Polystyrene bound oxidovanadium(IV) and dioxidovanadium(V) complexes of histamine derived ligand for the oxidation of methyl phenyl sulfide, diphenyl sulfide and benzoin

Article abstract of DOI:10.1039/b814297a

Ligand Hsal-his (I) derived from salicylaldehyde and histamine has been covalently bound to chloromethylated polystyrene cross-linked with 5% divinylbenzene. Upon treatment with [VO(acac)₂] in DMF, the polystyrene-bound ligand (abbreviated as PS-Hsal-his, II) gave the stable polystyrene-bound oxidovanadium(iv) complex PS-[V^IVO(sal-his)(acac)] 1, which upon oxidation yielded the dioxidovanadium(v) PS-[V^VO ₂(sal-his)] 2 complex. The corresponding non polymer-bound complexes [V^IVO(sal-his)(acac)] 3 and [V^VO₂(sal-his)] 4 have also been obtained. These complexes have been characterised by IR, electronic, ⁵¹V NMR and EPR spectral studies, and thermal as well as scanning electron micrograph studies. Complexes 1 and 2 have been used as a catalyst for the oxidation of methyl phenyl sulfide, diphenyl sulfide and benzoin with 30% H₂O₂ as oxidant. Under the optimised reaction conditions, a maximum of 93.8% conversion of methyl phenyl sulfide with 63.7% selectivity towards methyl phenyl sulfoxide and 36.3% towards methyl phenyl sulfone has been achieved in 2 h with 2. Under similar conditions, diphenyl sulfide gave 83.4% conversion where selectivity of reaction products varied in the order: diphenyl sulfoxide (71.8%) > diphenyl sulfone (28.2%). A maximum of 91.2% conversion of benzoin has been achieved within 6 h, and the selectivities of reaction products are: methylbenzoate (37.0%) > benzil (30.5%) > benzaldehyde-dimethylacetal (22.5%) > benzoic acid (8.1%). The PS-bound complex, 1 exhibits very comparable catalytic potential. These polymer-anchored heterogeneous catalysts do not leach during catalytic action, are recyclable and show higher catalytic activity and turnover frequency than the corresponding non polymer-bound complexes. EPR and ⁵¹V NMR spectroscopy was used to characterise methanolic solutions of 3 and 4 and to identify species formed upon addition of H₂O₂ and/or acid and/or methyl phenyl sulfide.

Full text of DOI:10.1039/b814297a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Polystyrene bound oxidovanadium(IV) and dioxidovanadium(V) complexes of
histamine derived ligand for the oxidation of methyl phenyl sulfide, diphenyl
sulfide and benzoin†
a
a
b
b
Mannar R. Maurya,* Aarti Arya, Amit Kumar and Jo a˜ o Costa Pessoa*
Received 18th August 2008, Accepted 24th November 2008
First published as an Advance Article on the web 3rd February 2009
DOI: 10.1039/b814297a
Ligand Hsal–his (I) derived from salicylaldehyde and histamine has been covalently bound to
chloromethylated polystyrene cross-linked with 5% divinylbenzene. Upon treatment with [VO(acac) ] in
2
DMF, the polystyrene-bound ligand (abbreviated as PS–Hsal–his, II) gave the stable polystyrene-bound
IV
oxidovanadium(IV) complex PS–[V O(sal–his)(acac)] 1, which upon oxidation yielded the
V
dioxidovanadium(V) PS–[V O
[
2
(sal–his)] 2 complex. The corresponding non polymer-bound complexes
IV
V
V O(sal–his)(acac)] 3 and [V O (sal–his)] 4 have also been obtained. These complexes have been
characterised by IR, electronic, V NMR and EPR spectral studies, and thermal as well as scanning
2
5
1
electron micrograph studies. Complexes 1 and 2 have been used as a catalyst for the oxidation of methyl
phenyl sulfide, diphenyl sulfide and benzoin with 30% H
2
2
O as oxidant. Under the optimised reaction
conditions, a maximum of 93.8% conversion of methyl phenyl sulfide with 63.7% selectivity towards
methyl phenyl sulfoxide and 36.3% towards methyl phenyl sulfone has been achieved in 2 h with 2.
Under similar conditions, diphenyl sulfide gave 83.4% conversion where selectivity of reaction products
varied in the order: diphenyl sulfoxide (71.8%) > diphenyl sulfone (28.2%). A maximum of 91.2%
conversion of benzoin has been achieved within 6 h, and the selectivities of reaction products are:
methylbenzoate (37.0%) > benzil (30.5%) > benzaldehyde-dimethylacetal (22.5%) > benzoic acid
(
8.1%). The PS-bound complex, 1 exhibits very comparable catalytic potential. These
polymer-anchored heterogeneous catalysts do not leach during catalytic action, are recyclable and
show higher catalytic activity and turnover frequency than the corresponding non polymer-bound
5
1
complexes. EPR and V NMR spectroscopy was used to characterise methanolic solutions of 3 and 4
and to identify species formed upon addition of H
2
2
O and/or acid and/or methyl phenyl sulfide.
Introduction
sation of homogeneous catalysts through their encapsulation
or immobilization has the advantage of easy separation from
the reaction mixture, leading to operational flexibility and their
facile regeneration compared to homogeneous catalysts. Recently
we have used chloromethylated polystyrene cross-linked with
The coordination chemistry of vanadium has received increasing
attention from researchers, particularly after the discovery of
vanadate-dependent enzymes, vanadium haloperoxidases from
various sea algae and terrestrial fungi. They are very active for
the oxidative halogenation and oxidation of organic substrates in
the presence of H
have been developed to understand the role of the enzyme and the
mechanism of the reaction. Generally, these functional models
are homogeneous in nature and decompose during the catalytic
reaction and thus are not suitable for industrial applications.
Encapsulation of monomeric complexes in microporous materials
such as zeolites and mesoporous materials e.g. MCM-41,
their immobilization onto polymer supports through covalent
attachment are means to overcome this problem. Heterogeni-
1
–3
4
divinylbenzene, one of the most widely employed macromolecular
13–16
supports,
for immobilization of model vanadium complexes
5,6
2
O
2
.
Various structural and functional models
and have tested their catalytic potential for a variety of oxidation
17–19
reactions.
