Oxidation of p-chlorotoluene and cyclohexene catalysed by polymer-anchored oxovanadium(iv) and copper(ii) complexes of amino acid derived tridentate ligands

Article abstract of DOI:10.1039/b804823a

3-Formylsalicylic acid (Hfsal), covalently bound to chloromethylated polystyrene (PS) and cross-linked with 5% divinylbenzene reacts with d,l-alanine and l-isoleucine to give the Schiff-base tridentate ligands PS-H ₂fsal-d,l-Ala and PS-H₂fsal-l-Ile, respectively. These anchored ligands upon reaction with VOSO₄ and Cu(CH ₃COO)₂?H₂O form the complexes PS-[VO(fsal-d,l-Ala)(H₂O)], PS-[Cu(fsal-d,l-Ala)(H₂O)], PS-[VO(fsal-l-Ile)(H₂O)] and PS-[Cu(fsal-l-Ile)(H₂O)]. The structures of these immobilized complexes have been established on the basis of scanning electron micrographs, spectroscopic (infrared, electronic and EPR), thermogravimetric and elemental analysis studies. The oxidation of p-chlorotoluene and cyclohexene has been investigated using these complexes as the catalysts in the presence of H₂O₂ as the oxidant. Reaction conditions have been optimised by considering the concentration of the oxidant, the amount of catalyst used and the temperature of the reaction mixture. Under the optimised conditions, p-chlorotoluene gave a maximum of 14% conversion using PS-[VO(fsal-d,l-Ala)(H₂O)] as the catalyst, with the main products having a selectivity order of: p-chlorobenzaldehyde >> p-chlorobenzylalcohol > p-chlorobenzoic acid > 2-methyl-5-chlorophenol > 3-methyl-6-chlorophenol. The oxidation of cyclohexene with PS-[VO(fsal-d,l-Ala)(H₂O)] proceeds with 79% conversion, which is followed by PS-[VO(fsal-l-Ile)(H₂O)] with 77% conversion, and the oxidation of cyclohexene by Cu-based catalysts occurs with considerably lower conversions (29-32%). The selectivity of the products follows the order: 2-cyclohexene-1-ol > cyclohexene oxide > cyclohexane-1,2-diol > 2-cyclohexene-1-one. Recycling studies indicate that these catalysts can be reused at least three times without any significant loss in their catalytic potential. However, EPR studies indicate that while the polymer supported V(iv)O-complexes do not change after being used, the EPR spectra of the Cu-complexes show significant changes. The corresponding non-polymer bound complexes [VO(fsal-d,l-Ala)(H₂O)], [Cu(fsal-d,l-Ala)(H ₂O)], [VO(fsal-l-Ile)(H₂O)] and [Cu(fsal-l-Ile)(H ₂O)] have also been prepared in order to compare their spectral properties and catalytic activities. The non-polymer bound complexes exhibit lower conversion, along with lower turn-over frequency as compared to their polymer-bound analogues. Several EPR, ⁵¹V NMR and UV-vis studies have been undertaken to detect the intermediate species, and outlines for the mechanisms of the catalytic reactions are proposed.

Full text of DOI:10.1039/b804823a

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Oxidation of p-chlorotoluene and cyclohexene catalysed by polymer-anchored
oxovanadium(IV) and copper(II) complexes of amino acid derived tridentate
ligands†
Mannar R. Maurya,*a Maneesh Kumar,a Amit Kumarb and Joa˜o Costa Pessoa*b
Received 20th March 2008, Accepted 2nd June 2008
First published as an Advance Article on the web 11th July 2008
DOI: 10.1039/b804823a
3-Formylsalicylic acid (Hfsal), covalently bound to chloromethylated polystyrene (PS) and cross-linked
with 5% divinylbenzene reacts with D,L-alanine and L-isoleucine to give the Schiff-base tridentate
ligands PS–H2fsal–D,L-Ala and PS–H2fsal–L-Ile, respectively. These anchored ligands upon reaction
with VOSO4 and Cu(CH3COO)2·H2O form the complexes PS–[VO(fsal–D,L-Ala)(H2O)],
PS–[Cu(fsal–D,L-Ala)(H2O)], PS–[VO(fsal–L-Ile)(H2O)] and PS–[Cu(fsal–L-Ile)(H2O)]. The structures
of these immobilized complexes have been established on the basis of scanning electron micrographs,
spectroscopic (infrared, electronic and EPR), thermogravimetric and elemental analysis studies. The
oxidation of p-chlorotoluene and cyclohexene has been investigated using these complexes as the
catalysts in the presence of H2O2 as the oxidant. Reaction conditions have been optimised by
considering the concentration of the oxidant, the amount of catalyst used and the temperature of the
reaction mixture. Under the optimised conditions, p-chlorotoluene gave a maximum of 14% conversion
using PS–[VO(fsal–D,L-Ala)(H2O)] as the catalyst, with the main products having a selectivity order of:
p-chlorobenzaldehyde >> p-chlorobenzylalcohol > p-chlorobenzoic acid > 2-methyl-5-chlorophenol >
3-methyl-6-chlorophenol. The oxidation of cyclohexene with PS–[VO(fsal–D,L-Ala)(H2O)] proceeds
with 79% conversion, which is followed by PS–[VO(fsal–L-Ile)(H2O)] with 77% conversion, and the
oxidation of cyclohexene by Cu-based catalysts occurs with considerably lower conversions (29–32%).
The selectivity of the products follows the order: 2-cyclohexene-1-ol > cyclohexene oxide >
cyclohexane-1,2-diol > 2-cyclohexene-1-one. Recycling studies indicate that these catalysts can be
reused at least three times without any significant loss in their catalytic potential. However, EPR studies
indicate that while the polymer supported V(IV)O-complexes do not change after being used, the EPR
spectra of the Cu-complexes show significant changes. The corresponding non-polymer bound
complexes [VO(fsal–D,L-Ala)(H2O)], [Cu(fsal–D,L-Ala)(H2O)], [VO(fsal–L-Ile)(H2O)] and
[Cu(fsal–L-Ile)(H2O)] have also been prepared in order to compare their spectral properties and
catalytic activities. The non-polymer bound complexes exhibit lower conversion, along with lower
turn-over frequency as compared to their polymer-bound analogues. Several EPR, 51V NMR and
UV-vis studies have been undertaken to detect the intermediate species, and outlines for the
mechanisms of the catalytic reactions are proposed.
vanadium and molybdenum, and a number of these complexes
Introduction
are good oxidants. Namely, vanadium peroxide complexes are
Vanadium and copper are important elements in biology and
effective oxidants in non-protic solvents under mild conditions
in inorganic and organic synthesis, and can heterogeneously
and not only catalyse the epoxidation of olefins, but also hydrox-
or homogeneously catalyse the selective oxidations of alkanes,
ylate aromatic hydrocarbons and alkanes to yield alcohols and
alkenes, arenes, alcohols, halides, sulfides, etc.1–3 Hydrogen perox-
ide is a clean and readily available oxidant and is, together with
molecular oxygen, one of the preferred oxidants for laboratory
or industrial reactions. The electron-rich peroxide group easily
forms complexes with metal ions of low dn configurations, such as
ketones.4
Immobilisation of the complexes in solid supports often presents
advantages for industrial applications, and the catalytic potentials
of polymer-immobilised metal complexes in organic transforma-
tions have been reviewed in detail by several groups.5–8 This type
of immobilisation of active metal complexes has evolved as a
promising strategy for combining the advantages of homogeneous
as well as heterogeneous catalysts, due to their easy separation
from the products by simple filtration, and to meet the industrial
demand of recyclability for continuous operation.9,10
aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee-
247667, India. E-mail: rkmanfcy@iitr.ernet.in
bCentro Qu´ımica Estrutural, Instituto Superior Te´cnico, TU Lisbon, Av Ro-
visco Pais, 1049–001 Lisboa, Portugal. E-mail: pcjpessoa@popsrv.ist.utl.pt
† Electronic supplementary information (ESI) available: EDAX, IR spec-
tral data, spin Hamiltonian parameters, 51V NMR chemical shifts and
electronic spectra. For ESI see DOI: 10.1039/b804823a
Amino acids immobilised on Merrifield resin were reported
by Petit and Jozefonvicz, and later by Valodker et al.11,12 Due
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to the presence of the reactive amino, as well as carboxylic acid
groups, special reagents were required to achieve immobilisation
either through the amino group or the carboxylato group. We
have simplified the procedure of immobilisation by reacting
chloromethylated polystyrene with 3-formylsalicylic acid in the
first step followed by condensation of an amino acid with immo-
bilised 3-formylsalicylic acid.13 In this way the ligand obtained has
more coordination sites available to bind to the metal ion, which in
turn may improve the catalytic activity and turn-over rates for the
catalysts. As a continuation of our work on the development of
such heterogeneous catalysts we recently reported PS–[VO(fsal–
b-Ala)(DMF)] (where H2fsal–b-Ala = Schiff base derived from
3-formylsalicylic acid and b-alanine, PS = polystyrene, DMF =
dimethylformamide) immobilised on polystyrene and explored for
their catalytic potential in the oxidation of a series of organic sub-
strates. We report here the immobilisation of oxovanadium(IV) and
copper(II) complexes of the dibasic tridentate oxygen and nitrogen
donor ligands, H2fsal–D,L-Ala and H2fsal–L-Ile (Scheme 1), along
with their catalytic activities for the oxidation of p-chlorotoluene
and cyclohexene.
