Oxidovanadium(IV) complexes of tetradentate ligands encapsulated in zeolite-Y as catalysts for the oxidation of styrene, cyclohexene and methyl phenyl sulfide

FULL PAPER

DOI: 10.1002/ejic.201100625

Oxidovanadium(IV) Complexes of Tetradentate Ligands Encapsulated in

Zeolite-Y as Catalysts for the Oxidation of Styrene, Cyclohexene and Methyl

Phenyl Sulfide

Mannar R. Maurya,*^[a]Priyanka Saini,^[a]Amit Kumar,^[b]and João Costa Pessoa*^[b]

Keywords: Heterogeneous catalysis / Vanadium / Oxidation / EPR spectroscopy

The reaction of oxidovanadium(IV)-exchanged zeolite-Y

with N,NЈ-ethylenebis(pyridoxyliminato) (H₂pydx-en, I),

N,NЈ-propylenebis(pyridoxyliminato) (H₂pydx-1,3-pn, II)

and H₂pydx-1,2-pn (III) in methanol heated at reflux leads to

the formation of the corresponding complexes, abbreviated

herein as [V^IVO(pydx-en)]-Y (4), [V^IVO(pydx-1,3-pn)]-Y (5)

and [V^IVO(pydx-1,2-pn)]-Y (6) in the supercages of zeolite-

6 h of reaction time. The selectivity of the various products

was similar for the catalyst precursors 4–6 and followed the

order: benzaldehyde Ͼ 1-phenylethane-1,2-diol Ͼ benzoic

acid Ͼ phenyl acetaldehyde. With cyclohexene, a maximum

conversion of 95.9% was achieved with 4, 94.5% with 5 and

94.2% conversion with 6, also in 6 h of reaction time. The

selectivity of the various products was similar for the three

Y. The neat complexes [V^IVO(pydx-en)] (1), [V^IVO(pydx-1,3- catalysts: 2-cyclohexen-1-one Ͼ 2-cyclohexen-1-ol Ͼ cyclo-

pn)] (2) and [V^IVO(pydx-1,2-pn)] (3) were also prepared.

hexane-1,2-diol. The oxidation of methyl phenyl sulfide was

Spectroscopic studies (IR, UV/Vis and EPR), elemental analy-

achieved with 4, 5 and 6 in 2.5 h of reaction time with 85.5,

ses, thermal studies, field-emission scanning electron micro- 82.1 and 80% conversion, with higher selectivity towards

graphs (FESEM) and X-ray diffraction patterns were used to

characterize these complexes. Oxidations of styrene, cyclo-

hexene and methyl phenyl sulfide were investigated using

these complexes as catalyst precursors in the presence of

H₂O₂as oxidant. Under the optimized reaction conditions, a

maximum of 85.5% conversion of styrene was obtained with

4, 84.6% conversion with 5 and 82.9% conversion with 6 in

sulfoxide. Overall, the encapsulated catalysts were signifi-

cantly more active than their neat counterparts and have the

further advantage of being recyclable. No relevant difference

in activity was found due to a change in the diamine in the

Schiff base ligands I–III. UV/Vis and ⁵¹V NMR spectroscopic

experiments with 1 confirmed the plausible formation of

V^VO(O₂)L as intermediates in the catalytic oxidations.

enebis(salicylimine)] ligands have provided opportunities to

encapsulate various transition-metal complexes in the nano-

cavity of zeolites and to develop catalytic processes for vari-

ous reactions. Poltowicz et al. encapsulated a whole range

of salen complexes {e.g., [Fe(salen)], [Mn(salen)], [Cu-

(salen)] and [Co(salen)]} in zeolite-X and studied their cata-

lytic activity for the oxidation of cyclooctane.^[8]Ratnasamy

and co-workers used copper(II) and manganese(III) com-

plexes of salen derivatives encapsulated in the cavity of zeo-

lite-X and zeolite-Y for the oxidation of styrene under aero-

bic conditions using tert-butyl hydroperoxide as oxidant.

The catalytic efficiency of these encapsulated complexes was

much higher than that of the neat complexes. Electron-

withdrawing substituents such as Cl, Br and NO₂on the

aromatic ring were found to enhance the rate of oxi-

dation.^[9,10]These complexes also catalyze the oxidation of

phenol and p-xylene.^[11,12]Epoxidations of cyclohexene, cy-

clooctene, 1-hexene and other various types of alkenes, ar-

enes and cycloalkenes, catalyzed by different complexes of

salen-type ligands, were also carried out.^[2,13–15][Co-

(salophen)]-Y [H₂salophen = N,NЈ-bis(salicylidene)benz-

ene-1,2-diamine] and related derivatives catalyzed the oxi-

Introduction

The advantages of zeolite-encapsulated metal complexes

(ZEMC) as catalysts in the past two decades have promoted

researchers to design such catalysts and investigate their

catalytic properties.^[1–7]The relatively large size of the en-

capsulated catalysts and their rigidity make it difficult for

them to escape from the zeolite cages. Thus, zeolite-encap-

sulated homogeneous catalysts may have the advantages of

solid heterogeneous catalysts while retaining most of their

original character. The relatively easy preparation, wide

variety and flexible nature of salen-type [salen = N,NЈ-ethyl-

[a] Department of Chemistry,

Indian Institute of Technology Roorkee,

Roorkee 247667, India

Fax: +91-1332-273560

E-mail: rkmanfcy@iitr.ernet.in

[b] Centro Química Estrutural, Instituto Superior Técnico,

Technical University of Lisbon,

Av. Rovisco Pais, 1049-001 Lisboa, Portugal

Fax: +351-21-8464455

E-mail: joao.pessoa@ist.utl.pt

Supporting information for this article is available on the

WWW under http://dx.doi.org/10.1002/ejic.201100625.

4846

Eur. J. Inorg. Chem. 2011, 4846–4861

Oxidovanadium(IV) Complexes of Tetradentate Ligands

dation of β-isophorone (βIP) to ketoisophorone (KIP) un- Besides restricting the movement of complexes inside the

der ambient conditions of temperature and pressure.^[16]Li- zeolite cages, this may (or may not) induce selectivity effects

quid-phase oxidation of phenol with H₂O₂was reported distinct from those of the corresponding Schiff base com-

using Cu^IIand V^IVO complexes of N,NЈ-bis(salicylidene)- plexes derived from the condensation of salicylaldehyde

propane-1,3-diamine (H₂sal-1,3-pn) and N,NЈ-bis(salicylid- with diamine derivatives.

ene)diethylenetriamine (H₂saldien).^[17–19]Zeolite-Y-encap-

It is expected that the access of the oxidant and substrate

sulated Cu^IIand V^IVO complexes of N,NЈ-bis(salicylidene)- to the active complex inside the zeolite cages is rather re-

cyclohexane-1,2-diamine (H₂sal-dach) catalyze the oxi- stricted, and further knowledge how this may affect the

dation of styrene, cyclohexene and cyclohexane ef- overall efficiency of the reactions is required. Moreover, the

ficiently.^[20]Similar homogeneous oxidovanadium(IV) com- inside of the zeolite cages has polarity and hydrogen-bond

plexes derived from 2,2Ј-dimethylpropanediamine catalyze formation capability quite distinct from organic solvents

the oxidation, by tert-butyl hydroperoxide, of cyclooctene such as acetonitrile, and it is relevant to check if, when

and styrene.^[21]Chiral [Mn^III(salen)]-type complexes immo- using aqueous H₂O₂as oxidant, this may favour the activity

bilized on inorganic–organic hybrid materials of zirconium when comparing encapsulated and nonencapsulated cata-

poly(styrene-phenylvinylphosphonate)phosphate were also lysts.

used as effective catalysts for asymmetric epoxidation of un-

functionalized olefins.^[22,23]

Results and Discussion

We reported several chiral V-salen and V-salan (H₂salan

= reduced Schiff bases) complexes and tested them as cata-

lysts in the oxidation of styrene, cyclohexene, cumene and

methyl phenyl sulfide using H₂O₂and tBuOOH as oxidants.

Overall, the V-salan complexes showed higher activity and

normally better selectivity in alkene oxidation and higher

activity and enantioselectivity for sulfoxidation than their

parent V-salen complexes.^[24]The catalytic potential, often

with high turnover numbers, of metal complexes encapsu-

lated in the nanocavity of zeolites prompted us to design

new zeolite-Y-encapsulated oxidovanadium(IV) complexes

of tetradentate ligands I, II and III (Scheme 1). We have

Synthesis and Characterization of Complexes

The reaction between equimolar amounts of [V^IVO-

(acac)₂] and H₂pydx-en (I), H₂pydx-1,3-pn (II), or H₂pydx-

1,2-pn (III) (Scheme 1) in dry methanol heated at reflux

yielded the V^IVO complexes [V^IVO(pydx-en)] (1), [V^IVO-

(pydx-1,3-pn)] (2) and [V^IVO(pydx-1,2-pn)] (3), respectively.

The general synthetic process may be outlined by Equation

(1).

[V^IVO(acac)₂] + LH₂Ǟ [V^IVOL] + 2Hacac

(1)

also prepared the corresponding neat oxidovanadium(IV) H₂L = H₂pydx-en: 1; H₂L = H₂pydx-1,3-pn: 2; H₂L = H₂pydx-1,2-

pn: 3.

complexes of these ligands to compare their catalytic poten-

tials.