7
Herein we report the preparation and characterisation of
chloromethylated polystyrene-bound oxidovanadium(IV) and
dioxidovanadium(V) complexes of Hsal–his (I), Scheme 1. The
corresponding non polymer-bound complexes of I, which are
considered as structural models of haloperoxidases, have also been
prepared. Spectral evidence is presented for peroxide binding, in
8
9–12
or
the presence of H
2
O
2
, to the vanadium centre. Catalytic potential
a
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee,
2
47 667, India. E-mail: rkmanfcy@iitr.ernet.in
b
Centro Qu ´ı mica Estrutural, Instituto Superior T e´ cnico, TU Lisbon,
Av Rovisco Pais, 1049-001, Lisboa, Portugal. E-mail: joao.pessoa@mail.
ist.utl.pt
†
Electronic supplementary information (ESI) available: Electronic spec-
51
tra, spin Hamiltonian parameters and V NMR chemical shifts. See DOI:
0.1039/b814297a
1
Scheme 1
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of these complexes has been demonstrated by studying the
oxidation of methyl phenyl sulfide, diphenyl sulfide and benzoin.
Preparation of complexes
IV
PS–[V O(sal–his)(acac)] 1. Polymer-anchored ligand PS–
Hsal–his (2.00 g) was allowed to swell in DMF (25 mL) for 2 h.
Experimental
A solution of [VO(acac) ] (5.30 g, 20 mmol) in 20 mL DMF was
2
added to the above suspension and the reaction mixture was heated
Materials and methods
◦
at 90 C for 14 h with slow mechanical stirring. After cooling to
room temperature, the dark black polymer-anchored complex was
separated by filtration, washed with hot DMF followed by hot
methanol and dried at 120 C in an air oven. (Found: C, 68.56; H,
Chloromethylated polystyrene [18.9% Cl (5.3 mmol Cl per gram
of resin)] cross-linked with 5% divinylbenzene was obtained as
a gift from Thermax Limited, Pune, India. Analytical reagent
◦
1
0.24; N, 7.45; V, 8.89%).
grade V
Ranbaxy, India), histamine hydrochloride, methyl phenyl sulfide,
diphenyl sulfide (Himedia, India, or Acros, UK), benzoin (SRL,
2
O
5
(Loba Chemie, India), 30% H
2
O
2
, salicylaldehyde
(
V
PS–[V O
2
(sal–his)] 2.
Method A. A solution of KVO
dissolving V (5.46 g, 30 mmol) in aqueous KOH (3.36 g,
0 mmol in 50 mL H O). PS–Hsal–his (2 g) was suspended in
3
was generated in situ by
2
0
India) and other chemicals were used as purchased. [VO(acac) ]
2
2
O
5
21
and Hsal–his were prepared according to the methods reported
in the literature.
6
2
the above solution and stirred mechanically for ca. 48 h at which
point the colour of the beads had changed to orange. They were
separated by filtration, washed with water followed by methanol
and dried in a desiccator. (Found: C, 67.13; H, 9.12; N, 6.96; V,
Elemental analyses of the ligands and complexes were ob-
tained by an Elementar model Vario-EL-III. IR spectra were
recorded as KBr pellets on a Nicolet NEXUS Aligent 1100
FT-IR spectrometer. Electronic spectra of the polymer-bound
complexes were recorded in Nujol on a Shimadzu 1601 UV-Vis
spectrophotometer by layering a mull of the sample on the inside
of one of the cuvettes while keeping the other one layered with
Nujol as reference. Spectra of non polymer-bound ligand and
complexes were recorded in methanol. The EPR spectra were
recorded with a Bruker ESP 300E X-band spectrometer. For the
polymer-anchored complex samples the spectra were measured at
room temperature and also at 77 K after swelling in DMF; for the
neat complexes the samples were frozen in either MeOH or DMF
in liquid nitrogen and the EPR spectra were measured at 77 K. The
spin Hamiltonian parameters were obtained by simulation of
the spectra with the computer program of Rockenbauer and
8
.41%).
IV
Method B. Complex, PS–[V O(sal–his)(acac)] 1 (1.5 g) was
suspended in methanol (40 mL) and air was bubbled through
the suspension for ca. 4 d. During this period the colour of the
beads slowly changed to orange. They were separated by filtration,
washed with water followed by methanol and dried in a desiccator.
(Found: C, 67.10; H, 9.14; N, 6.94; V, 8.43%).
IV
[V O(sal–his)(acac)] 3. Complex 3 was prepared according to
21
the reported procedure. Yield 65%. (Found: C, 53.21; H, 5.54;
N, 10.85; V, 12.83%. Calcd for C H N O V: C, 53.27; H, 5.78; N,
1
8
23
3
5
10.96; V, 13.29%).
V
22
51
[V O
2
(sal–his)] 4. Complex 3 (0.383 g, 1 mmol) was dissolved
Korecz. The V NMR spectra were recorded on a Bruker Avance
II+ 400 MHz (ultrashield magnet) instrument. Thermogravi-
metric analyses of the complexes were carried out using Perkin
Elmer’s PYRIS Diamond under oxygen atmosphere. The energy-
dispersive X-ray analyses (EDX) of anchored ligand and com-
plexes were recorded on a FEI Quanta 200 FEG. The samples
were coated with a thin film of gold to prevent surface charging, to
protect the surface material from thermal damage by the electron
beam and to make the sample conductive. The identity of the
products was confirmed using a GC-MS Perkin Elmer Clarus 500
and comparing the fragments of each product with the library
available. A Thermax Nicolet gas chromatograph with a HP-1
capillary column (30 m ¥ 0.25 mm ¥ 0.25 mm) was used to analyse
the reaction products and their quantifications were made on the
basis of the relative peak area of the respective product.
in 50 mL of methanol and after addition of aqueous 30% H
2
O
2
(
0.2 mL) air was gently passed through the solution for 4 d. During
IV
this period the V in 3 completely oxidized and the solution
became yellow. The yellow solid of 4 was obtained after reducing
the solvent volume to ca. 5 mL, which was filtered and dried in
vacuo. Yield 63%. (Found: C, 48.24; H, 3.96; N, 13.93; V, 16.42%.
Calcd for C12
H NMR (DMSO-d
CH=N–), 8.07 (s, 1H), 7.47 (s, 1H), 7.37 (s, 1H), 7.12 (s, 1H), 6.74
H
12
N
3
O
3
V: C, 48.50; H, 4.07; N, 14.14; V, 17.14%.)
, d/ppm): 12.82 (s, 1H,–NH), 8.70 (s, 1H,–
1
6
5
1
(
(
s, 1H), 6.72 (d, 1H) (aromatic), 3.91 (b, 4H,–CH
MeOD-d , d/ppm): -547).
