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

Article abstract of DOI:10.1002/ejic.201100625

The reaction of oxidovanadium(IV)-exchanged zeolite-Y with N,N′-ethylenebis(pyridoxyliminato) (H₂pydx-en, I), N,N′-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-Y. The 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. Spectroscopic studies (IR, UV/Vis and EPR), elemental analyses, thermal studies, field-emission scanning electron micrographs (FESEM) and X-ray diffraction patterns were used to characterize these complexes. Oxidations of styrene, cyclohexene and methyl phenyl sulfide were investigated using these complexes as catalyst precursors in the presence of H₂O₂ as oxidant. Under the optimized reaction conditions, a maximum of 85.5a% conversion of styrene was obtained with 4, 84.6% conversion with 5 and 82.9% conversion with 6 in 6 h of reaction time. The selectivity of the various products was similar for the catalyst precursors 4-6 and followed the order: benzaldehyde > 1-phenylethane-1,2-diol > benzoic acid > 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 catalysts: 2-cyclohexen-1-one > 2-cyclohexen-1-ol > cyclohexane-1,2-diol. The oxidation of methyl phenyl sulfide was achieved with 4, 5 and 6 in 2.5 h of reaction time with 85.5, 82.1 and 80a conversion, with higher selectivity towards sulfoxide. Overall, the encapsulated catalysts were significantly 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. Complexes [V^IVO(pydx-en)] (1), [V^IVO(pydx-1,3- pn)] (2) and [V^IVO(pydx-1,2-pn)] (3) are reported as well as their encapsulation in the cavity of zeolite-Y and their catalytic activity for the oxidation of styrene, cyclohexene and methyl phenyl sulfide.

Full text of DOI:10.1002/ejic.201100625

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 [VIVO(pydx-en)]-Y (4), [VIVO(pydx-1,3-pn)]-Y (5)
and [VIVO(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 [VIVO(pydx-en)] (1), [VIVO(pydx-1,3- catalysts: 2-cyclohexen-1-one Ͼ 2-cyclohexen-1-ol Ͼ cyclo-
pn)] (2) and [VIVO(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
H2O2 as 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
VVO(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 NO2 on 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
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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 H2O2 was reported distinct from those of the corresponding Schiff base com-
using CuII and VIVO 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 CuII and VIVO 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 [MnIII(salen)]-type complexes immo- using aqueous H2O2 as 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 H2O2 and 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 [VIVO-
(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 VIVO complexes [VIVO(pydx-en)] (1), [VIVO-
(pydx-1,3-pn)] (2) and [VIVO(pydx-1,2-pn)] (3), respectively.
The general synthetic process may be outlined by Equation
(1).
[VIVO(acac)2] + LH2 Ǟ [VIVOL] + 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
(ONNO2–) 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 [VIVO(pydx-en)] (1), [VIVO(pydx-1,3-
ties of zeolite-Y, three heterogenized complexes 4–6 were pn)] (2) and [VIVO(pydx-1,2-pn)] (3) in the nanocavities of
prepared.
zeolite-Y involved as a first step the exchange of [VIVO]2+
Ligands I–III include the Npyridine atom and the with Na+ of Na–Y in water to form zeolite-[VIVO]-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 [VIVO(pydx-
with surface –OH groups inside the cavities of zeolite-Y. en)]-Y (4), [VIVO(pydx-1,3-pn)]-Y (5) and [VIVO(pydx-1,2-
Eur. J. Inorg. Chem. 2011, 4846–4861
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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, [VIVO]–
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
VIVO 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, [CuII- 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 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) [VIVO]-Y and (c) [VIVO-
(pydx-en)]-Y (4).
Thermogravimetric Analysis
The thermal decomposition of [VIVO(pydx-1,3-pn)]-Y (5)
and [VIVO(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 [VIVO(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 VIVO2+
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, [VIVO]–Y and encapsu-
lated VIVO 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 VIVO2+
-
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
4848
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 4846–4861