Scheme 2 presents the structural formulas and binding

sets proposed for these complexes, and are based on their

spectroscopic characterization [IR, electronic, electron

paramagnetic resonance (EPR)] and elemental analyses.

The coordination of the ligands involves their dianionic

(ONNO^2–) form.

Scheme 1. Structural formulas of ligands and abbreviations used in

this work.

Several authors state^[25]that the flexibility of the diamine

backbone of the Schiff base ligand and the stereoelectronic

effects of substituents are crucial factors that govern the

reactivity of complexes. To test the influence of change in

the diamine moiety and to further understand how this fac-

tor is affected by encapsulation of the complexes in the cavi-

Scheme 2. Structural formulas of the neat complexes prepared in

this work.

Encapsulation of [V^IVO(pydx-en)] (1), [V^IVO(pydx-1,3-

ties of zeolite-Y, three heterogenized complexes 4–6 were pn)] (2) and [V^IVO(pydx-1,2-pn)] (3) in the nanocavities of

prepared.

zeolite-Y involved as a first step the exchange of [V^IVO]²⁺

Ligands I–III include the N_pyridineatom and the with Na⁺of Na–Y in water to form zeolite-[V^IVO]-Y spe-

–CH₂OH group of the pyridoxal moiety. This anticipates cies. This is followed by the reaction of the metal-exchanged

the probable formation of hydrogen bonds of these groups zeolite-Y with I, II or III in methanol to give [V^IVO(pydx-

with surface –OH groups inside the cavities of zeolite-Y. en)]-Y (4), [V^IVO(pydx-1,3-pn)]-Y (5) and [V^IVO(pydx-1,2-

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4847

M. R. Maurya, P. Saini, A. Kumar, J. Costa Pessoa

FULL PAPER

pn)]-Y (6). The remaining uncomplexed metal ions in zeo- fraction pattern of encapsulated metal complexes, [V^IVO]–

lite were removed by ion exchange with aqueous 0.01 m Y and Na–Y are essentially similar except for a slight

NaCl solution. Extraction of impurities with methanol was change in the intensity of the bands in encapsulated com-

carried out by Soxhlet extraction, thus removing the excess plexes. These observations indicate that the framework of

amount of free ligand. As the solids were submitted to very zeolite did not undergo any significant structural change

extensive extraction, we expect that the metal content found during encapsulation (i.e., the crystallinity of the zeolite-Y

(Table 1) after encapsulation is only due to the presence of is preserved). Absence or no formation of new peaks due

V^IVO complexes inside the cavities of the zeolite-Y. The di- to the presence of complexes was detected in encapsulated

agonal dimension of the closely related complex, [Cu^II- zeolites.

(pydx-en)]^[26]is around 12 Å, which suggests that they will

fit well into the super cages of the zeolite-Y and will not

pass through the apertures of around 7.4 Å.^[27]

Table 1. Chemical composition, physical and analytical data.

Colour

Metal content

[wt.-%]

[mmolg^–1]

[VO(pydx-en)]-Y (4)

[VO(pydx-1,3-pn)]-Y (5)

[VO(pydx-1,2-pn)]-Y (6)

light green

light cream

1.78

1.85

1.53

0.35

0.36

0.29

These encapsulated complexes were additionally charac-

terized by thermogravimetric patterns, field-emission scan-

ning electron micrograph (FESEM), energy-dispersive X-

ray analysis (EDX), powder X-ray diffraction (PXRD)

studies and EPR. The binding modes for these encapsulated

complexes are proposed by comparison with the corre-

sponding homogeneous model complexes.

Figure 1. PXRD patterns of (a) Na–Y, (b) [V^IVO]-Y and (c) [V^IVO-

(pydx-en)]-Y (4).

Thermogravimetric Analysis

The thermal decomposition of [V^IVO(pydx-1,3-pn)]-Y (5)

and [V^IVO(pydx-1,2-pn)]-Y (6) proceeds in two major steps.

An endothermic weight loss of around 21% in 5 and

around 20% in 6 occurs in the temperature range 100 to

300 °C, which is possibly due to the removal of intrazeolitic

water. The second step of endothermic weight loss starts

immediately after the first one and continues till around

600 °C to constant weight. A total weight loss of approxi-

mately 6% in 5 and 7% in 6 occurs in the temperature range

of 300–600 °C due to the slow decomposition of vanadium

complexes. The first decomposition step of complex

[VO(pydx-en)]-Y (4) is similar but continues to around

350 °C with a weight loss of 25%. The second step of endo-

thermic weight loss of approximately 16% starts immedi-

ately after the first step and consists of several substeps that

continue till around 650 °C to constant weight. This infor-

mation suggests the insertion of only a small amount of

metal complexes in the cavities of the zeolite-Y. This is in

agreement with the low percentage of metal content ob-

tained for encapsulated complexes (Table 1).

FESEM and EDX Study

Figure 2 presents the field-emission scanning electron

micrograph of [V^IVO(pydx-en)]-Y (4). It is clear from the

micrograph that zeolite-entrapped vanadium complexes

have well-defined crystals, and there is no indication of the

presence of any metal ions or complexes on the surface.

EDX plots support this conclusion as no vanadium or ni-

trogen contents were noted on the spotted surfaces in plots.

The average silicon and aluminium percentage on the spot-

ted surface, as evaluated semiquantitatively, were approxi-

mately 7.5 and 2.6%, respectively. The finding of around

1.1% sodium suggests the exchange of surface free V^IVO²⁺

ions by sodium ions during the re-exchange process (see

Exp. Sect.). Only a small amount of carbon (about 18.4%)

but no nitrogen suggests the presence of a trace amount of

solvent (methanol) from which it was finally washed after

Soxhlet extraction. No morphological changes on the sur-

face upon encapsulation of the complexes in the cavities

were noticed. Very similar observations were obtained for

the other two catalysts.

PXRD Studies

IR Spectral Studies

The PXRD patterns of Na–Y, [V^IVO]–Y and encapsu-

lated V^IVO complexes were recorded at 2θ values between

A partial list of IR spectroscopic data is presented in

Table 2. Comparison of the spectra of neat complexes with

5 and 70°. The PXRD patterns of representative V^IVO²⁺

-

exchanged zeolite and the zeolite-encapsulated metal com- the respective ligand provides evidence for the coordinating

plexes along with Na–Y are presented in Figure 1. The dif- mode of ligands in complexes. Globally the intensities of

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Eur. J. Inorg. Chem. 2011, 4846–4861

Oxidovanadium(IV) Complexes of Tetradentate Ligands

Figure 2. Field-emission scanning electron micrograph of [V^IVO(pydx-en)]-Y and the corresponding energy-dispersive X-ray analysis plot.

the peaks due to encapsulated complexes are weak because tensity at around 380 nm in neat complexes is due to both

of their low amount in zeolite matrix and the spectra of the (i) the ligand-to-metal charge-transfer (phenolate O to d or-

encapsulated as well as neat complexes showed essentially bitals of vanadium) band, and (ii) the nǞπ* transition of

similar bands. Ligands as well as complexes exhibit several the C=N moiety. The bands at around 275 nm and around

multiple bands of medium intensity that cover the region 235 nm may be assigned to πǞπ* and φǞφ* transitions,

2850–3000 cm^–1due to C–H bands. A medium-intensity

band around 3000 cm^–1in ligands, mostly due to intramo-

lecular ν(OH)_phenolichydrogen bonding, was not found in

the spectra of encapsulated as well as neat complexes, thus

indicating the destruction of the hydrogen-bond network

followed by the coordination of the phenol oxygen atom

after deprotonation. A sharp band that appeared at

1630 cm^–1(in I), at 1634 cm^–1(in II) and at 1631 cm^–1(in

III), due to the ν(C=N) (azomethine) shifts in complexes,

thereby suggesting the coordination of the azomethine ni-

trogen atom. All neat V^IVO complexes show a sharp band

between 973–986 cm^–1due to ν(V=O).^[28]This band was

not observed in encapsulated complexes, probably due to

poor loading of complexes in the zeolite matrix.

Table 3. Electronic spectroscopic data of ligands: neat and in com-

plexes.

Solvent λ^[a][nm]

H₂pydx-en (I)

MeOH 336, 254, 215

MeOH 418, 335, 253, 214

MeOH 336, 253, 211

MeOH 766, 591, 380, 263b, 234

DMSO 750b, 585, 385, 281b, 262

MeOH 787, 594, 383, 312b, 274b, 239

DMSO 750b, 590, 385, 280b, 262

MeOH 383, 311b, 274b, 239

DMSO 774, 538

H₂pydx-1,3-pn (II)

H₂pydx-1,2-pn (III)

[V^IVO(pydx-en)] (1)

[V^IVO(pydx-1,2-pn)] (3)

[V^IVO(pydx-1,3-pn)] (2)

[V^IVO(pydx-en)]-Y (4)

nujol

393, 344, 295, 221

415, 340, 273, 228

415, 288, 223

[V^IVO(pydx-1,3-pn)]-Y (5)

[V^IVO(pydx-1,2-pn)]-Y (6)

Table 2. IR spectroscopic data of ligand, neat and encapsulated

complexes.