complexes. In this work, we therefore explore both the adequate
capacity of V and Cu complexes to catalyse oxidation reactions
and the rich information that may be obtained from their EPR
spectra.
Experimental
Materials and methods
V2O5 (S.D. Fine Chemicals, India), cupric acetate monohydrate,
VOSO4·5H2O, salicylic acid, hexamine (Loba Chemie, India),
benzene, barium chloride, sodium hydroxide (Ranbaxy, India),
D,L-alanine and L-isoleucine (Spectrochem, India) were used as
obtained. Chloromethylated polystyrene [18.9% Cl, 5.3 mmol Cl
per gram of resin] having 5% cross-linking with divinylbenzene
was obtained as a gift from Thermax Limited, Pune, India. 3-
Formylsalicylic acid was prepared according to a method reported
in the literature.28
Elemental analyses of the compounds were carried out using
an Elementar model Vario-El-III. An inductively-coupled plasma
spectrometer (ICP, Labtam 8440 plasmalab) was used for the
analysis of copper and vanadium after leaching the metal ions
with conc. nitric acid and diluting with distilled water (for copper),
or with a very dilute aqueous KOH solution (for vanadium) to a
specific volume in volumetric flasks. Thermogravimetric analyses
of the complexes were carried out using a Perkin-Elmer (Pyris
◦
Diamond) instrument in air with a rate of 10 C min−1. Field
Scheme 1
emission scanning electron microscopy (FE-SEM) and energy
dispersive X-ray analysis (EDAX) plots of anchored complexes
were recorded using a FEI Quanta 200 FEG microscope, The
Netherlands. Chloromethylated polystyrene with a known chlo-
rine content (see above) was used as a reference. The samples were
dusted on alumina and coated with a thin film of gold to make
the surface conductive, to prevent surface charging and to protect
the surface material from thermal damage by the electron beam.
IR spectra were recorded as KBr pellets on a Nicolet NEXUS
Aligent 1100 series FT-IR spectrometer. Electronic spectra of both
the ligands and complexes were recorded in nujol on a Shimadzu
1601 UV-visible spectrophotometer by layering the mull of the
sample on the inside of one of the cuvettes. The inside of the
other cuvette was layered with Nujol and used as the reference.
The EPR spectra were recorded using a Bruker ESP 300E X-band
spectrometer. For the samples containing the polymer-anchored
complex, the spectra were measured at room temperature and
also at 77 K after swelling in DMF; for the non-polymer-bound
complexes, the samples in either DMF or MeOH were frozen 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 by Rockenbauer and Korecz.29
The 51V NMR spectra were recorded on a Bruker Avance III 400
Mhz instrument. A Thermax Nicolet gas chromatograph with a
HP-1 capillary column (30 m × 0.25 lm × 0.25 lm) was used to
analyse the reaction products and their quantifications were made
on the basis of the relative peak area of the respective product.
Each catalytic experiment was repeated twice for the validity of
data. The identity of the products was confirmed using GC-MS,
model Perkin-Elmer Clarus 500 by comparing the fragments of
each product with the library that was available.
Selective oxidation of p-chlorotoluene (PCT) to p-
chlorobenzaldehyde (CBD) is an important reaction as CBD is a
key intermediate in the synthesis of a variety of fine chemicals,
such as pharmaceutical drugs, dyes, optical brighteners and
agricultural chemicals.14 Traditionally, p-chlorobenzaldehyde
is produced mainly from the chlorination of p-chlorotoluene,
which forms p-chlorobenzalchloride and is then subjected to acid
hydrolysis.15 It can also be produced by the hydrogenation of
p-chlorobenzonitrile.16 Industrial methods use the oxidation of p-
chlorotoluene catalysed by soluble transition metal acetates to give
a mixture of p-chlorobenzyl alcohol and p-chlorobenzaldehyde,
with further oxidation of p-chlorobenzyl alcohol also yielding
p-chlorobenzaldehyde.17–20 Vanadium silicate molecular sieves
also catalyse a variety of reactions but only a limited number
of literature citations have been reported for the liquid-phase
oxidation of p-chlorotoluene using the clean and green oxidant
H2O2.21,22
The catalytic epoxidation of unsaturated hydrocarbons rep-
resents an industrially important synthetic method for the
production of speciality and fine chemicals. Polymer-supported
transition metal complexes with Schiff-bases have been shown
to be active catalysts for the epoxidation of olefinic compounds
in the presence of hydrogen peroxide/tert-butyl hydroperoxide as
oxidants.23–26
Vanadium(IV) is a d1 metal ion and copper(II) has a d9
configuration, and in many spectroscopic studies they have been
compared using the hole-formalism.27 Both have one unpaired
electron and electron paramagnetic resonance is a particularly
good and relatively simple spectroscopic method to study their
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Preparations
Preparation of [VO(fsal–D,L-Ala)(H2O)] 5. A solution of 3-
formylsalicylic acid (1.66 g, 0.010 mol) dissolved in 10 mL of
water was added to a solution of D,L-alanine (0.89 g, 0.010 mol)
and KOH (0.56 g, 0.010 mol in 20 mL). The reaction mixture was
stirred for 2 h and a yellow solution was obtained. An aqueous
solution of vanadyl sulfate (2.53 g, 0.010 mmol in 30 mL) was
added slowly and digested on a water bath for 3 h. After reducing
the volume to ca. 10 mL and keeping at ca. 10 ◦C overnight, a
green solid separated out which was filtered, washed with diethyl
ether and dried in a vacuum desiccator. Yield 55%. (Found: C
41.02, H 3.54, N 4.26, V 15.62. Calcd for C11H11O7NV: C 41.25,
H 3.44, N 4.38, V 15.94%).
Preparation of PS–Hfsal. Chloromethylated polystyrene
(10 g) was allowed to swell in DMF (50 mL) for 2 h. A solution
of 3-formylsalicylic acid (16.6 g, 0.10 mol) in DMF (40 mL) was
added to the above suspension followed by triethylamine (15 g) in
ethylacetate (50 mL), and the reaction mixture was then heated
to 90 ◦C for 10 h whilst stirring continuously. After cooling to
room temperature, the yellowish resin was separated by filtration,
washed with hot DMF (3 × 5 mL) and followed by hot ethanol.
The whole process was repeated with 5 g of 3-formyl salicylic acid
and the PS–Hfsal thus obtained was dried in an air oven at ca.
120 ◦C.
Preparation of [Cu(fsal–D,L-Ala)(H2O)] 6. The ligand H2fsal–
D,L-Ala (0.010 mol) was prepared in situ as described in the prepa-
ration for 5, except an aqueous solution of Cu(CH3COO)2·H2O
(2.0 g, 0.010 mol in 20 mL) was added. The reaction mixture was
digested on a water bath for 3 h. After being kept at ambient
temperature, the green solid that had slowly separated out was
filtered, washed with water followed by diethyl ether and dried in
a vacuum desiccator. Yield: 62%. (Found: C 41.33, H 3.37, N 4.36,
Cu 19.84. Calcd for C11H11O6NCu: C 41.70, H 3.48, N 4.42, Cu
20.07%).
