)ꢀO(1)ⁱ

O(2)ꢀCo(1)ꢀO(1)ⁱ

O(2)ꢀCo(1)ꢀO(1)

O(3)ⁱꢀCo(1)ꢀO(1)

Co(1)ꢀO(1)

180.0

87.71(8)

92.29(8)

87.71(8)

180.0

88.97(9)

91.03(9)

88.97(9)

O(3)ꢀ 90.02(9)

89.71(9).

180.000(1)

O(1)ⁱꢀCo(1)ꢀO(1)

Symmetry code: (i) −x+1, −y+1, −z

Fig. 1. An ORTEP view of complex 1 drawn at 50% ellipsoid

probability levels with the crystallographic numbering.

The asymmetric unit of

1 is composed of one DMF

(dimethylformamide) molecule and one 2ꢀhydroxyꢀ1ꢀnaphtaldehyde

anion. Two 2ꢀhydroxyꢀ1ꢀnaphtaldehyde ligands coordinate to Co(II)

as bidentate [O2, O3], chelating ligand via the deprotonated

carboxylic acid and phenoxy oxygen atoms, defining the equatorial

coordination plane. There is a distinct trend in Co–O bond lengths in

the complex with Co1ꢀO2 (carboxyl) < Co1ꢀO3(phenoxyl) < Co1ꢀ

O1(dimethylformamide). The complexes are held together through

hydrogen bonds (C3ꢀH3A…O3)( C3ꢀH3A: 0.9600 (Å), H3AꢀꢀꢀO3:

2.4800(Å), C3ꢀꢀꢀO3: 3.353(4) (Å), C3ꢀH3AꢀꢀꢀO3: 151.00°) making

a chain along [100] (Fig.2). The neighboring chains are then

connected by pi…pi interactions (Cg(3)…Cg(4), 4.161 Å, where

Cg(3): C4/C5/C7/C8/ C9/C10 and Cg(4): C7/C8/C11/C12/C13/C14)

stabilizing 3D framework (Fig. 3).

Fig. 2 Zigꢀzag chains of molecules of complex 1 with hydrogen

bonds drawn as dashed lines.

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_______________________________________________________ _______________________________________________________

The electronic absorption spectrum of complex 1 is shown in Figure

S1. The cobalt(II) complex 1 in DMF exhibits three bands at 266,

313 and 369 nm, and a broad band with low absorptivity at 416 nm.

The ﬁrst band observed at 266 nm can be assigned to the aromatic

ring transition π → π*. The other bands displaying at 313 and 369

are due to n → π* type electronic transitions. Broad band with low

absorptivity centred at 416 nm can be assigned to a d–d transition

from the 3d cobalt (II) metal center.³⁹

________________________________________

To evaluate the thermal stability of complex 1,

thermal

decomposition was studied (Fig. 5). The TGA curve shows two step

weight loss at 133 and 348 °C together with exothermic peaks in the

DTG curve due to the decomposition of the dimethylformamide and

organic ligand (obs.:89.5%, calc.: 89.2%) with the formation of

cobalt oxide.

Fig. 3. Packing structure of complex 1.

The FTꢀIR spectra of free 2ꢀhydroxyꢀ1ꢀnaphtaldehyde as ligand

and [Co(2ꢀhydroxy1ꢀnaphtaldehyde)₂(DMF)₂] complex 1 are shown

in Fig.4a and b, respectively. The bands appearing at 3100, 2855,

1647, 1180 and 770 cm^ꢀ1are due to the phenolic OH, CꢀH, aldehyde

C=O, phenolic CꢀO stretching and OꢀH bending, respectively

(Fig.4a). The absence of absorption bands at 3100 and 770 cm^ꢀ1due

to the stretching and bending of phenolic OH in the FTꢀIR spectrum

of complex 1 confirms the bond formation between cobalt ion and

phenolic oxygen via deprotonation. Moreover, the 10 cm^ꢀ1increase

in the wave number of band appearing at 1119 cm^ꢀ1is due to the

phenolic CꢀO absorption which also confirms the CO coordination to

the metal.³⁷The red shift observed in C=O wave number appearing

at 1641 cm^ꢀ1indicates the oxygen atom coordination to the cobalt

ion with no need to enolization.³⁸

Fig. 5. TGA and DTG curves of the complex 1.