[a] b indicates broad band.

ν(C=N) [cm^–1] ν(V=O) [cm^–1]

H₂pydx-en (I)

1630

1634

1631

1618

1625

1627

1619

1633

1630

–

984

983

973

–

H₂pydx-1,3-pn (II)

H₂pydx-1,2-pn (III)

[V^IVO(pydx-en)] (1)

[V^IVO(pydx-1,3-pn)] (2)

[V^IVO(pydx-1,2-pn)] (3)

[V^IVO(pydx-en)]-Y (4)

[V^IVO(pydx-1,3-pn)]-Y (5)

[V^IVO(pydx-1,2-pn)]-Y (6)

–

Electronic Spectral Studies

The electronic spectroscopic data of ligands and com-

plexes are presented in Table 3. The spectral profiles of neat

complexes are reproduced in Figure S1 (see the Supporting

Information), where those of encapsulated ones are in Fig-

ure 3. The electronic absorption spectra of [V^IVO(pydx-en)]

Figure 3. Electronic spectra of (a) [V^IVO(pydx-en)]-Y (4), (b) [V^IV-

O(pydx-1,3-pn)]-Y (5) and (c) [V^IVO(pydx-1,2-pn)]-Y (6) recorded

was described previously.^[29]The broad band of medium in- dispersed in Nujol.

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M. R. Maurya, P. Saini, A. Kumar, J. Costa Pessoa

FULL PAPER

respectively. These are sometimes designated by B band and widths due to incomplete rotational averaging of the g and

K band, respectively.^[30]In addition, two broad bands at A tensors.^[32]These broadened spectra, when compared

around 750 and 580 nm (in DMSO) appear in the visible with those of other V^IV-heterogenized complexes^[1]indicate

region and are assigned to band I (d_xyǞd_xz, d_yz) and band that the encapsulated V^IVcomplexes closely fit the zeolite

II (d_xyǞd_x₂_–y₂). All these bands show slight shifts in meth- cages; additionally, the compounds probably have their

anol.^[31]

movement restricted by hydrogen-bond formation between

Due to poor loading of complexes in the nanocavities of OH groups of zeolite and N_pyridineand/or –CH₂OH groups

zeolite-Y, and their intrinsic low intensity, the bands due of the pyridoxal moiety.

The EPR spectra were simulated^[33]and the obtained

spin Hamiltonian parameters g_zand A_zagree well with the

values estimated using the additivity relationship proposed

by Wüthrich^[34]and Chasteen^[32]with an estimated accu-

racy of Ϯ3ϫ10^–4cm^–1. The spin Hamiltonian parameters

obtained by simulation of the experimental EPR spectra are

included in Table 4, but for the donor groups under con-

sideration their predicted contributions to the parallel hy-

to d–d transitions are not observed in 4–6. However, all

encapsulated complexes exhibit three bands in the UV re-

gion and the band at around 400 nm, thereby suggesting the

presence of the V^IVO complexes in the cavities of zeolite-Y.

EPR Studies

The EPR spectra of “frozen” (77 K) solutions of 1, 2 and perfine coupling constant are rather similar {for O_phenolate

3 in MeOH or DMSO are depicted in Figure 4, and those ≈ 38.9ϫ10^–4cm^–1; for N_imine39Ϯ2ϫ10^–4cm^–1}.^[33–38]

of solids 4, 5 and 6 at room temperature in Figure 5. The Globally the hyperfine features of the spectra of 1–3 are

spectra of 1–3 are well resolved, whereas the spectra of 4– consistent with a binding set that involves the (O_phenolate

,

6 show broadening and changes in the peak-to-peak line _imine, N_imine, O_{phenolate equatorial}

N

)

.

Figure 4. First-derivative EPR spectra of frozen (77 K) solutions of (a) [V^IVO(pydx-en)] (1) (4 mm) in MeOH; (b) [V^IVO(pydx-1,2-pn)]

(3) (4 mm) in MeOH; and (c) [V^IVO(pydx-1,3-pn)] (2) (4 mm) in DMSO.

Figure 5. First-derivative EPR spectra at room temperature of zeolite-encapsulated complexes (solids) (a) [V^IVO]-Y; (b) [V^IVO(pydx-en)]-

Y (4); (c) [V^IVO(pydx-1,2-pn)]-Y (6); (d) [V^IVO(pydx-1,3-pn)]-Y (5).

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Oxidovanadium(IV) Complexes of Tetradentate Ligands

Table 4. Spin Hamiltonian parameters obtained^[25]by simulation of Catalytic Activity Studies

the experimental EPR spectra.

Oxidation of Styrene

Complex

Species

g_z

A_z

A_x, A_y

g_x, g_y

[ϫ10⁴cm^–1] [ϫ10⁴cm^–1

]

To test the catalytic potential of the complexes prepared,

the oxidation of styrene was chosen as one of the model

reactions. Thus, the oxidation of styrene catalyzed by 4–6

was carried out using aqueous 30% H₂O₂as an oxidant. It

mainly gave styrene oxide, benzaldehyde, 1-phenylethane-

1,2-diol, benzoic acid and phenylacetaldehyde along with

only minor amounts of other unidentified products, as

shown in Scheme 3.

(1) 4 mm solution

in MeOH

(4) solid

1.957

158.5

55.1

1.979

first species^[a]

≈1.96

≈157

≈159

156.6

≈55

≈60

48.3

≈1.976

≈1.975

1.985

second species^[a]≈1.96

(2) 4 mm solution

in DMSO

(5) solid

(3) 4 mm solution first species

in MeOH

1.956

≈ .95

1.959

≈166

161.7

≈59

48

≈1.981

≈1.978

second species

1.960

≈1.95

157.9

≈166

50

≈60

≈1.979

(6) solid

[a] Apparently, there are two species, but the corresponding A and

g parameters given are only rough approximations of their correct

values.

Hydrated zeolite-[V^IVO]-Y was previously reported and

its EPR spectrum recorded.^[39]It was concluded that the

V^IVO²⁺ions are largely located on type-III sites in the large

cavities bound to hydroxy or O atoms, the spin Hamiltonian

parameters being as follows: g_z= 1.938 and A_z

=

178ϫ10^–4cm^–1. The EPR spectrum of the prepared zeolite-

[V^IVO]-Y (Figure 5) shows rather broad lines, thus the spin

Hamiltonian parameters cannot be determined accurately.

They are g_z≈ 1.939 and A_z≈ 176ϫ10^–4cm^–1, which is in

good agreement with the data previously reported.^[39]

As mentioned above, encapsulation of 1–3 in zeolite-Y

involved the reaction of zeolite-[V^IVO]-Y with I, II or III

in methanol, followed by careful and exhaustive extraction

procedures to wash out any nonencapsulated vanadium

species. Therefore we expect that all V^IVin compounds 4–

6 corresponds to encapsulated complexes and no significant

amount of complexes is bound to the external surface of

the zeolite. The change of the EPR spectrum after introduc-

tion of a ligand and its similarity with the corresponding

neat complex is thus good proof of the encapsulation of the

V^IVO(ligand) species inside the pores of zeolite-Y.^[1]

Scheme 3. Main products obtained upon the catalytic oxidation of

styrene by the zeolite-encapsulated compounds 4–6.

These products have also been observed when the

polymer-supported

complex

PS-[VO(sal-ohyba)(dmf)]

(H₂sal-ohyba = Schiff base derived from salicylaldehyde

and o-hydroxybenzylamine; PS = polystyrene) was applied

as catalyst.^[40]These products of styrene oxidation were pre-

viously observed by others as well.^[41–45]

In the present work, our objective was to achieve suitable

reaction conditions for the maximum oxidation of styrene.

Thus, [V^IVO(pydx-en)]-Y (4) was taken as a representative

catalyst and different parameters, namely, the amount of

oxidant (mol of H₂O₂per mol of styrene), catalyst and tem-

perature of the reaction mixture were tested.

For three different molar ratios of styrene to aqueous

The EPR spectra recorded for 4, 5 and 6 at 77 K also 30% H₂O₂(e.g., 1:1, 1:2 and 1:3), the amount of styrene

show relatively broad lines but differ from the spectrum of (1.04 g, 10 mmol) and catalyst (0.010 g) were taken in

zeolite-[V^IVO]-Y used as precursor (Figure 5), thereby con- CH₃CN (5 mL), and the reaction was carried out at 80 °C.

firming the change in environment of the V^IVO centre. As The formation of products was regularly analyzed at similar

mentioned above, it is also probable that, besides the bind- time intervals. As illustrated in Figure 6, increasing the

ing to the ligands, hydrogen-bond interactions are estab- H₂O₂/styrene ratio from 1:1 to 2:1 improved the conversion

lished between the zeolite–OH groups and the –CH₂OH from around 60 to 86%. The oxidation improved only mar-

and N_pyridinemoieties of the ligands. Equatorial or axial ginally upon further increasing this ratio to 1:3, which sug-

binding of zeolite–OH groups to V^IVO is also possible and gested that a higher amount of oxidant does not improve

may explain the relatively high A_zvalues obtained for 5 and the oxidation of styrene, a 1:2 ratio being considered ade-

6. Notwithstanding, globally the EPR results for 4–6 con- quate.

firm the presence of the V^IVO complexes inside the nanop-

Similarly, for three different amounts (that is, 0.005,

ores of zeolite-Y and are consistent with their formulation 0.010 and 0.015 g) of catalyst for the fixed amount of styr-

as [V^IVO(pydx-en)]-Y (4), [V^IVO(pydx-1,3-pn)]-Y (5) and ene (1.04 g, 10 mmol), H₂O₂(2.28 g, 20 mmol), CH₃CN

[V^IVO(pydx-1,2-pn)]-Y (6), each with

a

binding set (5 mL) and temperature (80 °C), 0.005 g of catalyst gave

(O_phenolate, N_imine, N_imine, O_{phenolate equatorial}.