2
). V NMR
4
Catalytic activity studies
Oxidation of methyl phenyl sulfide, diphenyl sulfide and benzoin
was carried out in 50 mL reaction flasks.
Preparation of polymer-bound ligand, PS–Hsal–his II
Oxidation of methyl phenyl sulfide and diphenyl sulfide. The
polymer-anchored catalyst, after swelling in methanol for 2 h,
was used for the oxidation of methyl phenyl sulfide and diphenyl
sulfide. Methyl phenyl sulfide (1.24 g, 10 mmol) or diphenyl sulfide
Chloromethylated polystyrene (3.0 g) was allowed to swell in
DMF (40 mL) for 2 h. A solution of Hsal–his (4.60 g, 25 mmol)
in DMF (30 mL) was added to the above suspension followed
by triethylamine (4.50 g) in ethylacetate (40 mL). The reaction
(1.86 g, 10 mmol), 30% aqueous H
2
2
O (1.71 g, 15 mmol), and
◦
mixture was heated at 90 C for 15 h with slow mechanical
catalyst (0.045 g) in 10 mL acetonitrile were stirred at room tem-
perature and the reaction was monitored by withdrawing samples
at different time intervals and analysing them quantitatively by gas
chromatography. The identities of the products were confirmed
by GC-MS. Various parameters such as the amount of oxidant
stirring. After cooling to room temperature, the yellow resins were
separated by filtration, washed thoroughly with hot DMF followed
◦
by hot methanol and dried in an air oven at 120 C. (Found: C,
7
4.40; H, 11.61; N, 9.41%).
2
186 | Dalton Trans., 2009, 2185–2195
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and catalyst were considered in order to study their effect on the
reaction products.
Table 1 Ligand and metal loadings in polymer-bound complexes, and
ligand-to-metal ratio data
Ligand loading
(
resin)
Ligand :
Metal
(mmol g of resin) ratio
Oxidation of benzoin. In a typical oxidation reaction, benzoin
mmol g- of
1
Metal ion loading
-1
a
(
1.06 g, 5 mmol), aqueous 30% H
2
O
2
(1.71 g, 15 mmol) and catalyst
Compound
(
0.030 g) were mixed in 25 mL methanol. The reaction mixture
PS–[Hsal–his] I
2.24
1.66
—
1.74
—
1:0.95
was heated under reflux with stirring for 6 h. The progress of the
reaction was monitored as mentioned above. The effect of various
parameters such as temperature, amount of oxidant and catalyst
were checked to optimise the conditions for the best performance
of the catalyst.
IV
PS–[V O(sal–his)
(
acac)] 1
V
PS–[V O
2
(sal–his)] 2 1.77
1.65
1:1.07
a
.
1
51
Results and discussion
H and V NMR) studies. The corresponding polymer-bound
complex 2 is proposed to have a similar binding mode.
Synthesis, reactivity and solid state characteristics
Field emission scanning electron microscope (FE-SEM) and
energy-dispersive X-ray analysis (EDX) studies
Reaction of Hsal–his with chloromethylated polystyrene, cross-
linked with 5% divinylbenzene in DMF in the presence of triethy-
lamine gave the polymer-anchored ligand, PS–Hsal–his. The reac-
Field emission scanning electron micrographs (FE-SEM) and
energy-dispersive X-ray analysis (EDX) profiles for a single bead
of polymer-bound ligand and the vanadium complexes were
recorded to observe the morphological changes. Some of these
images along with the energy-dispersive X-ray analysis (EDX)
profile are reproduced in Fig. 1. As expected, the pure polystyrene
bead has a smooth and flat surface while the polymer-bound
ligand and complexes show a very slight roughening of the top
layer. This roughening is more noticeable in complexes 1 and 2
possibly due to the interaction of vanadium with the polymer-
bound ligand which results in the formation of a complex with
a fixed geometry. Accurate information on the morphological
changes in terms of exact orientation of ligands coordinated to
the metal ion has not been possible due to poor loading of
the metal complex. However, pure polymer beads show mainly
two components on the surface—carbon (80.7%) and chlorine
◦
tion was carried out at 90 C. At this temperature the ligand did not
decompose. Miller and Sherrington have used refluxing toluene
to carry out the anchoring of 2-(2-pyridyl)imidazole through
covalent attachment of the imine nitrogen to chloromethylated
23
polystyrene. During this process the –NH group of histamine
reacts with the –CH Cl group as shown in Scheme 2. The
2
remaining chlorine content of 1.5% (0.42 mmol Cl per gram
of resin) in the PS-bound ligand suggests ~92% loading of the
ligand.
Scheme 2 Reaction of Hsal–his with chloromethylated polystyrene.
(18.3%)—as evaluated semi-quantitatively by energy-dispersive
X-ray analysis. A considerable amount of N (ca. 8.1%) and a
IV
The anchored ligand on reaction with [V O(acac) ] in DMF
2
resulted in the formation of the oxidovanadium(IV) complex PS–
IV
[
V O(sal–his)(acac)] 1. The chloromethylated group does not
coordinate with the vanadium precursor. Aerobic oxidation of 1 in
methanol is very slow but gave the dioxidovanadium(V) complex
V
PS–[V O
2
(sal–his)] (2). Eqns 1 and 2 summarise the synthetic
procedures.
IV
PS–Hsal–his + [V O(acac)
2
]
IV
→
PS–[V O(sal–his)(acac)] + Hacac
(1)
(2)
IV
1
2
2
PS–[V O(sal–his)(acac)] + O
2
+ H O
2
V
→
2 PS–[V O
2
(sal–his)] + 2Hacac
Table 1 provides data of metal and ligand loading in polymer-
anchored complexes assuming the formation of PS–Hsal–his.
The data show that the metal to ligand ratio in polymer-bound
complexes is close to 1 : 1.
The structure of non polymer-bound oxidovanadium(IV) com-
plex 3 (refer to Scheme 5) has already been established by an
21
X-ray single crystal study by Pecoraro and co-workers. Aerobic
oxidation of 3 in the presence of a few drops of aqueous 30% H
2
O
2
results in the formation of dioxidovanadium(V) complex 4, which
is now characterised by elemental and spectroscopic (IR, UV-vis,
Fig. 1 FE-SEM (left) and energy-dispersive X-ray analysis (EDX) profile
V
(right) of (a) PS–Hsal–his II and (b) PS–[V O
2
(sal–his)] 2.