_____________________________________________________

Characterization of Fe₃O₄@SiO₂@APTMS@complex 1

Fe₃O₄nanoparticles (MNPs) were prepared by precipitation of iron

(II) and (III) ions in basic solution under nitrogen atmosphere.⁴⁰

Subsequent coating with silica to forming Fe₃O₄@SiO₂coreꢀshell

particles followed by modification with aminopropylꢀ

trimethoxysilane (APTMS), afforded MNPs designated as

Fe₃O₄@SiO₂@APTMS. Finally, immobilization of complex

1

within Fe₃O₄@SiO₂@APTMS resulted in the formation of the

desired product designated as Fe₃O₄@SiO₂@APTMS@complex 1,

perhaps by the exchange of the immobilized amine ligand with

coordinated DMF (Scheme 1).

Fig. 4. FTꢀIR spectra of (a) 2ꢀhydroxy 1ꢀnaphtaldehyde and (b)

[Co (2ꢀhydroxy1ꢀnaphtaldehyde)₂(DMF)₂] complex 1.

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Scheme 1. Preparationsteps of Fe₃O₄@SiO₂@APTMS@complex 1.

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Fig. 6 The FTIR spectra of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, (c) Fe₃O₄@SiO₂@APTMS, (d) complex 1, (e)

Fe₃O₄@SiO₂@APTMS@complex1 before using and (f) after using as catalyst.

__________________________________________________________________________________________

As shown in Fig. 6a, the broad band with relatively low intensity enables it to disperse rapidly when the magnetic ﬁeld is

centered at 3400 and 1600 cm^ꢀ1should be attributed either to the removed.^40,42,44

surface FeꢀOH groups, or the stretching and bending vibrations of

the adsorbed water. The stretching vibration of FeꢀO also appears at

555 cm^ꢀ1. The band appearing at 1091 cm^ꢀ1due to SiꢀO stretching

demonstates the presence of SiO₂components (Fig. 6b).^40ꢀ42. After

functionalization of silica coated magnetic nanoparticles with

APTMS, new bands displaying at 2870ꢀ2920 cm^ꢀ1are due to the NꢀH

and CꢀH stretching vibrations.⁴³The FTꢀIR spectrum of catalyst 1 is

shown in Fig. 6d. Upon immobilization of cobalt complex on the

modified nanoparticles, the appearance of two peaks at 1647 and

1180 cm^ꢀ1due to the ν(C=O) and ν(CꢀO), respectively confirms the

occurrence of immobilization.

To study the magnetic properties of magnetite nanoparticles, the

hysteresis loops of magnetite and functionalized magnetite

nanoparticles at room temperature were investigated by using

vibrating sample magnetometry (VSM). Magnetization curves of

Fe₃O₄,

Fe₃O₄@SiO₂,

Fe₃O₄@SiO₂@APTMS,

Fe₃O₄@SiO₂@APTMS@complex 1before and after using as catalyst

are shown in Fig. 7aꢀc, respectively. The magnetization curve of

Fe₃O₄exhibited superparamagnetic property with saturation

magnetization of about 61 emu/g. The silica coated Fe₃O₄magnetite

nanoparticles also showed superparamagnetic behavior with

decreased saturation magnetization of about 57 emu/g. After coating

Fig. 7 Magnetization curves of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, (c)

Fe₃O₄@SiO₂@APTMS, (d)Fe₃O₄@SiO₂@APTMS@complex 1

before and (e) after using as catalyst.

of

Fe₃O₄@SiO₂

nanoparticles

with

aminosilane,

the

superparamagnetic behavior of Fe₃O₄@SiO₂@APTMS was found

to be about 56 emu/g. Immobilization complex 1 within the

Fe₃O₄@SiO₂@APTMS surface exhibited superparamagnetic

property decrease to saturation magnetization of about 51emu/g.