)

only 64.4% conversion (Figure 7). A maximum of 85.5%

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M. R. Maurya, P. Saini, A. Kumar, J. Costa Pessoa

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the net amount of oxidant is the same, the H₂O₂concentra-

tion is also higher when the volume of solvent added is

lower.

Figure 6. Effect of the amount of H₂O₂on the oxidation of styrene

at 80 °C as a function of time. Reaction conditions: styrene (1.04 g,

10 mmol), [V^IVO(pydx-en)]-Y (0.010 g), acetonitrile (5 mL).

Figure 8. Effect of the amount of solvent on the oxidation of sty-

rene at 80 °C as a function of time. Reaction conditions: styrene

(1.04 g, 10 mmol), H₂O₂(2.27 g, 20 mmol), [V^IVO(pydx-en)]-Y

(0.010 g).

conversion was achieved upon increasing the amount to

0.010 g, whereas further increments of catalyst amount re-

sulted in lower conversion. Therefore, an amount of 0.010 g

of catalyst may be considered adequate to obtain the maxi-

mum conversion of styrene. The temperature of the reaction

mixture also influenced the performance of the catalyst;

running the reaction at 80 °C gave much better conversion

than at lower temperatures and also reduced the time re-

quired to achieve the maximum conversion.

Thus, for the maximum oxidation of 10 mmol of styrene,

the following conditions were considered an adequate bal-

ance: catalyst (0.010 g, around 3.5 μmol of V complex),

H₂O₂(2.27 g, 20 mmol), CH₃CN (5 mL) and reaction tem-

perature (80 °C). The other catalyst precursors, [V^IVO-

(pydx-1,3-pn)]-Y and [V^IVO(pydx-1,2-pn)]-Y, were also

tested under the above optimized reaction conditions (Fig-

ure S2 in the Supporting Information), and the results are

presented in Table 5. It is clear from the data that all cata-

lysts give comparable conversion and selectivity. This obser-

vation confirms that chain length of the amine residue does

not have a relevant influence on the performance of the

catalyst. Probably the critical issue in the present encapsu-

lated catalysts 4–6 is the mass transfer of reagents and/or

products in and out of the zeolite cages, and the differences

in the calculated time of flight (TOF) values between 4, 5

and 6 do not really represent relevant differences in their

catalytic activities, but instead result from the distinct

amount of complex present inside the zeolite cavities in 4,

5 and 6, which leads to distinct calculated TOF values.

The conversion of styrene and the selectivity of different

reaction products using [V^IVO(pydx-en)]-Y as catalyst un-

der the optimized reaction conditions have been analyzed

as a function of time and are presented in Figure 9. It is

clear from the plot that a selectivity of 75.5% of benzalde-

Figure 7. Effect of the amount of catalyst on the oxidation of sty-

rene at 80 °C as a function of time. Reaction conditions: styrene

(1.04 g, 10 mmol), H₂O₂(2.27 g, 20 mmol) in acetonitrile (5 mL).

Interestingly 5 mL of acetonitrile was found to be suf- hyde has been obtained at a conversion of 7.5% of styrene

ficient to effect maximum oxidation of styrene, with signifi- in 0.5 h of reaction time. This selectivity slightly improves

cantly lower conversions being observed with 10 or 15 mL in the first two hours, then slowly decreases with time and

(Figure 8). This is probably due to a much higher concen- reaches 74.7% with the increase of the conversion of styrene

tration gradient of reagents between the outside and inside to 85.5% in 6 h. With 24.5% at a reaction time of 0.5 h, the

of the zeolite cavities when the volume of solvent is lower, selectivity of styrene oxide decreases considerably with the

thus emphasizing the important role of mass transfer in re- conversion of styrene and reaches 3.9%. The formation of

actions that involve catalysts encapsulated in zeolite-Y. As the other three products (i.e., benzoic acid, 1-phenylethane-

4852

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Oxidovanadium(IV) Complexes of Tetradentate Ligands

Table 5. Percentage conversion of styrene, product selectivity and turn over frequency (TOF).

Catalyst

Conv.

[%]

TOF

Selectivity^[c][%]

[h^–1]^[b]

so

bza

phed

bzac

phaa

other

[V^IVO(pydx-en)]-Y (4)

[V^IVO(pydx-en)]-Y^[a]

[V^IVO(pydx-1,3-pn)]-Y (5)

[V^IVO(pydx-1,3-pn)]-Y^[a]

[V^IVO(pydx-1,2-pn)]-Y (6)

[V^IVO(pydx-1,2-pn)]-Y^[a]

[V^IVO]-Y

85.5

83.4

84.6

83.2

82.9

81.7

4.8

65.3

62.7

63.2

490^[d]

–

3.9

3.7

4.1

3.5

3.8

3.3

–

2.3

2.6

1.8

74.7

72.7

74.1

73.2

76.5

75.4

75.0

74.8

73.8

74.5

9.4

9.1

9.3

8.6

8.2

8.0

–

4.1

3.4

3.6

6.8

7.2

6.1

5.6

5.8

3.4

5.0

2.9

6.4

2.0

2.7

–

3.4

4.8

6.7

1.8

2.3

3.5

3.7

3.6

3.9

–

4.8

5.2

3.7

466^[d]

–

553^[d]

–

7.7

–

25.0

10.6

10.2

9.6

[V^IVO(pydx-en)] (1)

369^[e]

366^[e]

369^[e]

[V^IVO(pydx-1,2-pn)] (3)

[V^IVO(pydx-1,3-pn)] (2)

[a] First cycle of used catalyst after recycling. [b] TOF values calculated at 5 h of reaction time. [c] so: styrene oxide, bza: benzaldehyde,

phed: 1-phenylethane-1,2-diol, bzac: benzoic acid, phaa: phenyl acetaldehyde. [d] The different TOF values mostly reflect the distinct

amount of V^IVpresent in 0.010 g of catalysts 4–6 (see Table 1): ≈3.5 μmol (4), ≈3.6 μmol (5) and ≈2.9 μmol (6). [e] The amounts of

catalysts in 0.0015 g of catalysts are ≈3.5 μmol (1), ≈3.4 μmol (2) and ≈3.4 μmol (3).

1,2-diol and phenyl acetaldehyde) starts only after approxi- whereas others follow a similar order of selectivity to that

mately 2 h, and their overall formation is relatively low even observed for encapsulated complexes. A blank reaction un-

after 6 h of the reaction time.

der the above reaction conditions gave around 3% conver-

sion. Thus, neat as well as encapsulated complexes show

good performance, but the catalytic activity is significantly

higher in the case of the encapsulated complexes 4–6. In

addition to the easier hydrolysis of the neat complexes 1–3

in the presence of aqueous H₂O₂relative to their encapsu-

lated counterparts, it is possible that these may also form

oligomeric inactive species.^[31]Therefore, besides their

higher activity, the recyclable nature, higher stability and

no observed leaching of encapsulated complexes make them

significantly better over the neat ones.

Aspects related to the mechanism of reaction are dis-

cussed below. Benzaldehyde is clearly the favoured product.

It may result from the further oxidation of styrene oxide

formed in the first step by a nucleophilic attack of H₂O₂

on styrene oxide followed by cleavage of the intermediate

hydroperoxystyrene.^[1]The formation of benzaldehyde may

also result from direct oxidative cleavage of the styrene side-

chain double bond through a radical mechanism. The high

amount of water present in H₂O₂is partly responsible for

the possible hydrolysis of styrene oxide to 1-phenylethane-

1,2-diol (phed). It is plausible that relatively high amounts

of H₂O adsorb onto the zeolite surface, namely, inside the

pores, which would explain why phed forms in higher

amounts in the case of the zeolite-encapsulated catalysts 4–

6 relative to 1–3. Other products (e.g., benzoic acid forma-

tion through further oxidation of benzaldehyde) only form

in relatively low amounts in these reactions, although signif-

icantly higher in the case of reactions that involve neat com-

plexes 1–3 as catalysts. Similarly, the formation of phenylac-

etaldehyde through isomerization of styrene oxide is quite

low in all cases.

Figure 9. Conversion of styrene and variation in the selectivity of

different reaction products as a function of time using [V^IVO(pydx-

en)]-Y as catalyst: (a) conversion of styrene (filled square), (b) selec-

tivity of benzaldehyde (filled triangle), (c) selectivity of 1-phenyl-

ethane-1,2-diol (filled upside-down triangle), (d) selectivity of

benzoic acid (filled diamond), (e) selectivity of styrene oxide (filled

circle), (f) selectivity of other products (filled triangle to left).