Preparation of PS–H2fsal–D,L-Ala III. The polymer-bound,
PS–Hfsal (5.0 g) was allowed to swell for 3 h in water (100 mL).
A solution of D,L-alanine (4.71 g, 0.053 mol) and KOH (2.96 g,
0.053 mol) in water (20 mL) was added to the above suspension
and th◦e reaction mixture was allowed to stir in an oil bath at
ca. 90 C for 12 h. After cooling to room temperature, the solid
resin beads were filtered, washed thoroughly with water followed
by ethanol and dried under vacuum. (Found: C 75.28, H 8.08, N
3.01%).
Preparation of PS–H2fsal–L-Ile IV. The PS–H2fsal–L-Ile was
synthesised similarly using PS–Hfsal (5.0 g) with L-isoleucine
(6.94 g, 0.053 mol). (Found: C 71.72, H 9.61, N 3.29%).
Preparation of [VO(fsal–L-Ile)(H2O)]
7 and [Cu(fsal–L-
Ile)(H2O)] 8. The ligand H2fsal–L-Ile was prepared in situ as
stated above. Complexes of 7 and 8 were prepared in about 60%
yield, which was similar to 5 and 6, respectively, replacing H2fsal–
D,L-Ala with H2fsal–L-Ile. Complex 7: (found: C 46.61, H 4.82, N
3.66, V 13.81. Calcd for C14H17O7NV: C 46.42, H 4.73, N 3.87, V
14.06%). Complex 8: (found: C 46.93, H 4.87, N 3.82, Cu 17.46.
Calcd for C14H17O6NCu: C 46.86, H 4.78, N 3.90, Cu 17.71%).
Elemental analysis data for the ligands and complexes are
presented in the experimental section. The metal and ligand
loading in polymer-anchored complexes is presented in Table 1.
Preparation of PS–[VO(fsal–D,L-Ala)(H2O)] 1. The polymer-
anchored ligand PS–H2fsal–D,L-Ala (2.5 g) was allowed to swell
in methanol (30 mL) for 2 h. Vanadyl sulfate (6.70 g, 0.0265 mol)
dissolved in 30 mL of methanol/water (7:3) was added to the
above suspension and the reaction mixture was refluxed in an oil
bath with continuous mechanical stirring for 70 h. The greenish
resin beads were separated by filtration, washed thoroughly with
◦
hot methanol and dried in an air oven at ca. 120 C. (Found: C
65.55, H 7.58, N 1.82, V 5.43%).
Preparation of PS–[Cu(fsal–D,L-Ala)(H2O)] 2. A methanolic
solution of Cu(CH3COO)2·H2O (5.0 g, 0.025 mol in 20 mL) was
added to a suspension of PS–H2fsal–D,L-Ala (2.5 g) in methanol
(30 mL) as described above, and the reaction mixture was refluxed
in an oil bath for 72 h. After cooling to ambient temperature, the
coloured resin beads were filtered, washed with hot methanol and
dried in an air oven at ca. 120 ◦C. (Found: C 68.64, H 7.09, N 1.76,
Cu 6.47%).
Catalytic activity studies
All catalysts were allowed to swell in acetonitrile for 2 h prior to
their use for the catalytic reactions.
Oxidation of p-chlorotoluene. In a typical reaction, an aqueous
30% H2O2 solution (2.27 g, 0.020 mol), p-chlorotoluene (1.126 g,
010 mol) and the catalyst (0.025 g) were mixed in CH3CN (15 mL),
and the reaction mixture was heated at 80 ◦C with continuous
stirring in an oil bath. The progress of the reaction was monitored
as a function of time by withdrawing portions of the sample and
analyzing it quantitatively by gas chromatography. The identities
of the products were confirmed by GC-MS.
Preparation of PS–[VO(fsal–L-Ile)(H2O)] 3 and PS–[Cu(fsal–
L-Ile)(H2O)] 4. Complexes 3 and 4 were prepared similarly as
described for 1 and 2, respectively. Compound 3: (found: C 66.38,
H 7.85, N 1.56, V 4.56%). Compound 4: (found: C 67.77, H 9.03,
N 1.86, Cu 6.16%).
Table 1 Analytical data for polymer-loading of both the ligand and complexes
Compounds
Ligand loading/mmol g−1 resin
Metal ion loading/mmol g−1 resin
Ligand : metal ratio
PS–H2fsal–D,L-Ala
2.15
1.32
1.26
2.35
1.11
1.33
—
—
PS–[VO(fsal–D,L-Ala)(H2O)]
PS–[Cu(fsal–D,L-Ala)(H2O)]
PS–H2 fsal–L-Ile
PS–[VO(fsal–L-Ile)(H2O)]
PS–[Cu(fsal–L-Ile)(H2O)]
1.06
1.01
—
0.89
0.97
1.2 : 1
1.2 : 1
—
1.2 : 1
1.3 : 1
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Oxidation of cyclohexene. The catalytic oxidation of cyclohex-
ene was also carried out using all catalysts. Cyclohexene (0.82 g,
0.010 mol), aqueous 30% H2O2 (2.27 g 0.020 mol) and the catalyst
(0.025 g) were mixed in acetonitrile (20 mL), and the reaction
mixture was heated at 80 ◦C with continuous stirring. The reaction
products were analysed and identified as mentioned above.
have been established and reported in the literature.30–33 Scheme 3
presents the proposed structures of the anchored complexes,
isolated and characterised by IR, UV-vis, EPR, elemental analysis,
FE-SEM, EDAX and thermogravimetric studies.
Results and discussion
The synthesis of the polymer-anchored ligands involved two steps.
In the first step, chloromethylated polystyrene, cross-linked with
5% divinylbenzene reacted with 3-formylsalicylic acid in the pres-
ence of triethylamine in ethylacetate to give the polymer-anchored
3-formylsalicylic acid, PS–Hfsal. During the reaction, the COOH
group of 3-formylsalicylic acid reacts with the CH2Cl group
of the chloromethylated polystyrene. The second step involves
the reaction between PS–Hfsal and D,L-alanine or L-isoleucine
in the presence of potassium hydroxide to give the polymer-
anchored ligands, PS–H2fsal–D,L-Ala (III) and PS–H2fsal–L-Ile
(IV). The whole synthetic procedure considering PS–H2fsal–D,L-
Ala (III) as a representative example is presented in Scheme 2. The
loading of the ligands was confirmed from estimating the amount
of chlorine that remained present after forming the anchored
ligands. Thus, the remaining chlorine content of 0.5 and 0.4%
determined gravimetrically in PS–H2fsal–D,L-Ala and PS–H2fsal–
L-Ile, respectively suggests a nearly quantitative loading of ligands.
The polymer-anchored ligands III and IV reacted with VOSO4
in methanol–water and formed the polymer-bound oxovana-
dium(IV) complexes PS–[VO(fsal–D,L-Ala)(H2O)] (1) and PS–
[VO(fsal–L-Ile)(H2O)] (3). Similarly, complexes PS–[Cu(fsal–D,L-
Ala)(H2O)] (2) and PS–[Cu(fsal–L-Ile)(H2O)] (4) were isolated
from the reaction of III and IV with Cu(CH3COO)2. The synthetic
procedures, choosing fsal-L-Ile as an example, are represented by
eqn (1) and (2).
Scheme 3 Proposed structures for the polymer-anchored complexes: R =
–CH3 (D,L-Ala); R = –CH(CH2CH3)CH3 (L-Ile).
Field emission scanning electron microscope and energy dispersive
X-ray analysis studies
A comparative study using field emission scanning electron mi-
croscopy along with energy dispersive X-ray analysis was carried
out taking single beads of pure chloromethylated polystyrene,
polymer-anchored ligands and polymer-anchored metal com-
plexes. As expected, the pure polymer beads have a smooth and
flat surface and show two main components; carbon (80.7%) and
chlorine (18.3%) on the surface, as evaluated semi-quantitatively
by energy dispersive X-ray analysis. The introduction of a ligand
onto the polystyrene beads through covalent bonding causes a
light roughening of the top layer of beads. By EDAX analysis,
considerable amounts of N and O, and a low amount of Cl (ca.