_____________________________________________

The Xꢀray diffraction pattern of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, (c)

Recall

that

the

superparamagnetic

property

of

Fe₃O₄@SiO₂@APTMS,

(d)

complex

1

(e)

Fe₃O₄@SiO₂@APTMS@complex

1

prevents aggregation and

Fe₃O₄@SiO₂@APTMS@complex 1 before and (f) after using as

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catalyst are shown in Fig. 8aꢀf, respectively. As shown in Fig. 9a, it

Journal Name

can be seen that the obtained Fe₃O₄has highly crystalline cubic The TEM image of Fe₃O₄@SiO₂@APTMS@complex 1 presented in

spinel structure in agree with the standard Fe₃O₄(cubic phase) XRD Fig. 10A and B before and after using it as catalyst show the core

spectrum (JCPDSNo.19ꢀ0629).^45,46The similar set of characteristic and shell of the prepared nanoparticles with the average particle size

peaks observed for Fe₃O₄@SiO₂and Fe₃O₄@SiO₂@APTMS (Fig. of 10 nm.

9b,c) indicate the stability of the crystalline phase of silica coating

and surface functionalization. The XRD pattern of complex 1 and

Fe₃O₄@SiO₂@APTMS@complex

1 are shown in Fig. 9dꢀe,

respectively. Observation of two additional peak at 2tetha 15 and

18 for Fe₃O₄@SiO₂@APTMS@complex 1 should be attributed to

the immobilization of complex 1 on modified iron oxide magnetic

nanoparticles.

Fig. 8 XRD pattern for(a) Fe₃O₄, (b) Fe₃O₄@SiO₂,

(c)Fe₃O₄@SiO₂@APTMS (d) complex 1 (e)

Fe₃O₄@SiO₂@APTMS@complex 1before using

and(f )after using as catalyst.

Fig. 10 The TEM image of Fe₃O₄@SiO₂@APTMS@complex 1(A)

before, and (B) after using it as catalyst.

_______________________________________________________

Catalytic studies

The EDX of Fe₃O₄@SiO₂@APTMS@complex 1 confirmed the

presence of cobalt complex on modified iron nanomagnet(Fig. 9).

Reaction of norbornene epoxidation with TBHP as oxidant was

selected as model to investigate the catalytic activity of

Fe₃O₄@SiO₂@APTMS@complex 1 (designated as catalyst 2) and

other reaction parameters such as catalyst amount, reaction time and

solvent. The results are presented in Figs 11–13. Excellent reaction

yield was obtained using 25 mg of catalyst in refluxing CH₃CN

within 3 h. As seen in Fig. 11, increasing the amount of catalyst

from 5 to 25 mg increases the conversion from 33 to 100% with

100% selectivity. Therefore, all epoxidation reactions were carried

out using 25 mg of catalyst. Similarly, increasing the reaction time

from 0.5 to 3 h using 25 mg of catalyst was found to increase the

norbornene conversion from 17 to 100% with 100% selectivity (Fig.

12).

Fig. 9 The EDX image of Fe₃O₄@SiO₂@APTMS@complex 1.

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Fig. 13 The effect of solvent on the conversion and selectivity of

norbornene using Fe₃O₄@SiO₂@APTMS@complex 1 as catalyst 2.

_______________________________________________________

Fig. 11. Conversion and selectivcity of norbornen with different

amounts of Fe₃O₄@SiO₂@APTMS@complex 1as catalyst 2.

_______________________________________________________

ethylbenzene were investigated under opitimized reaction conditions

Table 2 Results obtained for epoxidation of different alkenes in the

presence

of

complex

1

(catalyst

1)

and

Fe₃O₄@SiO₂@APTMS@complex 1 (catalyst 2)

Substrate conversion^a(%) Epoxide selectivity (%) (TON)^b

Entry

Catalyst 1 Catalyst 2

Catalyst 1

Catalyst 2

1

100

100 (185)

100 (654)

2

3

4

100 (185)

85^c(13)

100 (654)

100 (66)

Fig. 12 Conversion and selectivity of norbornene at

different

times in the presence of the Fe₃O₄@SiO₂@APTMS@complex 1 as

catalyst 2.