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small amount of Cl (ca. 2.1%), were determined on the surface
Table 3 Spin Hamiltonian parameters
of the beads containing bound ligand. The polystyrene beads
4
-1
4
-1
IV
Complex
Solvent
Solid
g
ꢀ
A
ꢀ
(¥10 cm )
g
^
^
A (¥10 cm )
of immobilized metal complexes PS–[V O(sal–his)(acac)] and
V
PS–[V O
2
(sal–his)] also contain significant amounts of metal
IV
PS–[V O
1.949 163.8
1.980 58.6
along with nitrogen, suggesting the formation of a metal complex
with the anchored ligand at various sites.
(sal–his)(acac)] 1
DMF 1.952 164.8
MeOH 1.953 161.5
1.980 57.5
1.981 56.0
IV
[
V O(sal–his)
(
acac)] 3
TGA study
DMF 1.954 161.5
1.980 55.7
Thermogravimetric analysis under an oxygen atmosphere shows
945 cm- (in 3), and at 927 and 895 cm (in 4) similar to other
1
-1
the good stability of polymer-anchored complexes 1 and 2 up to ca.
◦
24
2
00 C and thereafter they decompose exothermically in several
examples reported in the literature.
steps. Quantitative measurement of weight loss at various stages
was not possible due to their overlapping nature. However, the
residues due to the metal oxides obtained as the end product
Electronic spectral study
◦
◦
at ca. 850 C (in 1) and at ca. 500 C (in 2) indicate that
the metal complexes are covalently bound to the polymer. The
decomposition of 1 is completed in four steps. Two overlapping
Electronic spectra (Fig. S1 and S2) and spectral data (Table S1) of
ligands and complexes are given in the ESI†.
◦
steps occur in the temperature range 200–395 C. At this stage the
EPR Spectroscopy study
decomposed product is quite stable. The next weight loss starts
◦
st
at 725 C and continues until the formation of end product. The
The 1 derivative EPR spectra have been recorded for “frozen”
observed residue of 29.8% is close to the calculated value of 29.2%
MeOH and DMF solutions of complex 3 and for 1 in the solid
state at room temperature. The EPR spectra of 1 and 3 are shown
in Fig. 2. The spectrum of 1 is characteristic of magnetically di-
◦
for V
2
O
5
at 850 C. The first weight loss step in non polymer-
◦
◦
bound complex 2 starts at ca. 200 C and is completed at 290 C
IV
with a weight loss of 42%. The second weight loss step amounting
luted V O-complex and the well-resolved EPR hyperfine features
◦
◦
to 7.6% starts at ca. 290 C and is completed at 375 C. The final
indicate that the vanadium(IV) centers are well dispersed in the
polymer matrix. Comparison with the spectra of 3 in MeOH and
DMF indicates that the coordination environments of 1 and 3
◦
◦
step starts at 375 C and is completed at ca. 450 C with a total
weight loss of 68.7%. The final residue of 31.3% (calcd 30.6%)
21
suggests the formation of V
2
O
5
as the end product.
are the same and as reported by Cornman et al. The value of
can be estimated using the additivity relationship proposed
A
ꢀ
2
5
26a
IR spectral study
by Wuethrich and Chasteen, with estimated accuracy of ±3 ¥
-
4
-1
10
cm , and we do not expect any significant influence from
A partial list of IR spectral data of the polymer-anchored ligand
and complexes along with non-polymer bound ones are listed
in Table 2. The Hsal–his (I) ligand exhibits a sharp band at
26b
the axial ligand. The spectra were simulated and the spin
Hamiltonian parameters obtained are included in Table 3. By
22
using the following partial contributions (Oacac, 37.6; Ophenolate, 38.9;
632 cm- due to n(C=N) (azomethine), and this band shifts to
1
1
-4
-1
27
N
imine, 41.6; Nimid, 45 ¥ 10 cm )equatorial(Oacac
)
axial
,
an estimated
-
1
-1
lower wavenumbers by 32 cm (in 3) and 21 cm in 4 suggesting
the coordination of azomethine nitrogen to the metal ion. The
polymer-anchored ligand PS–Hsal–his (II) exhibits a sharp band
at 1639 cm- due to the n(C=N) stretch, and in the polymer-
-4
-1
Aꢀ of 163.1 ¥10 cm is obtained. We can therefore conclude
that the spectra are consistent with such a binding mode. The
contribution of imidazole (Nimid) may depend on its orientation
with respect to the V=O bond, ranging from 40 (parallel, best
1
-
1
anchored complexes this band shows up at 1612–1617 cm . This
-4
27
orbital overlap) to 45 ¥ 10 (perpendicular). For the Nimid of the
observation suggests the coordination of the azomethine nitrogen
sal–his ligand, taking into account the known molecular structure
atom to the metal ion. The additional band observed at ca.
21
in the solid state, we expect the imidazole ring to be positioned
1
1
630 cm- in all complexes is possibly due to coordination of the
perpendicular to the V=O group, so that a contribution close to
imidazole nitrogen atom.
-4
4
5 ¥ 10 is predicted.
The polymer-bound complex 1 exhibits a sharp band at
86 cm- due to n(V=O) while 2 exhibits two such bands at 960
1
9
-
1
and 931 cm corresponding to nasym(O=V=O) and nsym(O=V=O)
modes, respectively. As shown in Table 2, the corresponding
21
non polymer-bound vanadium complexes display these bands at
Table 2 IR spectral data
Compound
(C=N)
(V=O)
Hsal–his I
1632
1600
1611
1639
1612
1617
—
945
927, 895
—
986
[
[
VO(acac)(sal–his)] 3
VO
2
(sal–his)] 4
PS-Hsal–his II
st
IV
Fig.
2
1
derivative EPR spectra of PS–[V O(sal–his)(acac)] 1:
PS–[VO(acac)(sal–his)] 1
(
a) solid at room temperature, (b) in contact with DMF at 77K; and
PS–[VO
2
(sal–his)] 2
960, 931
IV
[
VO (sal–his)(acac)] 3: (c) in MeOH at 77K, (d) in DMF at 77K.
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188 | Dalton Trans., 2009, 2185–2195

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Catalytic activity studies
The catalytic potential of the polymer-bound complexes 1 and 2 as
well as their non polymer-bound analogues were explored for the
oxidation of methyl phenyl sulfide, diphenyl sulfide and benzoin.