It is clear from Table 5 that the selectivity of the different

oxidation products formed varies in the order: benzalde-

hyde ϾϾ 1-phenylethane-1,2-diol Ͼ benzoic acid Ͼ styrene

oxide Ͼ phenylacetaldehyde. Upon recycling, the overall re-

sults are very similar: the activity decreases slightly and the

selectivity shows small changes, with an increase in the for-

mation of phenylacetaldehyde and of the minor unidenti-

fied products.

Neat complexes 1–3 are also active and exhibit 62.7–

65.3% conversion (Table 5) with rather similar selectivity.

However, the selectivity of the products differs between en-

capsulated and nonencapsulated catalysts. Indeed, the selec-

Oxidation of Cyclohexene

The oxidation of cyclohexene was also studied using

tivity for the most relevant product, benzaldehyde, is ap- these catalysts. The reaction gave mainly four different

proximately the same, but the relative amount of benzoic products, that is, 2-cyclohexen-1-ol, 2-cyclohexen-1-one and

acid is higher and that of 1-phenylethane-1,2-diol is lower, cyclohexane-1,2-diol, as shown in Scheme 4. The formation

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M. R. Maurya, P. Saini, A. Kumar, J. Costa Pessoa

FULL PAPER

of the allylic oxidation products 2-cyclohexen-1-one and 2-

As illustrated in Figure 11, under the above optimized

cyclohexen-1-ol reflects the preferential attack of the acti- reaction conditions {i.e., cyclohexene (0.82 g, 10 mmol),

vated C–H bond over the C=C bond.^[46]

[V^IVO(pydx-en)]-Y (0.005 g) and H₂O₂(2.27 g, 20 mmol) in

5 mL of acetonitrile} are adequate to effect 95.9% conver-

sion of cyclohexene at 80 °C. Remarkably, increasing the

solvent volume decreases this conversion drastically.

Scheme 4. Oxidation products of cyclohexene.

The different parameters (e.g., amount of oxidant, cata-

lyst, solvent and temperature of the reaction mixture) were

optimized for the maximum oxidation of cyclohexene, again

considering [V^IVO(pydx-en)]-Y (4) as a representative cata-

lyst. Three different molar ratios of cyclohexene to aqueous

30% H₂O₂(e.g., 1:1, 1:2 and 1:3) were considered while

keeping fixed the amounts of cyclohexene (0.082 g,

10 mmol), catalyst precursor (0.005 g), acetonitrile (5 mL)

and temperature at 80 °C. The percent conversion obtained

as a function of time is presented in Figure 10. A maximum

of 90.4% conversion was achieved at an cyclohexene-to-

H₂O₂molar ratio of 1:1. Increasing this ratio to 1:2 in-

creased this conversion to 95.9%, whereas 1:3 molar ratios

gave 96.9% conversion (Figure 10). As the 1:3 molar ratio

of substract/oxidant did not show significant change in the

conversion of cyclohexene, the cyclohexene-to-H₂O₂molar

ratio of 1:2 was considered to be adequate.

Figure 11. Effect of the amount of solvent on the oxidation of

cyclohexene at 80 °C as a function of time. Reaction conditions:

cyclohexene (0.82 g, 10 mmol), H₂O₂(2.27 g, 20 mmol) and [V^IVO-

(pydx-en)]-Y (0.005 g).

Thus, under the optimized conditions [i.e., cyclohexene

(0.82 g, 10 mmol), H₂O₂(2.27 g, 20 mmol), catalyst

(0.005 g) CH₃CN (5 mL) and temperature (80 °C)], the per-

formance of catalyst precursors 5 and 6 and of 1–3 was

also studied. The conversions after 4–6 h of reaction time

(Figure S4 in the Supporting Information) along with the

product selectivity after 6 h are presented in Table 6. The

activity of 2 is higher than that of 1 and 3, but this trend is

not preserved in their encapsulated counterparts.

Again the catalytic activity of the encapsulated com-

plexes is significantly higher than their neat counterparts

due to more extensive hydrolysis and/or formation of inac-

tive oligomeric species in the presence of aqueous H₂O₂. It

is also probable in the present reaction that mass transfer

in, out and within the zeolite pores determines the kinetics

of the whole catalytic process and that the TOF values for

4–6 do not particularly reflect the different catalytic activi-

ties between 4, 5 and 6; instead they mostly result from the

different amount of catalyst inside the pores of 4–6. There-

fore we conclude that the catalytic performance of all en-

capsulated catalysts is comparable for the oxidation of cy-

Figure 10. Effect of the amount of H₂O₂on the oxidation of cyclo-

hexene at 80 °C as a function of time. Reaction conditions: cyclo-

hexene (0.820 g, 10 mmol), [V^IVO(pydx-en)]-Y (0.005 g) and aceto-

nitrile (5 mL).

Similarly, for cyclohexene (0.082 g, 10 mmol), 30% H₂O₂clohexene. The selectivity of the products follows the order:

(2.27 g, 20 mmol) and acetonitrile (5 mL), three different 2-cyclohexen-1-one Ͼ 2-cyclohexen-1-ol Ͼ cyclohexane-

amounts of catalyst (e.g., 0.003, 0.005 and 0.0075 g) were 1,2-diol Ͼ cyclohexene oxide. After approximately 4 h, the

considered and the reactions were monitored at 80 °C. The conversion has almost attained its maximum and only in-

results obtained as a function of time are presented in Fig- creases marginally between 4 and 6 h of reaction time. Neat

ure S3 in the Supporting Information; they indicate that complexes exhibit much lower conversion along with lower

0.005 g of catalyst amount shows a maximum conversion turn over frequency (Table 6). Here, the selectivity of dif-

of 95.9%, whereas 0.003 and 0.0075 g of catalyst gave lower ferent products also differs and follows the order: 2-cy-

conversions (around 76 and 80%, respectively) in 5–6 h of clohexen-1-ol ϾϾ 2-cyclohexen-1-one Ͼ cyclohexene oxide

reaction time.

Ͼ cyclohexane-1,2-diol.

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Eur. J. Inorg. Chem. 2011, 4846–4861

Oxidovanadium(IV) Complexes of Tetradentate Ligands

Table 6. Conversion of cyclohexene and selectivity of various oxidation products after 6 h of reaction time.

Catalyst

Conv.

[%]

TOF

[h^–1]

Selectivity^[b][%]

c

a

b

d

others

[V^IVO(pydx-en)]-Y (4)

[V^IVO(pydx-en)]-Y^[a]

95.9

94.2

92.6

90.4

89.2

85.8

6.6

28.1

32.2

23.1

1374^[c]

–

1.6

2.8

2.5

3.0

2.8

3.0

49.4

9.4

5.9

10.9

32.9

51.6

43.9

45.4

42.9

47.8

10.0

66.8

57.3

59.1

50.8

43.0

46.7

35.1

50.2

35.0

25.0

21.0

32.7

24.2

9.7

1.6

3.9

13.4

3.3

10.9

15.6

2.8

4.9

1.0

2.9

3.1

0.1

3.3

–

[V^IVO(pydx-1,3-pn)]-Y (5)

[V^IVO(pydx-1,3-pn)]-Y^[a]

[V^IVO(pydx-1,2-pn)]-Y (6)

[V^IVO(pydx-1,2-pn)]-Y^[a]

Without catalyst

1276^[c]

–

1487^[c]

–

[V^IVO(pydx-en)] (1)

595^[d]

704^[d]

505^[d]

[V^IVO(pydx-1,3-pn)] (2)

[V^IVO(pydx-1,2-pn)] (3)

4.2

5.7

[a] First cycle of used catalyst after recycling. [b] (a) cyclohexene oxide, (b) 2-cyclohexen-1-ol, (c) 2-cyclohexen-1-one and (d) cyclohexane-

1,2-diol. [c] The different TOF values mainly reflect the distinct amount of V^IVpresent in 0.005 g of catalysts 4–6 (see Table 1): ca.

1.75 μmol (4), ca. 1.8 μmol (5) and ca. 1.45 μmol (6). [d] The amounts of catalysts in 0.0010 g of catalysts are ca. 2.4 μmol (1), ca. 2.3 μmol

(2) and ca. 2.3 μmol (3).

The conversion of cyclohexene and the selectivity of dif- methyl phenyl sulfide was tested with the precursor cata-

ferent reaction products using [VO(pydx-en)]-Y (4) as cata- lysts (i.e., V^IVO complexes encapsulated in the nanocavities

lyst under the optimized reaction conditions have been ana- of zeolite-Y). Oxidation of methyl phenyl sulfide in the

lyzed as a function of time and are presented in Figure 12. presence of H₂O₂gave two products: methyl phenyl sulfox-

The formation of 2-cyclohexen-1-ol started with a selectiv- ide and methyl phenyl sulfone, as shown in Scheme 5.

ity of approximately 60% which slowly decreased with time

and reached around 33.6% at the end of 6 h of reaction.

Similarly, the selectivity of cyclohexene oxide started with

17.3% and reached 1.6%, whereas 2-cyclohexen-1-one

started with around 36% selectivity and improved to

around 54.6%. The formation of cyclohexane-1,2-diol was

Scheme 5. Oxidation of organic sulfides.

initially nearly zero and increased up to 9.8% after 6 h. The

other catalysts gave similar selectivity trends.