0.3%, see Table S1 of ESI†) were determined on the surface of
the beads containing bound ligands. This indicates their nearly
quantitative loading. Images of the metal complexes’ polymer
beads show further roughening of the top layer, which is probably
due to the interaction of the metal ion with the ligand to
accommodate the fixed geometry of the complex. EDAX analysis
of these beads show a metal content along with C, N, O and Cl,
suggesting the formation of metal complexes with the anchored
ligand at various sites. The EDAX profiles of representative
samples are presented in Fig. 1.
PS–H2fsal–L-Ile + VOSO4 + 2KOH
→ PS–[VO(fsal–L-Ile)(H2O)] + K2SO4 + 2H2O
(1)
PS–H2fsal–L-Ile + Cu(CH3COO)2
→ PS–[Cu(fsal–L-Ile)(H2O)] + 2CH3COOH
(2)
Thermogravimetric study
The metal and ligand loading in the polymer-bound complexes
presented in Table 1 suggest a ligand to metal loading stoichiom-
etry of ca. 1 : 1 in all cases. This indicates that most of the
immobilised ligand is involved in the coordination to the metal
ion.
Similarly, the reaction of VOSO4 and Cu(CH3COO)2 with
equimolar amounts of ligands I and II in methanol/water yields
the non-polymer bound complexes [VO(fsal–D,L-Ala)(H2O)]
(5), [Cu(fsal–D,L-Ala)(H2O)] (6), [VO(fsal–L-Ile)(H2O)] (7) and
[Cu(fsal–L-Ile)(H2O)] (8). The structures of similar complexes
The thermal analyses of polymer-anchored metal complexes were
carried out in an air atmosphere with a heating rate of 10 oC min−1.
The polymer-anchored metal complexes are generally thermally
stable up to ca. 225 ◦C except for the small weight loss (ca.
2%) between 100–225 ◦C due to the loss of water. Thereafter,
they decompose in several small fragments with exothermic
weight losses at higher temperature. Both vanadium and copper
based catalysts decompose in the temperature range of 225–
500 oC. Quantitative measurement of weight losses at various
Scheme 2 Reaction scheme for the synthesis of polymer-bound complexes.
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Table 2 Electronic spectral data for the ligands and the complexes
Compounds
kmax/nma
PS–H2fsal–D,L-Ala
271, 241, 220
280, 221
277, 236, 219
275, 241, 224
274, 239, 222
277, 239, 224
772(s), 543, 358, 301, 258, 236, 210
660(s), 356, 302, 254, 231, 209
770(s), 530, 358, 303, 255, 237, 209
655(s), 304, 234, 209
PS–[VO(fsal–D,L-Ala)·H2O]
PS–[Cu(fsal–D,L-Ala)·H2O]
PS–H2fsal–L-Ile
PS–[VO(fsal–L-Ile)·H2O]
PS–[Cu(fsal–L-Ile)·H2O]
[VO(fsal-D,L-Ala)·H2O]
[Cu(fsal–D,L-Ala)·H2O]
[VO(fsal–L-Ile)·H2O]
[Cu(fsal–L-Ile)·H2O]
a s = shoulder band.
[VO(fsal–L-Ile)(H2O)] exhibit sharp bands at 984 and 955 cm−1,
=
respectively, due to m(V O). The non-polymer bound vanadium
and copper complexes display these spectral bands close to that of
the anchored analogues.
Electronic spectral study
The electronic spectra of the non-polymer-bound ligands and their
complexes were recorded in methanol while that of the polymer-
anchored ligands and their metal complexes were recorded in a
nujol mull. These spectra are shown in Fig. S1, left (see ESI)†
and the spectral data are presented in Table 2. Ligands I and
II exhibit three spectral bands at 215, 240 and 275 nm in the
UV region which are assigned to ␾ → ␾*, p → p* and n →
p* transitions, respectively. For the complexes, the first two bands
shift towards lower wavenumbers, with two other bands appearing
at ca. 255 and 300 nm in 5, 6 and 7. In addition, a new band of
medium intensity appears at ca. 360 nm, which may be assigned
either to a ligand to metal charge transfer band, or to a p → p*
transition originating mainly in the azomethine chromophore.32
The expected d–d bands are observed only in methanolic solutions
of higher concentrations of non-polymer bound complexes. The
spectroscopic patterns shown by the polymer-anchored complexes
(Fig. S1, right of ESI)† are essentially similar to those of the
non-polymer-bound complexes, except for the broadness of the
bands in nujol and only one band being observed for the n→p*
transition. The bands for lower energy (less intense) could not be
observed in the polymer-anchored complexes due to their poor
loading in the polymer matrix.
Fig.
1
EDAX profiles of (a) chloromethylated polystyrene, (b)
PS–H2fsal–D,L-Ala and (c) PS–[VO(fsal–D,L-Ala)(H2O)].
stages was not possible due to the overlapping nature of the
decompositions. However, the final residues of 5.2% (1), 6.7%
(2) 5.0% (3) and 6.3% (4) suggest the formation of the respective
metal oxides, CuO or V2O5.
IR spectral study
A partial list of IR spectral data for the ligands and the complexes
are listed in Table S2 of the ESI.† The absence of characteristic
strong bands that appear at 1264 and 673 cm−1 in chloromethylated
polystyrene due to the CH2Cl groups suggests the formation of an
ester linkage by the reaction of chloromethyl group of styrene
and the COOH group of 3-formylsalicyclic acid.34 This is further
supported by the appearance of a new band at 1674 cm−1 due to
the m(C O); the free carboxylic group of 3-formylsalicylic acid
exhibits m(C O) at 1663 cm . The IR spectra of the polymer-
anchored ligands exhibit a sharp band at 1630 cm−1 due to m(C N)
(azomethine group), and this band shifts to lower wavenumbers
and appears at 1600–1608 cm−1, suggesting the coordination of
the azomethine nitrogen to the metal.32,33 The coordination of
the phenolic oxygen could not be ascertained unequivocally as
the band at ca. 3400 cm−1 due to coordinated water interferes in
this region. The complexes PS–[VO(fsal–D,L-Ala)(H2O)] and PS–
EPR studies
The EPR spectra for powdered samples of fresh and used polymer-
anchored V(IV)O and Cu(II) complexes were recorded at room
temperature. After swelling the polymer-anchored V(IV)O and
Cu(II) complexes with DMF for ca. 1 h, these were ‘frozen’ in
liquid nitrogen and their EPR spectra were also recorded and
these powdered samples, in contact with the solvent, often give
better-resolved EPR spectra, but in the present case no significant
improvement in the resolution was obtained. Fig. 2 presents the
EPR spectra for samples of PS–[VO(fsal–D,L-Ala)(X)] (1) (X =
H2O and/or DMF). The spectra recorded for PS–[VO(fsal–L-
Ile)(X)] (3) (X = H2O and/or DMF) are also similar. The EPR
spectra of 1 and 3 are characteristic of magnetically diluted
=
−1 35
=
=
4224 | Dalton Trans., 2008, 4220–4232
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Fig. 2 Powder EPR spectra for samples of catalyst PS–[VO(fsal–
D,L-Ala)(X)] 1. Black (thick): 1 at room temperature; grey (thick): 1 after
being used in the catalytic reaction, at room temperature; black (thin): 1 in
contact with DMF at 77 K; grey (thin): 1 after being used in the catalytic
reaction, in contact with DMF at 77 K.
V(IV)O complexes and the resolved EPR pattern indicates that
the vanadium centres are well dispersed in the polymer matrix.
The spectra were simulated and the spin Hamiltonian param-
eters obtained25 for the vanadium containing samples and are
included in Table 3. All the EPR spectra are compatible with the
expected binding mode (see Scheme 3), i.e. (O−Phe, Nimine, COO−,
H2O or DMF)equatorial, (H2O or DMF)axial, and with the EPR spectra
recorded for similar complexes with N-salicylidene-aminoacidato
ligands previously reported.32,33 The slight differences in the several
spectra recorded for 1, and in the several spectra recorded for 3
may be explained by assuming small differences in the amount of
H2O or DMF coordinated to the V(IV)O centres. The EPR spectra
for the powdered samples of 1 or 3 after being used for catalytic
reactions (e.g. Fig. 2, grey spectra) show no significant differences
from those of the fresh catalysts.
Fig.