Oxidation of methyl phenyl sulfide and diphenyl sulfide. The
2
8
oxidation of sulfides is catalysed by a variety of vanadyl,
29
30
manganese and titanium complexes. Methyl phenyl sulfide
and diphenyl sulfide have electron-rich sulfur atoms which, on
electrophilic oxidation, give sulfoxide and, upon further oxidation,
sulfone (see Scheme 3).
Scheme 3 Oxidation of organic sulfides.
IV
V
Complexes PS–[V O(sal–his)(acac)] 1 and PS–[V O
have been used as catalysts for the oxidation of these sulfides
by using aqueous 30% H . Reaction conditions have been
2
(sal–his)]
2
2
O
2
optimised for the maximum oxidation of methyl phenyl sulfide
and diphenyl sulfide considering 2 as a representative catalyst while
varying the amount of oxidant and catalyst.
The effect of the H
phenyl sulfide is illustrated in Fig. 3(a). Using three different
concentrations of aqueous 30% H , viz. 10 mmol (1.14 g),
5 mmol (1.71 g) and 20 mmol (2.27 g) with fixed amounts of
2
O
2
concentration on the oxidation of methyl
2
O
2
1
V
methyl phenyl sulfide (1.24 g, 10 mmol) and PS–[V O
2
(sal–his)]
(
0.025 g) in CH CN (15 mL), the conversions of methyl phenyl
3
sulfide obtained were 63.8, 93.8 and 96.3%, respectively, in 2 h at
room temperature. No appreciable improvement in the conversion
is noted on increasing the oxidant to substrate molar ratios. Thus,
the oxidant to substrate ratio of 1.5 : 1 may be considered as the
most suitable at the expense of oxidant for the maximum oxidation
of methyl phenyl sulfide.
For three different amounts viz. 0.015, 0.025 and 0.035 g of
catalyst and methyl phenyl sulfide to H
2
2
O molar ratio of 1 : 1.5
under the reaction conditions given above, 0.015 g of catalyst gave
only 75.0% conversion. Increasing this amount to 0.025 g gave
a conversion of 93.8% while 0.035 g of catalyst has shown only
slight improvement in the conversion (Fig. 3(b)). Thus, 0.025 g of
catalyst was considered adequate to run the reaction under these
conditions.
Fig. 3 (a) Effect of amount of H
sulfide. Reaction conditions: methyl phenyl sulfide (1.24 g, 10 mmol),
2
O
2
on the oxidation of methyl phenyl
V
PS–[V O
2
(sal–his)] (0.025 g) in CH
3
CN (15 mL). (b) Effect of amount
V
of catalyst PS–[V O
Reaction conditions: methyl phenyl sulfide (1.24 g, 10 mmol), H
2
(sal–his)] on the oxidation of methyl phenyl sulfide.
(1.71 g,
2 2
O
1
5 mmol) in CH CN (15 mL).
3
Thus, the optimised reaction conditions obtained for the max-
imum oxidation of 10 mmol of methyl phenyl sulfide are: catalyst
has good catalytic potential for both substrates with high turn over
frequency. But the selectivity for the formation of methyl phenyl
sulfoxide is better (71.8%) than for diphenyl sulfoxide (63.7%).
The catalytic activity of non polymer-bound complexes
(
2
0.025 g), H
h are required to complete the reaction. Similarly for 10 mmol
of diphenyl sulfide, 0.045 g of catalyst and H : diphenyl sulfide
2
O
2
(1.71 g, 15 mmol), and CH
3
CN (15 mL). At least
2
O
2
V
IV
molar ratio of 3 : 1 in 15 mL of acetonitrile was found to be the
[V O
2
(sal–his)] 4 and [V O(sal–his)(acac)] 3 using the same mole
best to give a maximum of 83.4% conversion of diphenyl sulfide in
concentration as used for the polymer-anchored complexes under
reaction conditions established above has also been tested for com-
parison. Comparative profiles for the conversion of methyl phenyl
sulfide and diphenyl sulfide using neat as well as polymer-bound
complexes are also presented in Table 4. Conversions of both
sulfides using neat complex are also very good, but always lower
than their polymer-bound analogues. The selectivity for sulfoxide
IV
3
h of reaction time at room temperature. Catalyst, PS–[V O(sal–
his)(acac)] 1, under the above reaction conditions gave lower con-
version (Table 4) for methyl phenyl sulfide as well as for diphenyl
sulfide. Selectivity details for the products obtained for the oxida-
tion of methyl phenyl sulfide and diphenyl sulfide are presented in
V
Table 4. It is clear from the table that catalyst PS–[V O
2
(sal–his)]
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Table 4 Conversion of sulfides, TOF and product selectivity data
%
Selectivity
a
-1
Substrate
Catalyst
Conv. (%)
TOF (h )
Sulfoxide
Sulfone
V
Methyl phenyl sulfide
PS–[V O
2
(sal–his) 2
93.8
84.8
79.5
72.1
83.4
70.7
67.4
60.3
113.8
96.4
91.2
83.3
37.5
30.2
28.7
19.8
63.7
61.0
64.8
62.9
71.8
67.8
73.1
68.9
36.3
39.0
35.2
37.1
28.2
32.2
26.9
31.1
V
[V O
2
(sal–his)] 4
IV
PS–[V O(sal–his)(acac)] 1
IV
[
V O(sal–his)(acac)] 3
V
Diphenyl sulfide
PS–[V O
2
(sal–his)] 2
2
V O (sal–his)] 4
IV
V
[
PS–[V O(sal–his)(acac)] 1
IV
[
V O(sal–his)(acac)] 3
a
TOF values in moles of product per mole of catalyst.
formation is also lower with neat complex (Table 4). Blank reac-
tions using methyl phenyl sulfide (1.24 g, 10 mmol), aqueous 30%
did not show considerable improvement either in the oxidation of
benzoin or in the reduction in time to reach the steady-state in the
reaction processes.
H
2
2
O (1.71 g, 15 mmol) and acetonitrile (15 mL) resulted in 15.2%
conversion with selectivity sulfoxide : sulfone of 68 : 32. Blank reac-
tions for diphenyl sulfide under the reaction conditions above gave
only 5.5% conversion with sulfoxide : sulfone selectivity of 57 : 43.