Oxidovanadium(IV) Complexes of Tetradentate Ligands
Figure 2. Field-emission scanning electron micrograph of [VIVO(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–1 due to C–H bands. A medium-intensity
band around 3000 cm–1 in ligands, mostly due to intramo-
lecular ν(OH)phenolic hydrogen 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 VIVO complexes show a sharp band
between 973–986 cm–1 due 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)
[VIVO(pydx-en)] (1)
[VIVO(pydx-1,2-pn)] (3)
[VIVO(pydx-1,3-pn)] (2)
[VIVO(pydx-en)]-Y (4)
nujol
nujol
nujol
393, 344, 295, 221
415, 340, 273, 228
415, 288, 223
[VIVO(pydx-1,3-pn)]-Y (5)
[VIVO(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)
[VIVO(pydx-en)] (1)
[VIVO(pydx-1,3-pn)] (2)
[VIVO(pydx-1,2-pn)] (3)
[VIVO(pydx-en)]-Y (4)
[VIVO(pydx-1,3-pn)]-Y (5)
[VIVO(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 [VIVO(pydx-en)]
Figure 3. Electronic spectra of (a) [VIVO(pydx-en)]-Y (4), (b) [VIV-
O(pydx-1,3-pn)]-Y (5) and (c) [VIVO(pydx-1,2-pn)]-Y (6) recorded
was described previously.[29] The broad band of medium in- dispersed in Nujol.
Eur. J. Inorg. Chem. 2011, 4846–4861
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4849

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 VIV-heterogenized complexes[1] indicate
region and are assigned to band I (dxyǞdxz, dyz) and band that the encapsulated VIV complexes closely fit the zeolite
II (dxyǞdx2–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 Npyridine and/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 gz and Az agree well with the
values estimated using the additivity relationship proposed
by Wüthrich[34] and Chasteen[32] with an estimated accu-
racy of Ϯ3ϫ10–4 cm–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 VIVO 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 Ophenolate
3 in MeOH or DMSO are depicted in Figure 4, and those ≈ 38.9ϫ10–4 cm–1; for Nimine 39Ϯ2ϫ10–4 cm–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 (Ophenolate
,
6 show broadening and changes in the peak-to-peak line imine, Nimine, Ophenolate equatorial
N
)
.
Figure 4. First-derivative EPR spectra of frozen (77 K) solutions of (a) [VIVO(pydx-en)] (1) (4 mm) in MeOH; (b) [VIVO(pydx-1,2-pn)]
(3) (4 mm) in MeOH; and (c) [VIVO(pydx-1,3-pn)] (2) (4 mm) in DMSO.
Figure 5. First-derivative EPR spectra at room temperature of zeolite-encapsulated complexes (solids) (a) [VIVO]-Y; (b) [VIVO(pydx-en)]-
Y (4); (c) [VIVO(pydx-1,2-pn)]-Y (6); (d) [VIVO(pydx-1,3-pn)]-Y (5).
4850
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 4846–4861

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
gz
Az
Ax, Ay
gx, gy
[ϫ104 cm–1] [ϫ104 cm–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% H2O2 as 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
≈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-[VIVO]-Y was previously reported and
its EPR spectrum recorded.[39] It was concluded that the
VIVO2+ 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: gz = 1.938 and Az
=
178ϫ10–4 cm–1. The EPR spectrum of the prepared zeolite-
[VIVO]-Y (Figure 5) shows rather broad lines, thus the spin
Hamiltonian parameters cannot be determined accurately.
They are gz ≈ 1.939 and Az ≈ 176ϫ10–4 cm–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-[VIVO]-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 VIV in 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
VIVO(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, [VIVO(pydx-en)]-Y (4) was taken as a representative
catalyst and different parameters, namely, the amount of
oxidant (mol of H2O2 per 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-[VIVO]-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 VIVO 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 Npyridine moieties of the ligands. Equatorial or axial ginally upon further increasing this ratio to 1:3, which sug-
binding of zeolite–OH groups to VIVO is also possible and gested that a higher amount of oxidant does not improve
may explain the relatively high Az values 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 VIVO 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 [VIVO(pydx-en)]-Y (4), [VIVO(pydx-1,3-pn)]-Y (5) and ene (1.04 g, 10 mmol), H2O2 (2.28 g, 20 mmol), CH3CN
[VIVO(pydx-1,2-pn)]-Y (6), each with
a
binding set (5 mL) and temperature (80 °C), 0.005 g of catalyst gave
(Ophenolate, Nimine, Nimine, Ophenolate equatorial.
)
only 64.4% conversion (Figure 7). A maximum of 85.5%
Eur. J. Inorg. Chem. 2011, 4846–4861
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4851