3
EPR spectra for the powdered samples of the catalyst
PS–[Cu(fsal–D,L-Ala)(X)] 2. (A) Dry fresh catalyst at room temperature
(black), and at 77 K for the powder in contact with DMF (grey); (B)
dry fresh catalyst (black), and used dry catalyst (grey), both at room
temperature.
be explained by assuming small changes in the relative amount of
H2O and DMF coordinated to the Cu centres. At least two distinct
species are detected in each of the EPR spectra recorded and
Table S3, in the ESI,† includes the spin Hamiltonian parameters
calculated by simulation of the spectra. Although the values
obtained for the distinct species present in the fresh catalysts are
not very accurate, globally the EPR spectra are consistent with a
NO3 binding mode with some, but normally not very significant
distortion from a square planar arrangement of the donor atoms.
However, the EPR spectra of catalysts 2 and 4 after being used in
the catalytic reactions (e.g. grey curve in Fig. 3B) show significant
differences from the fresh catalysts, and at least 3 distinct species
are detected. In fact, weak features for g ≈ 1.8–1.6 indicate the
Fig. 3A and Fig. 3B present EPR spectra for PS–[Cu(fsal–D,L-
Ala)(X)] (2) (X = H2O or DMF). Only slight differences are
detected in the EPR spectra obtained at room temperature for
powdered samples of the fresh polymer-anchored Cu(II) complexes
2 or 4 and those of the corresponding catalysts after swelling in
DMF and freezing to 77 K (e.g. Fig. 3A). These differences may
Table 3 Spin Hamiltonian parameters obtained from the experimental EPR spectra recorded for powdered samples of 1 and 3, and of the corresponding
non-polymer-bound complexes 5 and 7 dissolved in DMF
Catalyst
Sample
gꢀ
Aꢀ/×10−4 cm−1
g⊥
A⊥/×10−4 cm−1
[VO(fsal–D,L-Ala)(X)], 5
1st species
In DMF
—
—
Fresh
Used
Fresh
Used
In DMF
—
—
—
—
—
1.948
1.947
1.943
1.943
1.944
1.943
—
1.947
1.947
1.944
1.943
1.945
1.944
169.4
171.1
168.7
169.5
170.2
170.8
—
168.6
171.3
168.6
168.7
170.1
170.4
1.980
1.980
1.978
1.978
1.979
1.979
—
1.980
1.980
1.978
1.978
1.974
1.979
61.4
61.4
60.1
60.8
59.1
60.0
—
61.8
61.3
60.0
61.6
59.2
59.8
2nd speciesa
PS–[VO(fsal–D,L-Ala)(X)], 1
PS–[VO(fsal–D,L-Ala)(X)], 1
PS–[VO(fsal–D,L-Ala)(X)], 1–DMFb
PS–[VO(fsal–D,L-Ala)(X)], 1–DMFb
[VO(fsal–L-Ile)(X)], 7
1st species
2nd speciesa
—
PS–[VO(fsal–L-Ile)(X)], 3
PS–[VO(fsal–L-Ile)(X)], 3
PS–[VO(fsal–L-Ile)(X)], 3–DMFb
PS–[VO(fsal–L-Ile)(X)], 3–DMFb
Fresh
Used
Fresh
Used
a The spin Hamiltonian parameters of the two species are very close, possibly they are isomeric species. b Spectrum recorded after swelling the solid in
DMF.
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presence of a dimeric complex; the corresponding features that
should appear at g ≈ 2.4–2.6 cannot be seen because of the broad
bands due to the presence of at least two monomeric complexes
and of some dipolar interactions operating. The features observed
at g ≈ 1.90 0.05 probably correspond to the gx components of
a highly distorted monomeric Cu complex, whose gz components
are at g ≈ 2.4 0.1.36 A second monomeric complex with gz ≈
2.05 is also present but the corresponding Aꢀ values cannot be
estimated due the unresolved pattern of the g ≈ 2.6–2.2 region of
the spectra for catalysts 2 and 4 after being used for the catalytic
reactions.
Catalytic activity
Oxidation of p-chlorotoluene. The oxidation of p-chloro-
toluene by aqueous H2O2 as the oxidant was studied using
the polymer-bound catalysts. The reaction gave five oxidation
products namely, p-chlorobenzaldehyde, p-chlorobenzyl alco-
hol, 3-methyl-6-chlorophenol, 2-methyl-5-chlorophenol and p-
chlorobenzoic acid (see Scheme 4). Among the catalysts studied,
PS–[Cu(fsal–D,L-Ala)(H2O)] was taken as a representative one and
different parameters e.g. amount of oxidant, catalyst and temper-
ature were studied to optimise the reaction conditions to achieve
maximum oxidation of p-chlorotoluene to p-chlorobenzaldehyde.
Scheme 4
The effect of H2O2 concentration on the oxidation of p-
chlorotoluene is illustrated in Fig. 4A. A conversion of ca. 8.7%
was achieved at a p-chlorotoluene to aqueous H2O2 molar ratio of
1 : 1 in 6 h when p-chlorotoluene (1.26 g, 0.010 mol), 30% H2O2
(2.27 g, 0.020 mol) and PS–[Cu(fsal–D,L-Ala)(H2O)] (0.025 g)
were placed in acetonitrile (20 mL) and reacted at 80 ◦C. This
conversion improved to ca. 10% on increasing the ratio to 1 :
3 and to ca. 15.4% at a p-chlorotoluene to H2O2 molar ratio of
1 : 4. These results indicated a linear increment in conversion
upon increasing the H2O2 concentration. However, the selectivity
of p-chlorobenzaldehyde decreases at higher concentration of
oxidant due to its further oxidation to p-chlorobenzoic acid.
No further increment in the oxidation of p-chlorotoluene was
observed beyond a 6 h contact time.
Similarly three different amounts of catalyst viz. 0.025, 0.035
and 0.050 g were taken at fixed reaction conditions for p-
chlorotoluene (1.26 g, 0.010 mol), H2O2 (2.27 g, 0.020 mol),
CH3CN (20 mL) and temperature (80 ◦C) to understand the effect
of the relative amount of catalyst on the reaction. As shown in
Fig. 4B, 0.025 g of catalyst is adequate and any further increment
in the amount of catalyst had no affect on the conversion under
the above conditions.
Fig. 4 Oxidation of p-chlorotoluene. (A) Effect of the H2O2 con-
centration: reaction conditions; PS–[Cu(fsal–D,L-Ala)(H2O)] (0.025 g),
p-chlorotoluene (1.26 g, 0.010 mol), acetonitrile (20 mL) and 80 ◦C.
(B) Effect of the relative amount of catalyst: reaction conditions;
p-chlorotoluene (1.26 g, 0.010 mol), 30% H2O2 (2.27 g, 0.020 mol),
acetonitrile (20 mL) and 80 ◦C. (C) Effect of temperature: reaction
conditions; PS–[Cu(fsal–D,L-Ala)(H2O)] (0.025 g), p-chlorotoluene (1.26 g,
0.010 mol), 30% H2O2 (2.27 g, 0.020 mol) and acetonitrile (20 mL).
(0.025 g), aqueous 30% H2O2 (2.27 g, 0.020 mol), CH3CN (20 mL)
and temperature (80 ◦C). The other polymer-bound catalysts
were also tested, using these experimental conditions, and the
corresponding results are presented in Table 4.
From the results presented in Table 4 it is clear that the
conversion of p-chlorotoluene follows the order: PS–[VO(fsal–D,L-
Ala)(H2O)] (13.8%) > PS–[VO(fsal–L-Ile)(H2O)] (12.4%) > PS–
[Cu(fsal–L-Ile)(H2O)] (8.8) > PS–[Cu(fsal–D,L-Ala)(H2O)] (8.7%).
The temperature of the reaction has an influence on the
oxidation of p-chlorotoluene. As illustrated in Fig. 4C, 80 ◦C
was the most favorable temperature under the above reaction
conditions to obtain the maximum conversion of p-chlorotoluene.
The conversion increases with the temperature.
Therefore, from these experiments, for the maximum oxidation
of 0.010 mol of p-chlorotoluene the best conditions are: catalyst
4226 | Dalton Trans., 2008, 4220–4232
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Table 4 Conversion of p-chlorotoluene and selectivity of various products after 6 h of reaction timea
%Selectivity
Catalyst
%Conv.