_______________________________________________________

62

100

50

The nature of solvent has an important role on the conversion and

product selectivity. In this study, both protic and aprotic solvents

were used to investigate the solvent effect on the norbornene

epoxidation. As seen in Fig. 13, protic solvents such as methanol and

ethanol were not suitable for norbornene epoxidation. These results

are similar to those previously reported for alkene epoxidation

studies.^47ꢀ49On the other hand, high conversion and selectivity

toward epoxide was achieved in aprotic solvents in the order of nꢀ

hexane < CHCl₃< CH₂Cl₂<CH₃CN. The observed trend which is

consistent with increasing order of dielectric constants clearly

indicates the increasing solvent polarity role toward high conversion

(100%) and high epoxide selectivity (100%) in norbornene

epoxidation. When epoxidation of norbornene was carried out in the

100

75^d(15)

80^e(53)

5

6

100

40

100

30

70^f(13)

35^h(98)

97^g(64)

presence

of

both

complex

1

(catalyst

1)

and

53^k(218)

Fe₃O₄@SiO₂@APTMS@complex 1 as catalyst 2, 100% conversion

and selectivity was observed within 6 h in CH₃CN. Inspired by

these results, other alkenes such as cyclooctene, styrene, αꢀmethyl

styrene, transꢀstilbene and cyclohexene and activated secondary

alkanes including fluorene, adamantane, diphenylmethane and

^aReaction conditions: catalyst: 25 mg, substrate: alkenes (10 mmol),

(transꢀstilbene, 1 mmol), alkanes (1mmol) (cyclooctane, 10 mmol),

TBHP (14 mmol), solvent (acetonitril, 5 ml), conditions (reflux at 80

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b

°C), time (1: 6 h , 2, 3 h). TON is the mmol of product to mmol of

vanadium present in catalyst. ^cAcetophenone was identified as

byproduct. ^dBenzaldehyde (10%) and benzoic acid (15%) were

identified as byproducts. ^eBenzaldehyde (7%) and benzoic acid

(13%) were identified as byproducts. ^fBenzaldehyde (10%) and

benzoic acid (20%) were identified as byproducts. ^gBenzaldehyde

(3%) was identified as byproduct. ^h2ꢀCyclohexeneꢀ1ꢀol (42%) and 2ꢀ

cyclohexeneꢀ1ꢀone (23%) were identified as byproducts. ^k2ꢀ

Cyclohexeneꢀ1ꢀol (35%) and 2ꢀcyclohexeneꢀ1ꢀone (12%) were

identified as byproducts.

_____________________________________________

To cast light onto the reaction mechanism,^50ꢀ52the norbornene

epoxidation and fluorene or adamantane oxidation reactions were

carried out in the presence of diphenylamine as a radical scavenger.

Observation of suppression of the reaction yields clearly revealed

that these processes have proceeded via a radical mechanism.^53ꢀ55

Based on the operation of a radical mechanism, one may rationalize

the reaction mechanism (Scheme 2). It seems likely that reaction

between catalyst and TBHP initially affords

a

Co^IIIꢀperoxy

______________________________________________________

compound. Subsequently, tꢀBuOO radical is released from Co^IIIꢀ

peroxy adduct and regenerating the catalyst (Scheme 2a).⁵⁶In the

next step, tꢀBuOO radical either is added to the alkene double bond

and forming tꢀbutoxperoxy I (path 1, Scheme 2a) or abstract an

active H from allylic CꢀH bond and generating cycloalkenyl radical

I' intermediate (path 1', Scheme 2a). Whereas elimination of tꢀ

BuOH from I affords the epoxide product (path 2, Scheme 2a),

reaction of radical I' with molecular O₂gives rise to

cycloalkenylperoxide radical II' (paths 2', Scheme 2a), which

subsequently generates 2ꢀcycloalkenylꢀ1ꢀol and 2ꢀcycloalkenylꢀ1ꢀ

one via a disproportionation reaction (paths 3', Scheme 2a). Since

aqueous solution of TBHP was used as oxidant, partial hydrolysis of

αꢀmethyl styrene, styrene and transꢀstilbene epoxides to the

relevant 1,2ꢀdiols perhaps catalyzed by some acidic free silanol

groups present on the support is inevitable (path 3, Scheme 2a).⁵⁷

Subsequently, the obtained 1,2ꢀdiols undergo extremely rapid

scission to acetophenone, benzaldehyde and benzaldehyde,

respectively (path 4, Scheme 2a).⁵⁸Recall that norbornene and

cyclooctene neither undergo hydrolysis since their 1,2ꢀdiol

degradations are inefficient, nor susceptible to allylic site oxidation

due to instability of the bridgehead radical intermediate of the former

(Scheme 2b)^59,60and orthogonality of the CꢀH orbital to the C=C

double bond in the latter (Scheme 2c). respectively.⁶¹Particularly

significant is cyclohexene in which either epoxidation or oxidation

take place concomitantly on C=C and CꢀH bonds, respectively.