Among the three encapsulated complexes studied, [V^IVO-

(pydx-en)]-Y (4) was chosen again as a representative one

and the reaction conditions were optimized by considering

the amount of catalyst precursor, oxidant and solvent to

obtain maximum conversion of thioanisole. The effect of

H₂O₂was studied by considering substrate-to-oxidant ra-

tios of 1:1, 1:2 and 1:3 for the fixed amount of catalyst

(0.010 g) and methyl phenyl sulfide (1.242 g, 10 mmol) in

acetonitrile (5 mL) and the reaction was monitored at room

temperature (Figure S5 in the Supporting Information). At

an aqueous H₂O₂/substrate ratio of 2:1, a maximum of

85.5% conversion of methyl phenyl sulfide was achieved in

2.5 h of reaction time. Decreasing this ratio to 1:1 decreased

the conversion of methyl phenyl sulfide considerably,

whereas increasing this ratio to 3:1 improved the conversion

but not much (Figure S5), and it facilitated the reaction to

achieve equilibrium in a shorter time. However, an aqueous

H₂O₂/substrate ratio of 2:1 may be considered adequate to

Figure 12. Conversion of cyclohexene and variation in the selectiv-

ity of the different reaction products as a function of time using

[V^IVO(pydx-en)]-Y (4) as catalyst: (a) conversion of cyclohexene

(filled square), (b) selectivity of cyclohexen-1-one (filled inverted obtain maximum conversion of methyl phenyl sulfide.

triangle), (c) selectivity of cyclohexen-1-ol (filled triangle), (d) selec-

Similarly, three different amounts of catalyst precursors,

tivity of cyclohexane-1,2-diol (filled diamond) (e) selectivity of cy-

that is, 0.005, 0.010 and 0.015 g, were taken while keeping

clohexene oxide (filled circle) and (f) selectivity of other products

methyl phenyl sulfide (1.242 g, 10 mmol) and 30% H₂O₂

(filled triangle to left).

(2.27 g, 20 mmol) in acetonitrile (5 mL) to check the effect

of amount of catalyst on the reaction. As may be seen in

Figure S6 in the Supporting Information, increasing its

Oxidation of Methyl Phenyl Sulfide (Thioanisole)

Several vanadium-dependent haloperoxidases catalyze amount from 0.005 g to 0.010 g increased the conversion

sulfoxidations^[47–49]in which the electron-rich sulfur atom from 82.4 to 85.5%, whereas 0.015 g of catalyst gave a mar-

of the sulfide undergoes electrophilic oxidation by H₂O₂to ginal increase to 87.0% conversion. Thus, 0.010 g of cata-

give the sulfoxide and, further, sulfone. Such oxidation of lyst precursor is adequate to obtain an optimum methyl

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M. R. Maurya, P. Saini, A. Kumar, J. Costa Pessoa

FULL PAPER

phenyl sulfide conversion of 85.5% in around 2.5 h of con- Table 7. Conversion percentage of methyl phenyl sulfide in 2.5 h

and selectivity of sulfoxide and sulfone.

tact time.

In this reaction, the volume of solvent again has a re-

markable role in the oxidation of methyl phenyl sulfide. As

may be seen in Figure 13, the use of 5 mL of acetonitrile

Catalyst

Conv.

[%]

TOF

[h^–1]

Selectivity [%]

sulfoxide sulfone

[V^IVO(pydx-en)]-Y (4)

85.5

83.5

82.1

80.4

80.0

77.8

≈3

980^[c]

–

79.6

75.7

75.2

73.2

73.6

73.4

≈3

20.4

24.3

24.8

26.8

26.4

26.6

≈0

gave much better conversion under above optimized reac- [V^IVO(pydx-en)]-Y^[a]

[V^IVO(pydx-1,3-pn)]-Y (5)

905^[c]

–

tion conditions of catalyst and oxidant than 10 or 15 mL.

[V^IVO(pydx-1,3-pn)]-Y^[a]

This may possibly be due to a much higher concentration

[V^IVO(pydx-1,2-pn)]-Y (6)

1067^[c]

–

gradient of reagents between the outside and inside of the

zeolite cavities when the volume of solvent is lower.

[V^IVO(pydx-1,2-pn)]-Y^[a]

No solid^[b]

–

906

884

916

Na–Y^[b]

≈3

≈0

[V^IVO(pydx-en)] (1)

[V^IVO(pydx-1,2-pn)] (3)

[V^IVO(pydx-1,3-pn)] (2)

80.3

75.8

78.5

82.5

80.8

81.3

17.5

19.2

18.7

[a] First cycle of used catalyst. [b] 10 mL of acetonitrile. [c] The dif-

ferent TOF values mainly reflect the distinct amount of V^IVpresent

in 0.005 g of catalysts 4–6 (see Table 1): ca. 3.5 μmol (4), ca.

3.6 μmol (5) and ca. 2.9 μmol (6). [d] The amounts of catalysts in

0.0015 g of catalysts are ca. 3.5 μmol (1), ca. 3.4 μmol (2) and ca.

3.4 μmol (3).

Figure 13. Effect of volume of solvent on the oxidation of methyl

phenyl sulfide at room temperature as a function of time. Reaction

conditions: methyl phenyl sulfide (1.242 g, 10 mmol), H₂O₂(2.2 g,

20 mmol) and [V^IVO(pydx-en)]-Y (0.010 g).

Therefore, from these experiments, the more adequate re-

action conditions for the maximum oxidation of methyl

phenyl sulfide at room temperature are considered to be:

catalyst (0.010 g), methyl phenyl sulfide (1.242 g, 10 mmol),

H₂O₂(2.27 g, 20 mmol) and acetonitrile (5 mL). The other

catalyst precursors 5 and 6 were also tested under the above

reaction conditions. The corresponding results are summa-

rized in Figure S7 in the Supporting Information and selec-

tivity details are presented in Table 7.

Figure 14. Conversion of methyl phenyl sulfide and variation in the

selectivity of the reaction products as a function of time using 4 as

catalyst: (a) conversion of methyl phenyl sulfide (filled circle), (b)

selectivity of methyl phenyl sulfoxide (filled square) and (c) selectiv-

ity of methyl phenyl sulfone (filled triangle).

From Table 7, it is clear that 80.0–85.5% of methyl

phenyl sulfide is converted into products using all catalysts

Considering the same molar concentration of neat cata-

for which the percentage conversion varied in the order: lyst precursors [V^IVO(pydx-en)] (1), [V^IVO(pydx-1,3-pn)] (2)

[V^IVO(pydx-en)]-Y (85.5%)

Ͼ

[V^IVO(pydx-1,3-pn)]-Y and [V^IVO(pydx-1,2-pn)] (3) under reaction conditions ap-

(82.1%) Ͼ [V^IVO(pydx-1,2-pn)]-Y (80.0%). The selectivity proximately equivalent to those above [i.e., catalyst (0.001 g

of the major product (i.e., sulfone) is similar but varies in for all), methyl phenyl sulfide (1.242 g, 10 mmol), H₂O₂

the order: [V^IVO(pydx-en)]-Y (79.6%) Ͼ [V^IVO(pydx-1,3- (2.27 g, 20 mmol) and acetonitrile (5 mL)], 75.8–80.3%

pn)]-Y (75.2%) Ͼ [V^IVO(pydx-1,2-pn)]-Y (73.6%).

conversion of methyl phenyl sulfide was achieved after 2.5 h

The conversion of methyl phenyl sulfide and the selectiv- for which selectivity of sulfoxide was around 82%, which is

ity of different reaction products using 4 as catalyst under slightly higher than obtained for the encapsulated com-

the optimized reaction conditions were obtained as a func- plexes. It is possible that the local polar environment inside

tion of time and are presented in Figure 14. It is clear from the zeolite pores, which favours hydrogen-bond formation

the plot that conversion of methyl phenyl sulfide starts with and adsorption of H₂O and H₂O₂, may explain the higher

the good selectivity (around 90%) for the formation of degree of oxidation of the sulfide in the case of catalysts 4–

methyl phenyl sulfoxide, but this is slowly converted into 6. Globally both neat as well as encapsulated complexes

the sulfoxide. The initial selectivity of methyl phenyl sulfone may be considered as good catalysts for the oxidation of

is around 10% and reaches around 20% after around 2.5 h. the organic sulfide; however, the stability and recyclable na-

Very similar trends were also obtained with the catalyst pre- ture of zeolite-Y-encapsulated complexes make them better

cursors 5 and 6.

over neat ones.

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Oxidovanadium(IV) Complexes of Tetradentate Ligands

Tests of Recyclability and Heterogeneity of the Zeolite-Y-

Encapsulated Catalytic Reactions

sulfides to sulfoxides and sulfones as well as the epoxidation

of alkenes and allylic alcohols.^[50]

The reactivity of [V^IVO(pydx-en)] with H₂O₂in DMSO

was monitored by electronic absorption spectroscopy. The

observed spectral changes are presented in Figure 15. Thus,

the progressive addition of a dilute H₂O₂solution in

DMSO to a solution of [V^IVO(pydx-en)] in DMSO results

in the flattening of bands that appear at 776 and 650 nm.