Fig. 4 presents the selectivity of products along with the
conversion of benzoin as a function of time (6 h) under the optimal
experimental conditions as concluded above, i.e. benzoin (1.10 g,
V
5
0
mmol), 30% H
2
O
2
(1.7 g, 15 mmol), PS–[V O
2
(sal–his)] (0.030 g,
Oxidation of benzoin. The selective oxidation of a-hydro-
xyketones to a-diketones is one of the important reactions
.014 mmol) and methanol (25 mL). It is clear from the plot that all
products (four identified and one unidentified) form from the con-
version of benzoin. With the highest selectivity of benzil (ca. 50%)
at the beginning, a continuous but slow decrease with time of its
selectivity has been observed which finally reaches 30.5% after 6 h.
The selectivity of benzoic acid and benzaldehyde-dimethylacetal
only marginally increases while that of methyl benzoate increases
considerably from 15 to 37%. Thus, with the maximum benzoin
oxidation of 91.2% after 6 h of reaction time, the selectivities of
the reaction products vary in the order: methyl benzoate (37%) >
benzil (30.5%) > benzaldehyde-dimethylacetal (22.5%) > benzoic
acid (8.1%). These data are summarised in Table 5.
31–35
in fine chemical synthesis.
The oxidation of benzoin has
attracted the attention of researchers because one of its oxidised
products, benzil, is a very useful intermediate for the synthesis
36
of heterocyclic compounds and benzylic acid rearrangements.
Here, the oxidation of benzoin was successfully achieved with the
V
catalyst PS–[V O
2
(sal–his)] 2 using 30% aqueous H
2
2
O as oxidant.
Before starting the catalytic run, the catalyst was allowed to swell
in methanol for 2 h so that the active sites of the catalyst in the
polymer cavity are easily accessible to the substrate and oxidant.
The products mainly obtained were benzil, methylbenzoate, ben-
zoic acid and benzaldehyde-dimethylacetal (Scheme 4).
Scheme 4 Oxidized products of benzoin: (a) benzil, (b) methyl benzoate,
(
c) benzoic acid and (d) benzaldehyde-dimethylacetal.
To optimise the reaction conditions for the maximum oxidation
of benzoin, the effect of oxidant was studied by considering
oxidant to substrate ratios of 2 : 1, 3 : 1 and 4 : 1 for the fixed
amount of catalyst (0.030 g) and substrate (1.06 g, 5 mmol) in
2
5 mL of refluxing methanol. At the oxidant to substrate ratio
of 3 : 1, maximum conversion of benzoin was achieved in 6 h of
reaction time.
Similarly, among four different amounts of catalysts e.g. 0.015,
Fig. 4 Time vs. product selectivity distribution plot for the conversion of
0
5
.030, 0.050 and 0.070 g for the fixed amount of benzoin (1.06 g,
mmol) and 30% H (1.7 g, 15 mmol) in 25 mL of methanol at
V
benzoin (a) using PS–[V O
2
(sal–his)] as a catalyst, (b) benzil, (c) benzoic
2
O
2
acid, (d) benzaldehyde-dimethylacetal, (e) methyl benzoate and (f) others
not identified).
the reflux temperature, the oxidation of benzoin was slow for the
first 1.5 h with 0.015 g of catalyst, then reached 71.8% in ca. 6 h.
Increasing the catalyst amount to 0.030 g resulted in significant
improvement; here the conversion reached 91.2% within 6 h of
reaction time, followed by no further improvement with time.
Further increasing the amount of catalyst to 0.050 or 0.070 g
(
IV
The catalytic activity of PS–[V O(sal–his)(acac)] 1 is not so
good giving 83.4% conversion of benzoin, the selectivity of the
different products being nearly the same. The performance of
neat complexes using the same mole concentrations as used for
2
190 | Dalton Trans., 2009, 2185–2195
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Table 5 Percent conversion of benzoin and selectivities of various oxidation products
a
Product selectivity (%)
TOF/h-
1
Catalyst
Conv. (%)
a
b
c
d
Others
V
PS–[V O
2
(sal–his)] 2
91.2
76.4
83.4
70.7
30.5
27.8
31.2
29.5
8.1
13.2
5.8
22.5
19.6
23.4
20.2
37.0
36.8
37.5
35.2
1.9
2.6
2.1
2.3
15.4
12.2
14.1
11.3
V
[V O
2
(sal–his)] 4
IV
PS–[V O(sal–his)(acac)] 1
IV
[
V O(sal–his)(acac)] 3
12.2
a
For details of abbreviations see Scheme 4.
polymer-anchored complexes has also been studied under the
optimised conditions. Thus, 0.0048 g (0.016 mmol) of catalyst
V
[
5
V O
2
(sal–his)] 4 was added to a mixture of benzoin (1.10 g,
(1.7 g, 15 mmol) in 25 mL of methanol,
mmol) and 30% H
2
O
2
and the reaction products were analysed as a function of time. It
was observed that the neat catalyst is also very active and gave
7
6.4% conversion in 6 h of reaction time. Here, selectivity of the
various products varies in the order: methylbenzoate (36.8%) >
benzil (27.8%) > benzaldehyde-dimethylacetal (19.6%) > benzoic
acid (13.2%) i.e. the same order as obtained for the polymer-bound
catalyst. A maximum of 70.7% conversion has been obtained with
IV
[
V O(sal–his)(acac)] 3 with similar selectivity under the condi-
tions above. The turnover frequencies for the polymer-anchored
complexes are 14.1 (for 1) and 15.4 (for 2) and are higher than those
obtained for the non polymer-bound complexes. Moreover, the
easy removal of the polymer-bound catalysts makes them better
compared with their non polymer-bound counterparts.
Reactivity of non-polymer bound complexes with H
2
O
2
IV
Solutions of [V O(sal–his)(acac)] 3 in methanol are sensitive
towards addition of H
2
2
O , as monitored by electronic absorption
spectroscopy, yielding oxoperoxo species. Fig. 5 presents the
spectral changes observed for 3. Thus, the progressive addition
IV
of a dilute H
2
O solution in methanol to a solution of [V O(sal–
2
his)(acac)] 3 in methanol results first in flattening of the band
appearing at 776 nm; upon further addition of one drop portions of
the H
2
2
O solution this band disappears. The intensity of the 532 nm
band slowly increases, while the band at 382 nm gradually shifts
to 394 nm. At the same time new bands appear at 319 and 257 nm,
while the intensity of the 265 nm band also increases. These
IV
changes indicate the interaction of [V O(sal–his)(acac)] with H
2
O
2
in methanol. Thedisappearance of d–d bands is inaccordance with
IV
the oxidation of the V O complex to an oxidoperoxovanadium(V),
and the band appearing at ca. 425 nm is probably due to a LMCT
band of the monoperoxo complex.