TOF/h−1
PS–[VO(fsal–D,L-Ala)(H2O)]
PS–[VO(fsal–D,L-Ala)(H2O)]b
PS–[Cu(fsal–D,L-Ala)(H2O)]
PS–[Cu(fsal–D,L-Ala)(H2O)]b
PS–[VO(fsal–L-Ile)(H2O)]
PS–[VO(fsal–L-Ile)(H2O)]b
PS–[Cu(fsal–L-Ile)(H2O)]
PS–[Cu(fsal–L-Ile)(H2O)]b
[VO(fsal–D,L-Ala)(H2O)]
[Cu(fsal–D,L-Ala)(H2O)]
[VO(fsal–L-Ile)(H2O)]
13.8
13.3
8.7
8.68
—
5.62
—
9.25
—
6.07
—
5.20
4.46
5.97
4.81
62.8
62.5
15.6
15.8
54.5
54.4
15.9
16.0
61.1
16.0
59.7
16.2
14.2
14.2
32.4
32.3
13.5
13.2
27.6
27.4
12.5
23.2
15.4
24.9
4.9
4.8
7.7
7.7
9.3
9.6
7.2
7.0
7.6
13.2
6.4
12.4
6.8
7.0
11.3
11.5
20.3
20.1
12.3
12.4
22.7
22.6
9.9
24.0
24.1
10.4
10.4
26.7
27.0
8.8
22.2
8.3
22.9
8.0
12.4
12.0
8.8
8.2
8.3
6.8
8.0
7.0
25.0
10.3
23.5
[Cu(fsal–L-Ile)(H2O)]
a Reaction conditions: p-chlorotoluene (0.010 mol), 30% H2O2 (0.020 mol), catalyst (0.025 g), CH3CN (20 mL) and temperature, 80 ◦C. b First cycle of
used catalyst.
Thus, the vanadium based catalysts are better than the copper
based catalysts. However, within each of the particular metal
systems both complexes present nearly equal catalytic potential.
The selectivity for the different reaction products also differs: for
the vanadium based polymer-anchored complexes, the selectivity
of the various products follows the order: p-chlorobenzaldehyde
>> p-chlorobenzyl alcohol > p-chlorobenzoic acid > 2-methyl-5-
chlorophenol > 3-methyl-6-chlorophenol, while for the polymer-
bound copper complexes the order is: p-chlorobenzyl alco-
hol > 2-methyl-5-chlorophenol > p-chlorobenzoic acid > p-
chlorobenzaldehyde > 3-methyl-6-chlorophenol.
Oxidation of cyclohexene. The oxidation of cyclohexene catal-
ysed by polymer-anchored complexes gave cyclohexene epoxide, 2-
cyclohexene-1-ol, 2-cyclohexene-1-one and cyclohexane-1,2-diol
as shown in Scheme 5.
Both H2O2 and TBHP are known to oxidise cyclohexene in the
presence of alumina-supported metal complexes. However, under
similar conditions H2O2 gave much better epoxidation results for
cyclohexene than TBHP. TBHP as an oxidant promotes the allylic
oxidation pathway, and epoxidation products only form in low
amounts, while with the use of H2O2 the oxidation occurs mainly
on the double bond and cyclohexene epoxide is obtained almost as
the sole product. The formation of the allylic oxidation products 2-
cyclohexene-1-one and 2-cyclohexene-1-ol reflects the preferential
The used catalysts were washed with acetonitrile, dried at 100 ◦C
and tested for their catalytic activities. The corresponding data is
also presented in Table 5 and show that there is not much loss
in catalytic activity. The non-polymer-bound complexes exhibit
lower conversions as well as lower turn-over frequencies.
attack of the activated C–H bond over the C C bond.37 Valentine
=
and co-workers have suggested that the species responsible for the
cyclohexene oxidation is the product formed from the cleavage of
Table 5 Conversion of cyclohexene and selectivity of the various oxidation products after 6 h of reaction time
%Selectivity
Catalyst
%Conv.
TOF/h−1
PS–[VO(fsal–D,L-Ala)(H2O)]
PS–[VO(fsal–D,L-Ala)(H2O)]a
PS–[Cu(fsal–D,L-Ala)(H2O)]
PS–[Cu(fsal–D,L-Ala)(H2O)]a
PS–[VO(fsal–L-Ile)(H2O)]
PS–[VO(fsal–L-Ile)(H2O)]a
PS–[Cu(fsal–L-Ile)(H2O)]
PS–[Cu(fsal–L-Ile)(H2O)]a
[VO(fsal–D,L-Ala)(H2O)]
[Cu(fsal–D,L-Ala)(H2O)]
[VO(fsal–L-Ile)(H2O)]
78.8
78.0
32.5
31.6
76.6
76.0
29.4
28.2
53.8
26.3
50.5
18.1
49.6
—
21.3
—
57.1
—
20.2
—
33.9
17.3
37.7
12.4
34.7
34.6
39.3
38.8
36.1
36.3
42.3
41.9
38.9
36.2
40.0
39.7
48.1
48.3
53.1
53.5
44.7
44.6
45.7
46.0
52.6
49.6
48.4
42.1
6.8
6.7
4.3
4.3
8.2
8.2
3.6
3.6
5.1
9.7
6.3
8.3
10.4
10.4
3.3
3.4
11.0
10.9
8.4
8.5
3.4
4.5
5.28
9.9
[Cu(fsal–L-Ile)(H2O)]
a First cycle of used catalyst.
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Scheme 5
the O–O bond, whereas epoxidation occurs via the direct reaction
of the olefin with the coordinated HOO−. Since the O–O bond of
HOOH is 5 kcal mol−1 stronger than TBHP, an HOO− complex is
expected to have a higher activation energy for O–O bond cleavage
than a TBHP complex and therefore, will have a longer lifetime38
and a higher probability of forming the epoxide.
We have optimised different parameters such as the amount of
oxidant, catalyst, solvent and temperature of the reaction mixture
for the maximum oxidation of cyclohexene while considering
PS–[VO(fsal–D,L-Ala)(H2O)] as the representative catalyst. Three
different cyclohexene to aqueous 30% H2O2 molar ratios viz. 1 : 1,
1 : 2 and 1 : 3 were considered at a fixed amount of cyclohexene
(0.082 g, 0.010 mol), PS–[VO(fsal–D,L-Ala)(H2O)] (0.025 g) and
acetonitrile (20 mL), with the reaction being carried out at 70 ◦C. A
maximum of 22.9% conversion was achieved at a cyclohexene
to H2O2 molar ratio of 1 : 1. Increasing this ratio to 1 : 2
increased the conversion to 70.4% while a 1 : 3 molar ratio gave
75.2% conversion (Fig. 5A). Analysis of the effect of substrate
to oxidant ratios suggests that increasing the amount of oxidant
decreases the formation of cyclohexene oxide whilst increasing the
formation of 2-cyclohexene-1-ol and cyclohexane-1,2-diol slightly.
Further increments in the oxidant amount has not resulted in
any significant change in conversion, which suggests that a large
amount of oxidant is not an essential condition to improve the
oxidation of cyclohexene. Therefore, a cyclohexene to H2O2 molar
ratio of 1 : 2 was considered to be adequate in obtaining the highest
cyclohexene conversion (ca. 70%).
Using (0.082 g, 0.010 mol) of cyclohexene, 30% H2O2 (2.27 g,
0.020 mol) and acetonitrile (20 mL), three different amounts
of catalyst viz. 0.015, 0.025 and 0.035 g were considered for
the oxidation of cyclohexene and the reaction was monitored
at 70 ◦C. The results obtained as a function of time, presented
in Fig. 5B, indicate that 0.035 g of catalyst yields a maximum
conversion of ca. 75%, while 0.025 and 0.015 g of catalyst gave ca.
70 and 63% conversion, respectively in 6 h of reaction time. Further
increments in the amount of catalyst showed no significant change
in the conversion.
Fig. 5C illustrates the effect of temperature on the oxidation
of cyclohexene as a function of time under the above optimised
reaction conditions. From the figure we conclude that 80 ◦C is the
best suited temperature for the conversion of cyclohexene. At this
temperature, a maximum of ca. 79% conversion of cyclohexene
was achieved in 6 h of contact time. At lower temperature, viz.
60 and 70 ◦C, the conversions obtained were ca. 56 and 70%,
respectively.