Since radical p and C=C πꢀorbitals of cyclohexene are closely

parallel to each other, it easily undergoes allylicꢀsite oxidation,

affording cyclohexenol and cyclohexenone (Scheme 2d).

(Tables 2 and 3). As seen in Table 2, the reaction results can be

categorized in three trends. Whereas norbornene or cyclooctene

were selectively converted to the corresponding epoxides (entries 1

and 2, Table 2), epoxidation of αꢀmethyl styrene, styrene and transꢀ

stilbene proceeded to the corresponding epoxides as well as another

byꢀproduct in each case (entries 3 to 5, Table 2). On the other hand,

cyclohexene afforded cyclohexene epoxide plus 2ꢀcyclohexeneꢀ1ꢀol

and 2ꢀcyclohexeneꢀ1ꢀone (entry 5, Table 2). Interestingly, fluorene,

adamantane, diphenylmethane and ethylbenzene were oxidized to

the corresponding ketones with 40ꢀ100% conversions and 100%

selectivities (Table 3).

Table 3 Results obtained for the oxidation of different alkanes in the

presence

of

complex1

(catalyst

1)

and

Fe₃O₄@SiO₂@APTMS@complex1 (catalyst 2)

Substrate conversion^a(%)

Product selectivity (%)

Epoxide (TON)^b

Entry

Catalyst 1

Catalyst 2

Catalyst 1

Catalyst 2

1

Fluorene

90

50

40

100 (18)

100 (65)

Finally, transformation mechanism of alkanes having active

secondary CꢀH bonds such as fluorene, adamantane, diphenyl

methane, and ethylbenzene to the corresponding ketones under the

catalytic effect of catalysts 1 and 2 is suggested (Scheme 2e).⁶²

2

3

The

stability

of

the

magnetically

recovered

Adamantane

100 (18) 100 (65)

Fe₃O₄@SiO₂@APTMS@complex 1 as catalyst 2 was confirmed

with spectroscopic studies and elemental analysis. The FTIR and

XRD spectra of catalyst 2 before and after using as catalyst are

similar. Moreover, as seen in VSM spectra after using

Fe₃O₄@SiO₂@APTMS@complex 1 as catalyst 2, the saturation

magnetization was found to be 48 emu/g. Therefore, it can be

concluded that there is no significant change in its magnetic property

and the magnetic core seems to be stable during epoxidation

40

Diphenyl methane

reaction.⁶³

Fe₃O₄@SiO₂@APTMS@complex 1 determined before and after

using it as catalyst are 3.66%, 41.91% and 3.63%, 41.90%,

Cobalt

and

iron

content

of

80^c(14.4)

100(65)

100

respectively. Observation of no catalytic activity in the filtrate

when it was subjected to the epoxidation conditions in the absence of

catalyst clearly indicated that the catalyst is truly heterogeneous in

nature.

Ethylbenzene

4

100 (18) 100 (65)

70

65

^aReaction conditions: catalyst, 25 mg, substrate (alkanes, 1mmol),

TBHP (1.4 mmol), solvent(acetonitril, 5 ml, reflux at 80 °C), time

(1: 6h, 2, 3 h).

The reusability of the catalyst 2 after recovery by magnetic

decantation and washing with solvent was examined in the

epoxidation of norbornene in another run by addition of fresh

norbornene similar to the initial reaction. As indicated in Fig. 14, a

decrease in conversion from 100 to 97% without any changes in

selectivity was observed after five runs.

^bTON is the mmol of product to mmol of vanadium present in

catalyst.

^cdiphenyl methanol was identified as byproduct.

8 | J. Name., 2012, 00, 1-3

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toward the formation of corresponding products (Tables 2 and 3), the

latter with higher TON as indicated in these Tables is considerable.

Fig. 14 The effect of recycling of Fe₃O₄@SiO₂@APTMS@complex

as catalyst (2) on epoxidation of norbornene.

_______________________________________________________

Whereas the catalytic activity of homogeneous catalyst 1 exhibits

high activity in comparison to that of the heterogeneous catalyst 2

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Scheme 2. Proposed reaction mechanism for the epoxidation of alkene and oxidation of alkane with the catalyst.