M. R. Maurya, P. Saini, A. Kumar, J. Costa Pessoa
FULL PAPER
the net amount of oxidant is the same, the H2O2 concentra-
tion is also higher when the volume of solvent added is
lower.
Figure 6. Effect of the amount of H2O2 on the oxidation of styrene
at 80 °C as a function of time. Reaction conditions: styrene (1.04 g,
10 mmol), [VIVO(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), [VIVO(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, [VIVO-
(pydx-1,3-pn)]-Y and [VIVO(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 [VIVO(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
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 4846–4861

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
[VIVO(pydx-en)]-Y (4)
[VIVO(pydx-en)]-Y[a]
[VIVO(pydx-1,3-pn)]-Y (5)
[VIVO(pydx-1,3-pn)]-Y[a]
[VIVO(pydx-1,2-pn)]-Y (6)
[VIVO(pydx-1,2-pn)]-Y[a]
[VIVO]-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
[VIVO(pydx-en)] (1)
369[e]
366[e]
369[e]
[VIVO(pydx-1,2-pn)] (3)
[VIVO(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 VIV present 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 H2O2 relative 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 H2O2 is 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 [VIVO(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
Eur. J. Inorg. Chem. 2011, 4846–4861
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4853

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]
[VIVO(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 [VIVO(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-
H2O2 molar 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-H2O2 molar
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 [VIVO-
(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 H2O2 on the oxidation of cyclo-
hexene at 80 °C as a function of time. Reaction conditions: cyclo-
hexene (0.820 g, 10 mmol), [VIVO(pydx-en)]-Y (0.005 g) and aceto-
nitrile (5 mL).
Similarly, for cyclohexene (0.082 g, 10 mmol), 30% H2O2 clohexene. 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.
4854
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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
[VIVO(pydx-en)]-Y (4)
[VIVO(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
–
–
–
–
[VIVO(pydx-1,3-pn)]-Y (5)
[VIVO(pydx-1,3-pn)]-Y[a]
[VIVO(pydx-1,2-pn)]-Y (6)
[VIVO(pydx-1,2-pn)]-Y[a]
Without catalyst
1276[c]
–
1487[c]
–
–
[VIVO(pydx-en)] (1)
595[d]
704[d]
505[d]
[VIVO(pydx-1,3-pn)] (2)
[VIVO(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 VIV present 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., VIVO 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 H2O2 gave 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, [VIVO-
(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
H2O2 was 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
[VIVO(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 H2O2 to 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
Eur. J. Inorg. Chem. 2011, 4846–4861
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4855

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
[VIVO(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- [VIVO(pydx-en)]-Y[a]
[VIVO(pydx-1,3-pn)]-Y (5)
905[c]
–
tion conditions of catalyst and oxidant than 10 or 15 mL.
[VIVO(pydx-1,3-pn)]-Y[a]
This may possibly be due to a much higher concentration
[VIVO(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.
[VIVO(pydx-1,2-pn)]-Y[a]
No solid[b]
–
–
906
884
916
Na–Y[b]
≈3
≈3
≈0
[VIVO(pydx-en)] (1)
[VIVO(pydx-1,2-pn)] (3)
[VIVO(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 VIV present
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 [VIVO(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 [VIVO(pydx-en)] (1), [VIVO(pydx-1,3-pn)] (2)
[VIVO(pydx-en)]-Y (85.5%)
Ͼ
[VIVO(pydx-1,3-pn)]-Y and [VIVO(pydx-1,2-pn)] (3) under reaction conditions ap-
(82.1%) Ͼ [VIVO(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: [VIVO(pydx-en)]-Y (79.6%) Ͼ [VIVO(pydx-1,3- (2.27 g, 20 mmol) and acetonitrile (5 mL)], 75.8–80.3%
pn)]-Y (75.2%) Ͼ [VIVO(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.
4856
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 4846–4861