The spectral changes during a similar titration of 4 with
H
2
O
2
in methanol is shown in Fig. S3 of the ESI†. With low
amounts of H added no appreciable changes in band positions
were observed, but further additions of H yielded a final
spectrum which is very similar to that obtained in the titration
2
O
2
Fig.
5
UV-Vis spectral changes observed during titration of
2
O
2
[
VO(sal–his)(acac)] 3 with H
2
O
2
. (a) The spectra were recorded after
-4
successive additions of one drop portions of H
0% H
2
O
2
(6.6 ¥ 10 mmol of
IV
of [V O(sal–his)(acac)] 3 with H
the same oxidoperoxovanadium(V) species form upon addition
of H to methanolic solutions of either 3 or 4. Upon further
additions of H (2 mmol of 30% H dissolved in 5 mL of
2
O
2
, thus demonstrating that
-3
3
2
O
2
dissolved in 10 mL of methanol) to 50 mL of ca. 10
M
solution of 3 in methanol. (b) The equivalent titration, but with lower
IV
-4
2
O
2
concentrations of a [V O(sal–his)(acac)] 3 solution (ca. 10 M); the inset
2
O
2
2
O
2
shows an enlargement of the 300–500 nm region.
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methanol), the band appearing at 322 nm slowly disappears
with band tailing, possibly due to the formation of a diperoxo
species.
Stepwise addition of 2 equiv. of H
2
2
O (0.5, 1, 1.5 and 2 equiv)
to a methanolic solution of 3 leads to slight increases in A
ꢀ
4
-1
from 161.8 to 164 ¥10 cm , suggesting some change in the
coordination/solvation environment of 3 (Fig. 6). Simultaneously
the intensity of the spectrum decreases, and after the addition of
the 2 equiv of H
2
O the EPR intensity becomes ca. 1/5 that of
2
5
1
the initial solution. The V NMR of these solutions confirmed
the presence of compound 4 (and also 5, see below). Subsequent
addition of 2 equiv. of methyl phenyl sulfide gave spectra with the
same values of parameters g and A, the EPR signal increasing to
~
50% of that of the initial solution, indicating the reversibility of
the redox process occurring during the catalytic reaction.
51
V
Fig.
(
(
7
V
NMR spectra for [V O
b) 0.5 equiv. H ; (c) 1.5 equiv. H
e) 2.0 equiv. methyl phenyl sulfide; (f) after 20 h. All spectra were recorded
2
(sal–his)] 4: (a) in MeOH,
2
O
2
2 2
O ; (d) 1.0 equiv. methyl phenyl sulfide;
including an external reference of aqueous vanadate at pH ~12 (peak at
ca. -536 ppm).
Fig. 6 Treatment of compound 3 with 30% H
2 2
O followed by the addition
of methyl phenyl sulfide; (a) in MeOH; (b) 0.5 equiv. H
2
O
2
; (c) 2.0 equiv.
2 2
H O ; (d) 1.0 equiv. methyl phenyl sulfoxide; (e) 2.0 equiv. methyl phenyl
sulfoxide; (f) 2.0 equiv. methyl phenyl sulfoxide (after 20 h).
V
(sal–his)]-
bisperoxovanadium imidazole monoanion, [V O(O
1
2
)
2
2
0 (see Scheme 5), with the sp nitrogen atom of imidazole
coordinated to the vanadium center, which is consistent with
previously reported chemical shifts for similar coordination
V
51
The V system was also investigated by using V NMR
spectroscopy to detect intermediate species formed during the
catalytic cycle with both 3 and 4, choosing the oxidation of methyl
phenyl sulfide as a model reaction. Complex 4 in methanolic
37,38,39a
modes.
of H VO
On further addition of 0.5 equiv. portions of H
Another possibility for this peak is the formation
(
3-x)- 39b
x
2
(O
2
)
2
.
solution (3 mM) shows one strong resonance at d = -547 ppm
2
O three new
2
V
which we assign to [V O
2
(sal–his)] 4. This solution displays
signals at d = -715 ppm (11), -668 ppm (12) and -648 ppm (13)
are detected, which increase in intensity (Fig. S4 of the ESI†)
and correspond to various types of peroxo complexes (tentative
assignments are presented in Scheme 5).
The addition of methyl phenyl sulfide to the same reaction
mixture resulted in the disappearance of the peaks of 6, 10, 11,
12 and 13 (Fig. S4 and Scheme 5). The fact that a V NMR
spectrum similar to the initial one (the solution of 4 in MeOH) is
now obtained, confirms the reversibility of the processes involving
complex 4, namely the regeneration of 4 after the addition of
another minor (2.3%) signal at d = -558 ppm (Fig. 7a). Both values
V
37
are expected for V O complexes having a O/N donor set. We
2
tentatively assign the -558 ppm peak to the dioxidovanadium(V)
V
species [V O
2
(sal–his)(MeOH or H
2
O)] 5.
Upon successive additions of 30% H
relative intensity of the d = -547 ppm resonance decreases, and
after the addition of 1.5 equiv. of H , a peak is detected at
2
O
2
(0.5 equiv. steps) the
5
1
2
O
2
V
d = -579 ppm, which we assign to [V O(O)
2
(sal–his)] 6. These
observations are in agreement with the UV-Vis data discussed
above.
H
2
O , formation of an oxidoperoxo complex and its reaction with
2
This reaction mixture was divided into two portions. In the
first portion, upon addition of methyl phenyl sulfide, the peroxo-
vanadium(V) species 6 disappears immediately, with concomitant
production of 4 (Fig. 7e), indicating that 6 is one of the relevant
the sulfide.
Solutions of 4 in methanol are also sensitive to pH changes,
5
1
which were also monitored by V NMR spectroscopy. Addition
of 1 equiv. of HCl to a methanolic solution of 4 resulted in a
reduction in intensity of the d = -547 ppm and d = -558 ppm
peaks, while a new signal at d = -524 ppm was detected (Fig. S5
of the ESI†). Further addition of 1 equiv. of HCl gave a spectrum
with only one intense signal at d = -524 ppm. We assign this
5
1
species in the reaction with the sulfide. The final V NMR
spectrum was identical to the initial spectrum of 4, but the global
5
1
intensity of the V NMR signals decreased.