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in

2 mL EtOH) into 1,3,5ꢀtriazineꢀ2,4,6ꢀtriamine (melamine)

(1mmol, 126 mg in 2mL H₂O) in the presence of sodium acetate

(2mmol, 164 mg in 4 mL H₂O) and heating at reflux condition. After

appearing a clear yellow solution, CoCl₂.6H₂O (1mmol, 238 mg in 4

mL EtOH) was added and the solution was heated at reflux for 5 h.

The yellow resultant solid was filtered, washed with water, ethanol

and diethyl ether and dried in air at room temperature. Xꢀray quality

crystals of the complex 1 was obtained by slow evaporation of

saturated resultant solid solution in dimethylformamide (82% yield).

Decomp. point.275 ˚C, Anal. Calc. for C₂₈H₂₈CoN₂O₆(M = 547.45g

mol^ꢀ1): C, 61.43; H, 5.16; Co, 10.76; N, 5.12;. Found: C, 61.39; H,

5.14; Co, 10.45; N, 5.9. UV–Vis (DMF, λmax/nm): 409, 362, 313,

266.

Conclusions

Co(II) complex was prepared with CoCl₂.6H₂O and 2ꢀhydroxyꢀ1ꢀ

naphtaldehyde in the presense of melamine designated as [Co(2ꢀ

hydroxy1ꢀnaphtaldehyde)₂(DMF)₂] Complex 1. The molecular

structure of complex 1 was determined by singleꢀcrystal Xꢀray

crystallography. The Co atom in complex 1 crystallizes in the

triclinic P^ꢀ1space group and adopts a distorted octahedral 6ꢀ

coordinated geometry. Two molecules of 2ꢀhydroxyꢀ1ꢀ

naphtaldehyde coordinate as bidentate chelating ligands in

equatorial position and two DMF molecules in axial positions are

bound to Co(II) atom. Complex 1 immobilized on the modified iron

Preparation of magnetic nanoparticles Fe₃O₄(MNPs)

oxide

Fe₃O₄@SiO₂@APTMS@Complex 1.

complex and Fe₃O₄@SiO₂@APTMS@complex

nanomagnet

was

designated

Catalytic activities of

were

as

In order to prepare

magnetite nanoparticles Fe₃O₄(MNPs), a

solution of FeCl₂.4H₂O (4mmol, 860 mg) and FeCl₃(8mmol, 1400

mg) in deionized water (40 mL) was stirred at 85°C for 30 minutes

under nitrogen gas. Upon addition of ammonium hydroxide (25 %,

3 mL), the solution color turned from light brown to black. After

stirring for 20 minutes under these conditions, the solution was

cooled to room temperature followed by filtration. The magnetite

precipitate was washed with H₂O and sodium chloride solution

(0.02M)⁴⁰.

1

investigated in the epoxidation of alkenes and oxidation of activated

secondary alkanes with TBHP as homogenous catalyst 1 and

heterogeneous catalyst 2, respectively. It was found that the

selectivity and TON number of heterogeneous system is higher than

that of homogenous one. Easy preparation, mild reaction conditions,

high yield, ease of catalyst separation and recyclability of the solid

catalyst candidates Fe₃O₄@SiO₂@APTMS@complex 1 as a useful

heterogeneous catalysis system.

Preparation of silica coated magnetite nanoparticles SCMNPs

For preparation of SCMNPs, the magnetite nanoparticles (1 g)

were dispersed in deionized water in a 250 mL roundꢀbottom ﬂask

with sonication and then an aqueous solution of TEOS (10% (v/v),

80 mL) was added, followed by glycerol (50 mL). The pH of the

suspension was adjusted to 4.5 using glacial acetic acid, and the

mixture was then stirred and heated at 90 ˚C for 2 h under a nitrogen

atmosphere. After cooling to room temperature, the silica coated

magnetite nanoparticles was separated from the reaction mixture

using a permanent magnet and washed several times with distilled

water and methanol⁴⁰.

Experimental

General remarks

All materials were commercial reagent grade without further

purification. FTꢀIR spectra were recorded on a Bruker Tensor 27 FTꢀ

IR spectrometer using KBr pellets over the range of 4,000–400 cm^ꢀ1.