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 [VIVO(pydx-en)] with H2O2 in DMSO
was monitored by electronic absorption spectroscopy. The
observed spectral changes are presented in Figure 15. Thus,
the progressive addition of a dilute H2O2 solution in
DMSO to a solution of [VIVO(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 [VIVO(pydx-en)] with
H2O2 in methanol, thus forming [VVO(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 VVO2L compounds react with
H2O2 to give VVO(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 [VIVO(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
[VIVO(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% H2O2 dissolved in 5 mL of DMSO] to 10 mL
of 1.6ϫ10–4 m solution in DMSO. The inset shows similar spectra
recorded with around 7.55ϫ10–3 m solution of 1 in DMSO.
Eur. J. Inorg. Chem. 2011, 4846–4861
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4857

M. R. Maurya, P. Saini, A. Kumar, J. Costa Pessoa
FULL PAPER
Scheme 6. Tentative assignments of VV species formed in methanolic solutions of [VIVO(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
VIV or VV species 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 VV and VIV-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.
VIVO to VV complexes.
Thus, recyclability of the catalyst precursors mainly de-
Similar features were observed with [VIVO(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 [VIVO(pydx-en)] (1) were re- [VVO2(pydx-aebmz)] (7) and [VVO2(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 [VIVO(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: [VIVO(pydx-
nances at δ = –543 and –549 ppm build up, which we tenta- chen)] (9) and [VIVO(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% H2O2 aqueous 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% H2O2 solution, 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 [VVO2(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 [VIVO(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
VVO(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
4858
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 4846–4861

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 [VVO(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
[VIVO(pydx-en)]-Y (4), [VIVO(pydx-1,3-pn)]-Y (5) and compatible with the preferred formation of benzaldehyde.
[VIVO(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 [VIVO(pydx-en)] (1), [VIVO(pydx-
1,3-pn)] (2) and [VIVO(pydx-1,2-pn)] (3) were also prepared.
The encapsulated complexes 4–6 efficiently catalyze the
oxidation of styrene by H2O2 and with higher conversions
than the corresponding neat complexes 1–3 or the related
neat complexes [VIVO(pydx-chen)] (9) and [VIVO(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 [VIVO(pydx-chen)] (9) and [VIVO(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 H2O2 of 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-Kα target. 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] 51V 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 VVOCl3 as 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.
Eur. J. Inorg. Chem. 2011, 4846–4861
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4859