The second portion of the reaction mixture (see Fig. S4 of the
ESI†) after ca. 20 h gave only one intense signal corresponding
to 4, indicating that formation of monoperoxovanadium(V) was
to the protonation of the imidazole N atom, the sal–his ligand
V
becoming bidentate and the solvent also coordinating [V O
2
(sal-
reversible. Addition of 0.5 equiv. of H
2
O
2
regenerated the signal
(sal–his)] 6, along with a new signal at
d = -729 ppm. We tentatively assign this signal to the formation of
Hhis)(MeOH)] (d = -524 ppm) 7 (see Scheme 5). A rather similar
V
21 IV
corresponding to [V O(O)
2
explanation was given by Pecoraro and coworkers for the V O
with the same ligand system.
2
192 | Dalton Trans., 2009, 2185–2195
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51
Scheme 5 Proposed reaction scheme (see text) and tentative assignments of the V NMR chemical shifts involving oxidovanadium(V)-, dioxidovana-
51
dium(V)-, monoperoxovanadium(V)- and bisperoxovanadium(V)-species formed in methanolic solutions of 3 and 4 based on V NMR spectroscopy, on
addition of aqueous H , HCl and methyl phenyl sulfide (this may cause reduction to 3). S indicates solvent.
2
O
2
As compound 3 is paramagnetic, no signal was observed by
V NMR spectroscopy when dissolved in methanol. However,
Mechanism of sulfide oxidation
5
1
V
It is well known that V -peroxo compounds mediate oxygenation
after addition of 0.5 equiv. of H
2
O
2
, three signals (d = -494, -547
reactions including the oxidation of sulfides to sulfoxides and
sulfones and the epoxidation of alkenes and allylic alcohols.
The sulfur atom of methyl phenyl sulfide is electron-rich
and undergoes electrophilic oxidation giving the sulfoxide. We
clearly demonstrated that complexes 3 and 4 are able to generate
monoperoxo and even bisperoxovanadium(V) species on treatment
and -558 ppm) appear (Fig. 8), and we tentatively assign them to
40
V
V
[
(
5
V O(OH)(sal-Hhis)(MeOH)] (d = -494 ppm) 8, [V O
2
(sal–his)]
V
d = -547 ppm) 4 and {[V O
2
(sal–his)(MeOH)] (d = -558 ppm)
(Fig. 8). Upon further addition of 0.5 equiv. of H
2
2
O another
three signals at: d = -579, -524 and -572 ppm were detected,
V
which we tentatively assign as [V O(O
2
)(sal–his)] (d = -579 ppm)
V
V
with H
is likely that hydroperoxovanadium(V) complexes also form as
aqueous H is being added and the pH decreases, enhancing the
2
2
O . The peroxo complexes being stable and detectable, it
6
, {[V O
2
(sal-Hhis)(MeOH)] (d = -524 ppm) 7 and [V O(O
2
)(sal-
+
Hhis)] (d = -572 ppm) 9. Further addition of 0.5 equiv. portions
2
O
2
of H
above), and [VO(O
2
O
2
resulted in the formation of the peak at -729 ppm (see
-
electrophilicity of the coordinated peroxo ligand and facilitating
the nucleophilic attack by the sulfide. An outline of the catalytic
cycle for oxidation of methyl phenyl sulfide, which has also been
2
)
2
(H
2
O)(MeOH)] (d = -648 ppm), 13.
Upon addition of methyl phenyl sulfide (spectra h–j in Fig. 8),
the monoperoxovanadium(V) and bisperoxovanadium(V) species
21,27,41
5
1
proposed by other authors
is given in Scheme 6.
are consumed, and the final V NMR spectrum showed only
two major signals identical to the initial spectrum of methanolic
solutions of 4, showing that the V catalyst is regenerated after
V
Conclusions
the consumption of H
2
O
2
. Besides having an easily measurable
5
1
V NMR spectrum, for this solution a reasonably intense EPR
The compound Hsal–his I derived from salicylaldehyde and
histamine has been covalently bonded to chloromethylated
polystyrene cross-linked with 5% divinylbenzene. Upon reaction
spectrum was also recorded. This confirmed the formation of 3,
and even after 5 d a similar EPR spectra could be obtained.
In Scheme 5 we summarize our observations regarding the
reaction processes involving 3 and 4 in MeOH solutions, as
IV
IV
with [V O(acac)
2
] the complex PS–[V O(sal–his)(acac)] 1 was
obtained, which, upon oxidation, yielded the dioxidovanadium(V)
5
1
V
measured by V NMR and EPR spectroscopy.
PS–[V O
2
(sal–his)] 2. The corresponding non polymer-bound
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formed upon addition of H
2
O
2
and/or acid, similar oxido, peroxo
5
1
or peroxo species being detected by V NMR spectroscopy
starting either with solutions of 3 or of 4. Addition of methyl
phenyl sulfide to solutions containing peroxo species promoted the
IV
oxidation of the sulfide and regenerated the formation of the V
species formed upon dissolution of 4 or oxidation of 3. The EPR
IV
spectra also confirm that [V O(sal–his)(acac)] 3 is also present in
these solutions if the starting product is complex 3. These studies
confirm that most of these reactions involving complexes 3 or 4 are
reversible, therefore supporting the catalytic nature of the sulfide
oxidation processes.
Acknowledgements
Department of Science and Technology, Government of India,
New Delhi is gratefully acknowledged for financial support of
the work. A.A. thanks IITR for the MHRD fellowship. The
authors also wish to thank FEDER, Funda c¸ a˜ o para a Ci eˆ ncia
e a Tecnologia, POCI 2010 (namely PPCDT/QUI/55985/
2
004, PPCDT/QUI/56946/2004, and SFRH/BPD/34835/2007
programs).
5
1
IV
Fig. 8
V NMR spectra for [V O(sal–his)(acac)] 3: (a) in MeOH,
b) 0.5 equiv. H ; (c) 1.0 equiv. H ; (d,e) 1.5 equiv. H
f) 2.0 equiv. H ; (g) 2.5 equiv. H ; (h) 1.0 equiv. methyl phenyl sulfide;
References
(
(
(
(
2
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IV
V
complexes [V O(sal–his)(acac)] 3 and [V O (sal–his)] 4 have also
been prepared and characterised. Complexes 1 and 2 have been
used as catalysts for the oxidation of methyl phenyl sulfide,
diphenyl sulfide and benzoin with aqueous H
2
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