Therefore, under the optimised conditions i.e. cyclohexene
(0.82 g, 0.010 mol), H2O2 (2.27 g, 0.020 mol), catalyst (0.025 g)
CH3CN (20 mL) and temperature (80 ◦C), the performance of
the other catalysts was also studied and the conversions after
6 h of reaction time and the product selectivity are presented
in Table 5. It is clear from the table that the vanadium based
Fig. 5 Oxidation of cyclohexene. (A) Effect of H2O2: reaction conditions;
cyclohexene (0.082 g, 0.010 mol), PS–[VO(fsal–D,L-Ala)(H2O)] (0.025 g),
acetonitrile (20 mL) and 70 ◦C. (B) Effect of amount of catalyst: reaction
conditions; cyclohexene (0.082 g, 0.010 mol), 30% H2O2 (2.27 g, 0.020 mol),
acetonitrile (20 mL) and 70 ◦C. (C) Effect of temperature: reaction
conditions; cyclohexene (0.082 g, 0.010 mol), PS–[VO(fsal–D,L-Ala)(H2O)]
(0.025 g), 30% H2O2 (2.27 g, 0.020 mol) and acetonitrile (20 mL).
polymer-anchored catalysts are better than the copper based ones
for the oxidation of cyclohexene, and within each metal system
the catalytic performances are quite similar. The selectivity of
4228 | Dalton Trans., 2008, 4220–4232
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the products follows the order: 2-cyclohexene-1-ol > cyclohexene
oxide > cyclohexane-1,2-diol > 2-cyclohexene-1-one.
The catalytic activity of the used catalysts, after washing
with acetonitrile and drying at 100 ◦C, was also tested and
the corresponding data are presented in Table 5. These results
indicate that the catalytic activity of the used catalysts is similar
to that of the fresh ones. However, non-polymer bound complexes
exhibit lower conversions along with lower turn-over frequencies
(Table 5), but the selectivity for the major products, 2-cyclohexene-
1-ol and cyclohexene oxide, is the same.
Fig. 6 Titration of [VIVO(fsal–D,L-Ala)(H2O)] 5 with H2O2. The spectra
were recorded after successive additions of one-drop portions of H2O2
(1 mL of 30% H2O2 dissolved in 10 mL of methanol) to 10 mL
of ca. 10−4 M methanolic solution of [VO(fsal–D,L-Ala)(H2O)]. The
inset shows equivalent titrations, but with higher concentrations of a
[VO(fsal–D,L-Ala)(H2O)] solution (ca. 10−3 M).
Considerations about the reaction mechanism and intermediate
species
Peroxo-complexes may be able to transfer oxygen to aromatic
hydrocarbons. However, alkanes are much less readily hydroxy-
lated than aromatic hydrocarbons, and mixtures of alcohols and
ketones are formed. The mechanism of oxygen transfer of peroxo–
metal complexes to nucleophilic substrates has been a matter of
considerable debate, and overall two main alternatives have been
proposed. One proceeds through a simple bimolecular mechanism
involving the nucleophile and the peroxo–metal complex. The
other is a mechanism that involves the formation of an inter-
mediate resulting from the coordination of the substrate to the
metal.
It is known that [V(IV)O(sal–Am)(X)]-type complexes (Am =
amino acid with non-coordinating side chains, X = H2O, MeOH)
oxidise easily; namely [V(IV)O(sal–L-Ala)(H2O)] upon oxidation
in methanol forms [V(V)O(sal–L-Ala)(MeO)(MeOH)].39 Vana-
dium(V) complexes are also known to form oxoperoxovana-
dium(V) complexes in the presence of excess H2O2.2,39–41
In the absorption spectrum, the complex [V(V)O(sal–L-
Ala)(MeO)(MeOH)] has kmax at ∼360 and 500 nm,39 due to
p → p* (azomethine, see above) and charge-transfer transi-
tions, respectively, and upon addition of t-butyl-hydroperoxide
a relatively intense band appears at ca. 450 nm. [V(V)O(sal–
L-Val)(MeO)(MeOH)] in methanol also presents42 bands at ca.
420 nm (CT), and the corresponding V(IV)O complex a d–d band
at ca. 700 nm.33
Fig. 7 Titration of [Cu(fsal–L-Ile)(H2O)] (8) with H2O2. The spectra were
recorded after the successive additions of one drop portions of H2O2 (1 mL
of 30% H2O2 dissolved in 10 mL of methanol) to 10 mL of ca. 10−4
M
methanolic solution of [Cu(fsal–L-Ile)(H2O)]. The inset shows equivalent
titrations, but with higher concentrations of a [Cu(fsal–L-Ile)(H2O)]
solution (ca. 10−3 M).
species. Similar spectral changes have been reported for [VO(fsal–
b-Ala)(H2O)] (H2fsal–b-Ala = a Schiff base derived from 3-
formylsalicylic acid and b-alanine) upon treatment with H2O2.13
The band appearing at ca. 410–420 nm is probably due to a
LMCT band of the monoperoxo complex39,40,43,44 but is probably
superimposed with the CT transition from the phenolate group at
about the same energy (or slightly lower), as observed in [VVO(sal–
L-Am)(MeO)(MeOH)] complexes.33
To get information on the reaction pathway and the interme-
diate species formed during the oxidation of the substrates, a
methanolic solution of the non-polymer-bound complex 5 was
treated with a MeOH solution of H2O2, and the progress of the
reaction was monitored by UV-vis. Fig. 6 presents the spectral
changes observed. Thus, the addition of one-drop portions of
H2O2 to a ca. 10−4 M solution of [VO(fsal–D,L-Ala)(H2O)] in
MeOH resulted in only a slight reduction of the intensity of the
band appearing at 356 nm, along with the gradual appearance of
a new band at ca. 410–420 nm. The bands appearing at 249 nm
were slowly converted into weak shoulders that then increased
in intensity, while the intensity of the 238 nm bands increased.
The intensity of the 208 nm band also increases. At higher
concentration (ca. 10−3 M) [VO(sal–D,L-Ala)(H2O)] 5 also exhibits
two bands at 543 and 772 nm, as shown in the inset of Fig. 6. Upon
addition of H2O2 the bands appearing at ∼772 nm slowly flatten,
while the intensity of the 543 nm band is progressively increased
with broadening and finally disappeared.
The 51V NMR spectrum of a 4 mM solution of VIVO(fsal–
D,L-Ala)(X) 5 (in 80% MeOH and 20% MeOD), upon stand-
ing in contact with air for ca. 7 days, shows one peak at
−553 ppm and two close, equally intense peaks at −562 and
−566 ppm. We assign these two latter peaks to the stereoisomers
that may form for [VVO(fsal–D,L-Ala)(MeO)(MeOH)] 9. For
[VVO(naph–L-Phe)(1,3-butO)(1,3-butOH)] (naph-L-Phe = N-(2-
oxidonaphthal)–Phe–O− and 1,3-butOH = 1,3-butanediol) four
peaks in the range −553 to −585 ppm have been reported
in the 51V NMR spectra,45 and in the case of [VVO(naph–
L-Ala)(BuSO)(BuSOH)] (BuSOH = D,L-sec-butyl alcohol) three
peaks were found in the range −583 to −590 ppm and were also
assigned to the stereoisomers of the Schiff base V(V) complexes.46
+
The peak at ca. −553 ppm possibly corresponds to VO2 (V1),
We interpret these results as due to the formation of [VVO(fsal–
D,L-Ala) (MeO)(MeOH)] (9) and of an oxoperoxovanadium(V)
indicating some significant hydrolysis of the V(V) complexes after
one week of contact of the solution with air (Table S4, ESI).†
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Upon progressive addition of H2O2 to this oxidised solution of
5, the relative intensities of the peaks change: the −553 ppm peak
decreases its relative intensity, and after 8 equiv. of H2O2 it is no
longer detected, while a peak at −581 ppm increases its relative
intensity. This latter peak may be assigned to the formation of
[VVO(fsal–D,L-Ala)(O2)(MeOH)]− (see Table S4, ESI).† Overall
these 51V NMR spectra confirm the results and interpretation of
Fig. 6.
After 9 equiv. of H2O2 have been added to this oxidised solution
of 5, styrene (used here as a model of an alkene) was progressively
added to this mixture, namely a total of 2, 3, 5, 8, 10, 15, 18,
22, 30, 35, 40, and 45 equiv. of styrene were added and the
corresponding 51V NMR spectra were recorded (see Table S4,
ESI).† No new peaks were detected but the global intensity of
the 51V NMR spectra progressively decreased upon addition of
styrene. For the solutions where 40 or 45 equiv. of styrene have
been added almost no 51V NMR signal was detected. Therefore,
upon addition of styrene the oxidation of this substrate occurred,
with the concomitant reduction of all V(V) species present.