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.
[VIVO(pydx-en)] (1): A solution of H2pydx-en (1.792 g, 5 mmol) in
methanol (50 mL) was treated with [VIVO(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 [VIVO(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.
[VIVO(pydx-1,3-pn)] (2) and [VIVO(pydx-1,2-pn)] (3): These com-
plexes were prepared similarly using the respective ligand.
Acknowledgments
[VIVO(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.
[VIVO(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.
[VIVO]-Y [oxidovanadium(IV)-exchanged zeolite-Y]: A filtered solu-
tion of VIVOSO4·5H2O (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 VIVO2+ ion con-
tent and dried at 150 °C for 24 h. Yield: 14.9 g (99%). %V (ICP-
MS): 4.6.
[1] M. R. Maurya, A. Kumar, J. Costa Pessoa, Coord. Chem. Rev.
2011, 255, 2315–2344.
[2] C. Bowers, P. K. Dutta, J. Catal. 1990, 122, 127–279.
[3] R. Raja, P. Ratnasamy, Appl. Catal. A 1996, 143, 145–158.
[4] E. Paez-Mozo, N. Gabriunas, R. Maggi, D. Acosta, P. Ruiz, B.
Delmon, J. Mol. Catal. A 1994, 91, 251–258.
[5] C. R. Jacob, S. P. Varkey, P. Ratnasamy, Appl. Catal. A 1999,
182, 91–96.
[VIVO(pydx-en)]-Y (4): A mixture of [VIVO]-Y (3.0 g) and H2pydx-
en (3.0 g) was mixed in methanol (40 mL) and the reaction mixture
was heated at reflux for 15 h in an oil bath with stirring. The re-
sulting material was suction filtered and dried. The crude mass was
extracted with methanol using a Soxhlet extractor until the solid
was free from unreacted ligand and free metal complex on the sur-
face. After filtering, the free VIVO2+ ions present in the zeolite-Y
were removed by stirring with aqueous 0.1 m NaCl solution
(200 mL) for 8 h. It was finally washed with distilled water until no
precipitate of AgCl was observed in the filtrate upon treatment
with AgNO3.
[6] M. R. Maurya, S. J. J. Titinchi, S. Chand, Appl. Catal. A 2002,
228, 177–187.
[7] T. Joseph, D. Srinivas, C. S. Gopinath, S. B. Halligudi, Catal.
Lett. 2002, 83, 209–214.
[8] J. Poltowicz, K. Pamin, E. Tabor, J. Haber, A. Adamski, Z.
Sojka, Appl. Catal. A 2006, 299, 235–242.
[9] S. P. Verkey, C. Ratnasamy, P. Ratnasamy, J. Mol. Catal. A
1998, 135, 295–306.
[10] S. P. Verkey, C. Ratnasamy, P. Ratnasamy, Microporous Meso-
porous Mater. 1998, 22, 465–474.
[11] S. P. Verkey, C. Ratnasamy, P. Ratnasamy, Appl. Catal. A 1999,
182, 91–96.
[VIVO(pydx-1,3-pn)]-Y (5) and [VIVO(pydx-1,2-pn)]-Y (6): Com-
plexes [VIVO(pydx-1,3-pn)]-Y (5) and [VIVO(pydx-1,2-pn)]-Y (6)
were prepared following a procedure similar to the one described
for [VIVO(pydx-en)]-Y (4).
[12] S. Deshpande, D. Srinivas, P. Ratnasamy, J. Catal. 1999, 188,