The intensity of the band at around 385 nm slowly de-

creases with a slight shift towards lower wavelength and

stabilizes with the formation of a band at around 380 nm,

whereas the intensity of other bands that appear in the UV

region (at around 260 and around 280 nm) increases. These

changes indicate the interaction of [V^IVO(pydx-en)] with

H₂O₂in methanol, thus forming [V^VO(O)₂(pydx-en)], for

The recyclability of encapsulated complexes was exam-

ined by considering the oxidation of styrene. After contact

for 6 h, the reaction mixture was filtered, and the solid was

washed with acetonitrile and dried at approximately 120 °C.

It was subjected to further catalytic reaction under similar

conditions. As may be seen in Table 5, not much loss in

catalytic activity was observed, thus indicating that most of

the complex is still present in the cavity of the zeolite-Y. In

another experiment, after approximately 6 h of reaction

time, the filtrate collected after separating the used catalyst

was placed into the reaction flask and the reaction was con-

tinued for another 4 h after adding fresh oxidant. The gas

chromatographic analysis showed no significant improve-

ment in conversion, and this confirms that the reaction did

not proceed upon removal of the solid catalyst. The oxi-

dation of styrene is therefore heterogeneous in nature, and

no leaching of the active complex occurred during the cata-

lytic cycle. Similar experiments were done for the oxidation

of cyclohexene (Table 6) and methyl phenyl sulfide

(Table 7). The recycle experiments for cyclohexene show not

much loss in catalytic activity, whereas in the case of methyl

phenyl sulfide it gave around 2% less conversion than the

original catalyst.

Reactivity of Complexes and Possible Catalytic Reaction

Pathway

It is known that several V^VO₂L compounds react with

H₂O₂to give V^VO(O₂)L, and it is normally assumed that

the corresponding hydroperoxido complex is the active cat-

alyst that mediates oxygenations, including the oxidation of

Figure 16. ⁵¹V NMR spectra (a) of a solution of [V^IVO(pydx-en)]

(1) (ca. 4 mm) in methanol; the pH is around 7.5; (b) solution of

(a) after 1 h; (c) addition of 0.5 equiv. of aqueous 30% H₂O₂(total)

to the solution of (b); (d) after addition of 1.0 equiv. of 30% H₂O₂

(total) to the solution of (c); (e) after addition of 2.0 equiv. of 30%

H₂O₂(total) to the solution of (d) the pH is around 6.5; (f) after

addition of 3.0 equiv. (30%) H₂O₂(total) to the solution of (e) the

pH is around 6.5; (g) addition of methyl phenyl sulfide (10 equiv.)

to the solution of (f) after 3h at room temperature.

Figure 15. UV/Vis spectral changes observed during titration of

[V^IVO(pydx-en)] (1) with H₂O₂. The spectra were recorded upon

stepwise additions of five drops portions of H₂O₂[1.178 g

(10.39 mmol) of 30% H₂O₂dissolved in 5 mL of DMSO] to 10 mL

of 1.6ϫ10^–4m solution in DMSO. The inset shows similar spectra

recorded with around 7.55ϫ10^–3m solution of 1 in DMSO.

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FULL PAPER

Scheme 6. Tentative assignments of V^Vspecies formed in methanolic solutions of [V^IVO(pydx-en)] (1), upon addition of aqueous H₂O₂

solution, and upon addition of methyl phenyl sulfide. The tetradentate ligand is schematically represented as O–N–N–O for most of the

V^IVor V^Vspecies presented. ⁵¹V NMR spectroscopic assignments are partly based on results reported for other systems^[24,40]upon

addition of aqueous H₂O₂.

which bands at around 360–450 nm are expected to appear V^Vand V^IV-L complexes are the only V-containing species

(see ref.^[40]and references cited therein). The disappearance detected, thus indicating that the integrity of the ligand is

of d–d bands is in accordance with the oxidation of the preserved or may be restored after the oxidant is consumed.

V^IVO to V^Vcomplexes.

Thus, recyclability of the catalyst precursors mainly de-

Similar features were observed with [V^IVO(pydx-1,2-pn)] pends on the more or less easy separation of the complex

upon similar treatment (Figure S8 in the Supporting Infor- from the reaction mixture that contains the products.

mation).

We recently reported the catalytic oxidation of styrene by

The ⁵¹V NMR spectra of [V^IVO(pydx-en)] (1) were re- [V^VO₂(pydx-aebmz)] (7) and [V^VO₂(pydx-ambmz)] (8)

corded to establish their behaviour in solution and their (Scheme 7; aebmz = 2-aminoethylbenzimidazole, ambmz =

speciation upon modifying the composition of the solutions 2-aminomethylbenzimidazole),^[40]as well as with two cata-

(see Figure 16). A fresh solution of [V^IVO(pydx-en)] (1) lyst precursors similar to 4 in which the diamines were 1,2-

(around 4 mm) in methanol does not show any ⁵¹V NMR diaminocyclohexane (chen) and 1,2-diphenylethylenedi-

spectroscopic peak. But after approximately 15 min, reso- amine (dpen) instead of ethylenediamine: [V^IVO(pydx-

nances at δ = –543 and –549 ppm build up, which we tenta- chen)] (9) and [V^IVO(pydx-dpen)] (10).^[24b]The reaction

tively assign to species CI and CII (see Scheme 6). Upon gave identical products with similar selectivity, although for

successive additions of portions of a 30% H₂O₂aqueous the present systems the relative amounts of benzaldehyde

solution, the relative intensity of the δ = –543 and –549 ppm are significantly higher than in the case of 7 and 8 (around

peaks decrease and several new peaks build up. Namely, 55% versus approximately 75% in this study) and lower

after addition of around 0.5 equiv. of H₂O₂, a peak at δ = than in the case of 9 and 10 (around 85% and 92%, respec-

–578 ppm emerges; it corresponds to the monoperoxido tively^[24b]). Benzaldehyde is the product of the C–C bond

CIII, and upon further additions (1.0, 2.0 and 3.0 equiv.) of cleavage of styrene, which usually forms under free-radical

the 30% H₂O₂solution, new peaks appear at δ = –603 conditions.^[50]This may be considered an indication that in

(CIV) and –644 (CIV), which increase intensity at the ex- the present systems the mechanism follows a similar path-

pense of the resonances of CI and CII. Upon addition of way.

methyl phenyl sulfide (MPS), after around 3 h the peaks at

δ = –644 and –578 ppm that correspond to the peroxide

complexes CIII and CIV disappear and the spectrum (Fig-

ure 16g) shows the resonances at δ = –543 and –549 ppm

that correspond to CI and CII as the main peaks.

These results indicate that during the catalytic reactions

by peroxide, intermediate peroxido species indeed form.

Upon addition of methyl phenyl sulfide, the oxidant is con-

sumed and the catalytic precursors [V^VO₂(pydx-en)(S)_n] (n

Scheme 7.

For the oxidation of hydrocarbons with transition-metal

= 0, 1) are restored. The EPR spectra measured (Figure S9 complexes as catalysts in the presence of H₂O₂, two general

in the Supporting Information) indicate that this solution groups of mechanisms are usually considered: the radical

also contains [V^IVO(pydx-en)] (1).

and nonradical ones, both of which involve metal–peroxido

Thus, overall these UV/Vis and ⁵¹V NMR spectroscopic intermediates. The radical mechanism is based on the for-

experiments show that upon addition of aqueous H₂O₂, mation of the free HOO^·and HO^·radicals,^[51–53]the latter

V^VO(O)₂L species form, thereby supporting the possibility directly oxidizing hydrocarbons to give alkylperoxides,

of hydroperoxide complexes being the active catalyst. After which then undergo decomposition to the final oxidation

addition of the substrate (e.g., methyl phenyl sulfide) both products. Among the nonradical pathways, the Sharpless

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Oxidovanadium(IV) Complexes of Tetradentate Ligands

mechanism is usually considered to be the most favourable

one for the epoxidation of olefins.^[54,55]

The higher activity observed for the encapsulated com-

plexes 4–6 and the selectivity trend of obtaining products

The radical and nonradical (Sharpless-type) mechanism with a higher degree of oxidation when using the encapsu-

of epoxidation were recently studied by DFT methods for lated complexes, relative to the corresponding neat catalysts,

a model complex of 7 (ligand IV) and ethene as substrate. may result from favourable adsorption of water and H₂O₂

The radical pathway was found to be more favourable than to the polar surface of the zeolite cages, thus making the

the non-radical route. It is indeed possible that in the pres- oxidant more easily available for reaction with the sub-

ent systems the radical route might also be more favourable strates.

than the nonradical (Sharpless) pathway.

The catalytic reactions probably proceed through the for-

mation of [V^VO(O)₂(Schiff base)] intermediates, followed by

the corresponding hydroperoxido complexes. At least in the

case of styrene, by comparison with a previous mechanistic

study with a distinct system^[40]it is suggested that a radical

Conclusion

Oxidovanadium(IV)-exchanged zeolite-Y complexes pathway is preferred for the epoxidation step, this being

[V^IVO(pydx-en)]-Y (4), [V^IVO(pydx-1,3-pn)]-Y (5) and compatible with the preferred formation of benzaldehyde.