Magnetic susceptibility measurements were carried out using a

vibrating sample magnetometer (VSM) (BHVꢀ55, Riken, Japan) in

the magnetic field range of ꢀ8000 to 8000 Oe at room temperature.

Cobalt and iron were determined by atomic absorption on a Chermo

double beam instrument. Transmission electron microscopy (TEM)

images were taken by ZeissꢀEM10Cꢀ100 KV. Epoxidation products

were analyzed by GC and GCꢀMass using Agilent 6890 series with a

FID detector, HPꢀ5, 5% phenylmethylsiloxane capillary and Agilent

5973 network, mass selective detector, HPꢀ5 MS 6989 network GC

preparation of aminoropropyl modiﬁed SCMNPs

(Fe3O4@SiO₂@APTMS)

The obtained SCMNPs (2 g) were suspended in ethanol (100 mL)

and then aminopropyltrimethoxysilane (APTMS) (2 mL) was added

under dry nitrogen atmosphere. The mixture was heated at reﬂux for

12 h and the resultant solid was magnetically separated, washed with

methanol to remove the unreacted residue of silylating reagent and

X-ray crystallography

then dried at 80 ˚C^40,61

.

Crystallographic data were collected on

a MAR345 dtb

diffractometer equipped with image plate detector using MoꢀKα

radiation (0.71073 Å). The structure was solved by direct methods

using SHELXSꢀ97 and refined using the fullꢀmatrix leastꢀsquares

method on F², SHELXLꢀ97⁶⁴. All nonꢀhydrogen atoms were refined

anisotropically. Hydrogen atoms were added at ideal positions and

refined using a riding model. The data were deposited in Cambridge

crystallographic data center with deposition number CCDC

1403895. Crystallographic data, details of collected data and

structure reﬁnement are listed in Table S1. Selected bond lengths

and angles are shown in Table 1.

Immobilization of [Co (2-hydroxy1-naphtaldehyde)₂(DMF)₂]

complex onto

(Fe₃O₄@SiO₂@APTMS)

To a stirring solution of complex 1 (1mmol, 546mg in 20 mL

EtOH), Fe₃O₄@SiO2 @APTMS (1 g) was added and the mixture

heated at 90 ˚C for 12 h. The solid was then magnetically separated,

washed with ethanol and dried at room temperature to afford

Fe₃O₄@SiO₂@APTMS@complex

1 as a brown solid and

designated as catalyst 2.

Preparation of [Co (2-hydroxy1-naphtaldehyde)₂(DMF)₂]

complex (1)

Catalytic oxidation, general procedure

All oxidation and epoxidation reactions were carried out in a

round bottom flask equipped with a magnetic stirrer and a waterꢀ

cooled condenser. Typically, catalyst (25 mg) and substrate (10

[Co (2ꢀhydroxy1ꢀnaphtaldehyde)₂(DMF)₂] complex (1) was

prepared from inducing 2ꢀhydroxy1ꢀnaphtaldehyde (1mmol, 172 mg

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mmol) were mixed in CH₃CN (5 mL). TBHP (14 mmol, 1.6mL) was 20 Sujandi, E. A. Prasetyanto, S.ꢀ C. Han, S.ꢀE. Park, Bull. Korean

then added and the mixture was heated at reflux for several hours.

After separation of the catalyst by means of an external magnet, the

Chem. Soc., 2006,27, 1381ꢀ1385.

filtrate was subjected to GC and GC–Mass.

21 N. Ch. Jana, S. Adak, P. Brandão, T. K. Mandal, A. Panja,

Polyhedron, 2016, 107, 48ꢀ56.

22 T. Tezgerevska, K. G. Alley, C. Boskovic, Coord. Chem. Rev.,

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Moreau, F.D. Renzo, A. Galarneau, F. Fajula, Coord. Chem.

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Acknowledgements

The ﬁnancial support from Alzahra University is gratefully

acknowledged.

25 A. Choplin, F. Quignard, Coord. Chem. Rev. 1998, 178–180,

1679ꢀ1702.

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Synthesis, characterization and immobilization of a new cobalt(II) complex on modified magnetic nanoparticles as catalyst for epoxidation of alkenes and oxidation of activated alkanes

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