261–269.
[13] D. Chatterjee, A. Mitra, J. Mol. Catal. A 1999, 144, 363–367.
[14] M. S. Niassary, F. Farzaneh, M. Ghandi, L. Turkian, J. Mol.
Catal. A 2000, 157, 183–188.
Catalytic Activity: The catalytic potential of the compounds pre-
pared was tested by choosing the oxidation of styrene, cyclohexene
and methyl phenyl sulfide as model reactions. Thus, [VIVO(pydx-
en)]-Y (4), [VIVO(pydx-1,3-pn)]-Y (5) and [VIVO(pydx-1,2-pn)]-Y
(6) were used as catalyst precursors. All catalytic reactions were
carried out in a 50 mL flask fitted with a water-circulated con-
denser.
[15] M. Silva, C. Freire, B. de Castro, J. L. Figueiredo, J. Mol. Ca-
tal. A 2006, 258, 327–333.
[16] T. Joseph, S. B. Halligudi, C. Satyanarayan, D. P. Sawant, S.
Gopinathan, J. Mol. Catal. A 2001, 168, 87–97.
[17] M. R. Maurya, S. J. J. Titinchi, S. Chand, Appl. Catal. A 2002,
228, 177–187.
[18] M. R. Maurya, S. J. J. Titinchi, S. Chand, J. Mol. Catal. A
2003, 201, 119–130.
[19] M. R. Maurya, M. Kumar, S. J. J. Titinchi, H. S. Abbo, S.
Chand, Catal. Lett. 2003, 86, 97–105.
[20] M. R. Maurya, A. K. Chandrakar, S. Chand, J. Mol. Catal. A
2007, 270, 225–235.
[21] S. Rayati, M. Koliaei, F. Ashouri, S. Mohebbi, A. Wojtczak,
A. Kozakiewicz, Appl. Catal. A 2008, 346, 65–71.
[22] R. Bai, X. Fu, H. Bao, W. Ren, Catal. Commun. 2008, 9, 1588–
1594.
The progress of the reactions was monitored by gas chromatog-
raphy by withdrawing small aliquots at chosen time intervals. The
identity of the products was confirmed by GC–MS. For each type
of reaction, the effects of various parameters, such as amounts of
oxidant and catalyst as well as the temperature of the reaction mix-
ture were studied to check their effect on the conversion and selec-
tivity of the reaction products.
[23] B. Gong, X. Fu, J. Chen, Y. Li, X. Zou, X. Tu, P. Ding, L.
Ma, J. Catal. 2009, 262, 9–17.
[24] a) P. Adão, J. Costa Pessoa, R. T. Henriques, M. L. Kuznetsov,
F. Avecilla, M. R. Maurya, U. Kumar, I. Correia, Inorg. Chem.
2009, 48, 3542–3561; b) P. Adão, M. R. Maurya, U. Kumar, F.
Avecilla, R. T. Henriques, M. L. Kusnetsov, J. Costa Pessoa, I.
Correia, Pure Appl. Chem. 2009, 81, 1279–1296.
Oxidation of Styrene: In a typical reaction, styrene (0.52 g, 5 mmol)
and aqueous 30% H2O2 (1.70 g, 15 mmol) were dissolved in aceto-
nitrile (10 mL) and the temperature of the reaction mixture was set
to 80 °C. The catalyst (0.010 g) was added to the above reaction
mixture with stirring and reaction was considered to have begun.
For the neat catalysts the amount used was 0.0015 g.
4860
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 4846–4861