[V^IVO(pydx-1,2-pn)]-Y (6) were synthesized and charac-

terized by IR, UV/Vis and EPR spectroscopy, elemental

Experimental Section

analyses, thermal studies, FESEM and XRD. The corre-

sponding neat complexes [V^IVO(pydx-en)] (1), [V^IVO(pydx-

1,3-pn)] (2) and [V^IVO(pydx-1,2-pn)] (3) were also prepared.

The encapsulated complexes 4–6 efficiently catalyze the

oxidation of styrene by H₂O₂and with higher conversions

than the corresponding neat complexes 1–3 or the related

neat complexes [V^IVO(pydx-chen)] (9) and [V^IVO(pydx-

dpen)] (10),^[24b]benzaldehyde being the main product

formed (around 75% under the “optimized” conditions).

The encapsulated complexes 4–6 are also efficient catalysts

for the oxidation of cyclohexene by H₂O₂, and under the

“optimized” conditions, 2-cyclohexen-1-one (around 45%)

and 2-cyclohexen-1-ol (around 40%) are the main products

formed, followed by cyclohexane-1,2-diol. The encapsulated

complexes 4–6 again show significantly higher conversions

than the corresponding neat complexes 1–3 and the related

neat complexes [V^IVO(pydx-chen)] (9) and [V^IVO(pydx-

dpen)] (10), as well as distinct selectivity. Neat complexes 9

and 10 gave mainly cyclohexane-1,2-diol (around 70–75%),

followed by 2-cyclohexen-1-ol (around 14–18%) and cyclo-

Materials and Methods: Analytical reagent-grade oxidovanadi-

um(IV) sulfate (Loba Chemie, India), 30% aqueous H₂O₂(Qualig-

ens, India), pyridoxal hydrochloride (Himedia, India), methyl

phenyl sulfide (Alfa Aeaser, U.S.A.), styrene (Acros Organics,

U.S.A.), 1,2-diaminoethane, 1,2-diaminopropane and 1,3-diami-

nopropane (S.D. fine chemicals, India) were used as received. Zeo-

lite-Y (Si/Al ≈ 10) was obtained from Indian Oil Corporation (R&

D), Faridabad, India. All other chemicals and solvents used were

of AR grade. Ligands H₂pydx-en (I), H₂pydx-1,3-pn (II) and

H₂pydx-1,2-pn (III) were prepared as described in the literature.^[29]

Instrumentation and Characterization Procedures: Elemental analy-

ses of the ligands and complexes were obtained with an Elementar

Vario-EL-III instrument. Vanadium was analyzed using inductively

coupled plasma (ICP; Labtam 8440 plasma lab) after leaching the

metal ions with very dilute aqueous KOH solution to a specific

volume in a volumetric flask. Electronic spectra of encapsulated

complexes were recorded in Nujol with a Shimadzu 1601 UV/Vis

spectrophotometer by layering the mull of the sample inside of one

of the cuvettes while keeping the other one layered with Nujol as

reference. Spectra of neat complexes were recorded in methanol.

hexane-1,2-diol (around 4–9%).^[24b]The neat complexes 1– IR spectra were recorded as KBr pellets with a Nicolet NEXUS

Aligent 1100 series FTIR spectrometer after grinding the sample

with KBr. Thermogravimetric analyses of the complexes were car-

ried out with a Perkin–Elmer (Pyris Diamond) instrument in air

with a heating rate of 10 °Cmin^–1. Scanning electron micrographs

(SEMs) of encapsulated catalysts were recorded with a Leo instru-

ment model 435VP; the samples were dusted on alumina and

coated with a thin film of gold to prevent surface changing and to

protect the surface material from thermal damage by the electron

beam. In all analysis a uniform thickness of about 0.1 mm was

maintained. Energy-dispersive X-ray analysis (EDX) was obtained

3 gave higher selectivity for 2-cyclohexen-1-ol and lower for

2-cyclohexen-1-one.

The encapsulated complexes 4–6 catalyze the oxidation

by H₂O₂of methyl phenyl sulfide efficiently. Under the con-

ditions studied, the encapsulated complexes gave conver-

sions of approximately 80–85%, with formation of the cor-

responding sulfoxide (around 75%) and sulfone (around

25%).

According to our data for the oxidation of styrene, cyclo-

hexene and thioanisole with catalysts precursors 1–3 or 4– with an FEI Quanta 200 FEG microscope. X-ray powder dif-

fractograms of solid catalysts were recorded with a Bruker AXS

D8 Avance X-ray powder diffractometer with a Cu-K_αtarget. EPR

spectra were recorded with a Bruker ESP 300E X-band spectrome-

ter. The spin Hamiltonian parameters were obtained by simulation

of the spectra with the computer program of Rockenbauer and

Korecz.^{[33] 51}V NMR spectra were obtained with a Bruker Avance

III 400 MHz spectrometer with the common parameter settings.

NMR spectra were recorded in CD₃OD, and δ(⁵¹V) values are ref-

erenced relative to neat V^VOCl₃as external standard. A thermo-

electron gas chromatograph with an HP-1 capillary column (HP-1,

30 mϫ0.25 mmϫ0.25 μm) was used to analyze the reaction prod-

6, no clear trend of different catalytic reactivity was found

upon the change of the diamine of the Schiff base ligand.

Additionally, for the heterogenized catalysts it appears that

mass-transfer phenomena, namely, the restricted access of

the oxidant and substrate to the active complex, and poss-

ibly also diffusion of products out of the zeolite, are crucial

factors for the overall efficiency of the reactions. In particu-

lar, the effect of temperature is significant, probably mostly

due to faster mass transfer at 80 °C than at lower tempera-

ture, and not because of thermodynamic/kinetic reasons.

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M. R. Maurya, P. Saini, A. Kumar, J. Costa Pessoa

FULL PAPER

ucts. The identity of the products was confirmed by GC–MS with

a Perkin–Elmer Clarus 500 instrument.

Oxidation of Cyclohexene: Aqueous 30% H₂O₂(2.27 g, 20 mmol),

cyclohexene (0.82 g, 10 mmol) and catalyst (0.005 g) were mixed in

CH₃CN (5 mL) and the reaction mixture was heated at 80 °C with

continuous stirring in an oil bath for approximately 6 h. For the

neat catalysts the amount used was 0.0010 g.

[V^IVO(pydx-en)] (1): A solution of H₂pydx-en (1.792 g, 5 mmol) in

methanol (50 mL) was treated with [V^IVO(acac)₂] (1.33 g, 5 mmol)

dissolved in methanol (10 mL), and the reaction mixture was

heated at reflux in an oil bath for 4 h. During this period a green

solid of [V^IVO(pydx-en)] slowly separated out. After cooling the

mixture, it was filtered off, washed with methanol and dried. Yield

78.1%. C₁₈H₂₀N₄O₅V (423.32): calcd. C 51.07, H 4.76, N 13.24;

found C 49.8, H 4.8, N 13.4.

Oxidation of Methyl Phenyl Sulfide: Methyl phenyl sulfide (1.24 g,

10 mmol), aqueous 30% H₂O₂(2.27 g, 20 mmol) and catalyst

(0.010 g) were dissolved in CH₃CN (5 mL) and the reaction mix-

ture was stirred at room temperature for approximately 2.5 h. For

the neat catalysts the amount used was 0.0010 g.

[V^IVO(pydx-1,3-pn)] (2) and [V^IVO(pydx-1,2-pn)] (3): These com-

plexes were prepared similarly using the respective ligand.

Acknowledgments

[V^IVO(pydx-1,3-pn)] (2): Yield 88.3%. C₁₉H₂₂N₄O₅V (437.35):

M. R. M. and P. S. thank the Department of Science and Technol-

ogy, Government of India, New Delhi for financial support of the

work. J. C. P. and A. K. thank FEDER, Fundação para a Ciência

e Tecnologia, PEst-OE/QUI/UI0100/2011 and grant SFRH/BPD/

34835/2007.

calcd. C 52.18, H 5.07, N 12.81; found C 51.5, H 5.3, N 12.2.

[V^IVO(pydx-1,2-pn)] (3): Yield 65.4%. C₁₉H₂₂N₄O₅V (437.35):

calcd. C 52.18, H 5.07, N 12.81; found C 51.8, H 5.0, N 12.7.

[V^IVO]-Y [oxidovanadium(IV)-exchanged zeolite-Y]: A filtered solu-

tion of V^IVOSO₄·5H₂O (9 g, 36 mmol) dissolved in distilled water

(100 mL) was added to a suspension of Na–Y (15 g) in distilled

H₂O (800 mL), and the reaction mixture was heated at 90 °C with

stirring for 24 h. The light-green solid was filtered, washed with hot

distilled water until the filtrate was free from any V^IVO²⁺ion con-

tent and dried at 150 °C for 24 h. Yield: 14.9 g (99%). %V (ICP-

MS): 4.6.

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For the neat catalysts the amount used was 0.0015 g.

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Received: June 21, 2011

Published Online: September 19, 2011

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www.eurjic.org

4861

Article Doi

Oxidovanadium(IV) complexes of tetradentate ligands encapsulated in zeolite-Y as catalysts for the oxidation of styrene, cyclohexene and methyl phenyl sulfide

DOI: 10.1002/ejic.201100625

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