In a distinct experiment, to a 4 mM solution of [VIVO(fsal–
D,L-Ala)(X)] 5 (in 80% MeOH and 20% MeOD), which was kept
overnight in contact with air, 1 equiv. of H2O2 was added. The
51V NMR spectrum of this solution showed peaks at −553, −562
and −566 ppm. Upon addition of amounts of styrene (dissolved
in methanol) no new peaks were detected, but the intensity of all
peaks decreased as styrene was added.
10 equiv.) to the solution of 8 containing 10 equiv. of styrene also
did not change the pattern of the EPR spectra.
The addition of a methanolic solution of H2O2 to 10 mL of a ca.
10−3 M methanolic solution of [Cu(fsal–L-Ile)(H2O)] 8 resulted in
the reduction in the intensity of the d–d band at 655 nm without
changing its position, but further addition of H2O2 did not reduce
its intensity further (inset of Fig. 7). The addition of H2O2 to a
dilute solution of 8 (ca. 10−4 M) caused only slight increments
in the intensities of the 211 and 305 nm bands without changing
their position, while a considerable increase in the band maximum
was observed in the 234 nm band (Fig. 7). A weak shoulder also
appeared at 345 nm. These changes in the UV-vis spectra indicate
the interaction of the peroxo group with the Cu(II) centre in the
complex.
At least three types of peroxo species viz. side-on Cu(II)–(l-
g -peroxo)–Cu(II), bis(l-oxo-Cu(III)) and Cu(II)–O–O–H (copper
2
hydroperoxide) have been reported in the literature during catalytic
actions.48 As copper complexes supported on polymers are ex-
pected to be monomeric, and they all have one weakly coordinated
ligand (water) with exchangeable properties, the formation of
[(HOO)–Cu(II)–(fsal–D,L-Am)] as an intermediate species may be
expected upon interaction with H2O2. These intermediates are
expected to transfer the oxygen atoms to the substrates to give the
products. The presence of a weak, broad band at ca. 655 nm even
after H2O2 addition is possibly due to charge transfer due to Cu
hydroperoxide complex formation.49
The oxoperoxovanadium(V) species actively participates in the
oxidation by transferring one of its peroxo oxygens to the substrate.
Whether the mechanism involves an intermediate complex with
coordination of styrene, followed by an alkylperoxo species,
then yielding the products, or proceeds without the intermediate
binding of the substrate, is not clear. In fact, the EPR spectra
of the 4 mM methanolic solutions of [VIVO(fsal–L-Ile)(H2O)] 7,
did not change much after progressive addition of styrene (ca.
2 to 10 equiv.). After 10 equiv. of styrene had been added, the
pattern of the spectrum was very similar to that of 7, but the spin
Hamiltonian parameters had slightly changed (from gꢀ = 1.946,
g⊥= 1.980, Aꢀ= 172.0 and A⊥= 62.4 × 10−4 cm−1 for 7 in MeOH,
to gꢀ = 1.946, g⊥ = 1.980, Aꢀ = 171.5 and A⊥ = 61.9 × 10−4 cm−1
for 7 + 10 equiv. of styrene). Although for other systems we had
clear evidence for the binding of styrene to the V(IV) centre,47 here
the evidence for the coordination of styrene is weak. Addition of
different amounts of H2O2 (1, 2, 3, 5, 7 equiv.) to this solution of 7
containing 10 equiv. of styrene also did not change the pattern
of the EPR spectra, but its intensity decreased and after the
addition of 7 equiv. of H2O2 the EPR signal was almost zero.
The Aꢀ values apparently showed a very small decrease, but as the
intensity of the spectra decreased this cannot be clearly stated or
associated to the binding of a peroxo, hydroperoxide or peroxy
species.
A possible mechanism, following what was outlined by Mimoun
et al.,50 involves the formation of a V(V)-oxoperoxo complex,
which upon coordination of the substrate, and insertion of the
bound alkene into one of the metal–peroxo bonds, followed by the
collapse of the resulting peroxometallocycle yields the epoxide.
However, no data was obtained to support the coordination of the
arene. A bimolecular mechanism involving the nucleophilic attack
of the substrate at the protonated peroxide ligand is also possible.
With the Cu(II) complexes the formation of peroxo or hy-
droperoxo species is not so favourable and the conversions are
lower than with the vanadium complexes. The redox reactions
involving Cu(II)/Cu(III) species are also not so easy as with the
V(IV)/V(V) complexes. Therefore, the mechanism possibly involves
the nucleophilic attack of the substrate at the protonated peroxide
ligand, L-Cu(II)–OOH being the active catalyst.
The mechanism of the oxidation of alkanes and alcohols is
normally considered consistent with single transfer steps.2 For the
vanadium complexes the formation of an oxoperoxovanadium(V)
is usually assumed. For hydroxylation of benzene either: (i)
a mechanism in which the oxoperoxovanadium(V) undergoes
•
intramolecular electron transfer to give a V(IV)–(O2 −) biradical,
which inserts into aromatic or aliphatic C–H bonds1 or (ii) the re-
action occurs via the radical anion of the vanadium(V) complex.51
Other authors have also reported radical mechanisms involving
alkylhydroperoxide intermediates for oxidation of alkanes, alkenes
and arenes.52
Progressive additions of different amounts of H2O2 (2, 4, 5
equiv.) to a 4 mM methanolic solution of 7 also did not change
the EPR signal, but decreased its intensity.
The EPR spectra of 4 mM methanolic solutions of [Cu(fsal–L-
Ile)(H2O)] 8, showed no change upon the progressive addition of
styrene (1, 3, 5, 8, 10 equiv.). After 10 equiv. of styrene had been
added, the pattern of the spectrum was identical to that of 8, and
there was no indication to support the coordination of styrene
to the Cu centre. Addition of different amounts of H2O2 (2 to
Recyclability and heterogeneity of the reaction
The used catalysts PS–[VO(fsal–D,L-Ala)(H2O)] and PS–[Cu(fsal–
D,L-Ala)(H2O)], after separating from the reaction mixture, wash-
ing with acetonitrile and drying at ca. 120 ◦C, were further applied
for catalytic reactions under similar conditions in both cases. No
4230 | Dalton Trans., 2008, 4220–4232
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significant loss in the activity occurred, though the EPR study
suggests a slight modification in the coordinating mode of the
copper-based catalysts after use. The filtrate collected after each
catalytic reaction was placed further into the reaction flask and
the reaction was continued at the optimised conditions for 3 h
after adding fresh oxidant. The gas chromatographic analysis of
the reaction mixture showed no increase in the conversion. This
confirms that the reaction did not proceed upon the removal of the
solid catalyst and hence the reaction was heterogeneous in nature.
In addition, the filtrates tested by ICP-MS for all catalysts for the
presence of any leached metal ions showed negative results.
The non-polymer bound complexes are considerably less ef-
ficient than their polymer-bound counterparts. We expect this
to be due mainly to a global concentration effect, as the non-
polymer bound complexes partially decompose in solution due to
hydrolysis of the ligand and oxidation of V(IV). In contrast, the
polymer-bound complexes are stable towards decomposition and
hydrolysis.
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Polymer-anchored oxovanadium(IV) and copper(II) complexes
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or L-isoleucine have been prepared and characterised. These
complexes were used with success as catalysts for the oxidation
of p-chlorotoluene and cyclohexene. The reaction conditions
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formation of such species being previously suggested,38,45 and the
evidence for their formation was also obtained here. The recov-
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patterns and these suggest that their original state is maintained
after recovery. These polymeric supported catalysts are recyclable
and do not leach into solution in any of the above catalytic
reactions showing their heterogeneous nature. The EPR spectra
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state is not preserved after their use in the catalytic procedures.
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Acknowledgements
The Council of Scientific and Industrial Research, New Delhi is
gratefully acknowledged for financial support of the work. The
authors also wish to thank FEDER, Fundac¸a˜o para a Cieˆncia e a
Tecnologia, POCI 2010 (namely PPCDT/QUI/55985/2004 and
PPCDT/QUI/56946/2004 programs).
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