Oxidovanadium(IV) Complexes of Tetradentate Ligands
[25] P. K. Saha, B. Dutta, S. Jana, R. Bera, S. Saha, K. Okamoto,
S. Koner, Polyhedron 2007, 26, 563–571.
[26] M. R. Maurya, B. Singh, P. Adão, F. Avecilla, J. Costa Pessoa,
Eur. J. Inorg. Chem. 2007, 5720–5734.
[27] M. Salavati-Niasari, J. Inclusion Phenom. Macrocyclic Chem.
2009, 65, 349–360.
[28] M. R. Maurya, Coord. Chem. Rev. 2003, 237, 163–181.
[29] I. Correia, J. Costa Pessoa, M. T. Duarte, R. T. Henriques,
M. F. M. Piedade, L. F. Veiros, T. Jackusch, A. Dornyei, T.
Kiss, M. M. C. A. Castro, C. F. G. C. Geraldes, F. Avecilla,
Chem. Eur. J. 2004, 10, 2301–2317.
[41]
[42]
D. C. Crans, A. D. Keramidas, S. S. Amin, O. P. Anderson,
S. M. Miller, J. Chem. Soc., Dalton Trans. 1997, 2799–2812.
K. Srinivasan, P. Michaud, J. K. Kochi, J. Am. Chem. Soc.
1986, 108, 2309–2320.
R. Irie, Y. Ito, T. Katsuki, Synlett 1991, 265–266.
V. Hulea, E. Dumitriu, Appl. Catal. A 2004, 277, 99–106.
S. B. Kumar, S. P. Mirajkar, G. C. G. Pais, P. Kumar, R. Kumar,
J. Catal. 1995, 156, 163–166.
J. D. Koola, J. K. Kochi, J. Org. Chem. 1987, 52, 4545–4553.
D. Rehder, Bioinorganic Vanadium Chemistry, Wiley, New
York, 2008.
M. A. Anderson, A. Willets, S. Allenmark, J. Org. Chem. 1997,
62, 8455–8458.
[43]
[44]
[45]
[46]
[47]
[30] D. H. Williams, I. Fleming, Spectroscopic Methods in Organic
Chemistry, McGraw-Hill, London, 5th ed., 1995, pp. 18–21.
[31] L. F. Vilas Boas, J. Costa Pessoa, in: Comprehensive Coordina-
tion Chemistry (Eds.: G. Wilkinson, R. D. Gillard, J. A. Mc
Cleverty), Oxford, Pergamon, 1987, pp. 453–583.
[32] N. D. Chasteen, in: Biological Magnetic Resonance (Ed.: J.
Reuben), Plenum, New York, 1981, 53.
[48]
[49] H. B. ten Brink, H. E. Schoemaker, R. Wever, Eur. J. Biochem.
2001, 268, 132–138.
[50] a) A. G. J. Ligtenbarg, R. Hage, B. L. Feringa, Coord. Chem.
Rev. 2003, 237, 89–101; b) A. Butler, M. J. Clague, G. E. Meis-
ter, Chem. Rev. 1994, 94, 625–638.
[51] a) G. B. Shul’pin, Y. N. Kozlov, G. V. Nizova, G. Süss-Fink, S.
Stanislas, A. Kitaygorodskiy, V. S. Kulikova, J. Chem. Soc.,
Perkin Trans. 2 2001, 1351–1371; b) Y. N. Kozlov, V. B. Ro-
makh, A. Kitaygorodskiy, P. Buglyó, G. Süss-Fink, G. B.
Shul’pin, J. Phys. Chem. A 2007, 111, 7736–7752.
[52] R. Z. Khaliullin, A. T. Bell, M. Head-Gordon, J. Phys. Chem.
B 2005, 109, 17984–17992.
[53] M. V. Kirillova, M. L. Kuznetsov, V. B. Romakh, L. S. Shul’p-
ina, J. J. R. Fraústo da Silva, A. J. L. Pombeiro, G. B. Shul’pin,
J. Catal. 2009, 267, 140–157.
[33] A. Rockenbauer, L. Korecz, Appl. Magn. Reson. 1996, 10, 29–
43.
[34] K. Wüthrich, Helv. Chim. Acta 1965, 48, 1012.
[35] D. Rehder, C. Weidemann, A. Duch, W. Priebsch, Inorg. Chem.
1988, 27, 584–587.
[36] N. Butenko, I. Tomaz, O. Nouri, E. Escribano, V. Moreno, S.
Gama, V. Ribeiro, J. P. Telo, J. Costa Pesssoa, I. Cavaco, J. In-
org. Biochem. 2009, 103, 622–632.
[37] I. Cavaco, J. Costa Pessoa, D. Costa, M. T. L. Duarte, R. D.
Gillard, P. M. Matias, J. Chem. Soc., Dalton Trans. 1994, 149–
157.
[38] J. Costa Pessoa, M. J. Calhorda, I. Cavaco, I. Correia, M. T.
Duarte, V. Felix, R. T. Henriques, M. F. M. Piedade, I. Tomaz,
J. Chem. Soc., Dalton Trans. 2002, 4407–4415.
[54] D. V. Deubel, G. Frenking, P. Gisdakis, W. A. Herrmann, N.
Rosch, J. Sundermeyer, Acc. Chem. Res. 2004, 37, 645–652.
[55] M. L. Kuznetsov, J. Costa Pessoa, Dalton Trans. 2009, 5460–
5468.
[39] G. Martini, M. F. Ottaviani, G. L. Seravalli, J. Phys. Chem.
1975, 79, 1716–1720.
[40] M. R. Maurya, M. Bisht, A. Kumar, M. L. Kuznetsov, F. Avec-
illa, J. Costa Pessoa, Dalton Trans. 2011, 40, 6968–6983.
Received: June 21, 2011
Published Online: September 19, 2011
Eur. J. Inorg. Chem. 2011, 4846–4861
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4861