New thiosemicarbazide and dithiocarbazate based oxidovanadium(iv) and dioxidovanadium(v) complexes. Reactivity and catalytic potential

Article abstract of DOI:10.1039/c9nj01486a

The Schiff bases {H₃dfmp-(smdt)₂} (I), {H₃dfmp-(sbdt)₂} (II) and {H₃dfmp-(tsc)₂} (III) are synthesized by reaction of 2,6-diformyl-4-methylphenol (H₃dfmp) and S-methyldithiocarbazate (smdt), S-benzyldithiocarbazate (sbdt) and thiosemicarbazide (tsc), respectively. Addition of [V^IVO(acac)₂] to solutions of these compounds in methanol leads to the formation of the oxidovanadium(iv) complexes [V^IVO{Hdfmp-(smdt)₂(CH₃OH)}] (1), [V^IVO{Hdfmp-(sbdt)₂(CH₃OH)}] (2) and [V^IVO{Hdfmp-(tsc)₂(CH₃OH)}] (3). All these V^IVO-compounds can be oxidized to the corresponding dioxidovanadium(v) (V^VO₂) complexes in methanolic solution upon aerial oxidation in the presence of KOH. The isolated compounds are K[V^VO₂{Hdfmp-(smdt)₂}] (4), K[V^VO₂{Hdfmp-(sbdt)₂}] (5) and K[V^VO₂{Hdfmp-(tsc)₂}] (6). The Cs⁺ salts of these complexes i.e. Cs[V^VO₂{Hdfmp-(smdt)₂}] (7), Cs[V^VO₂{Hdfmp-(sbdt)₂}] (8) and Cs[V^VO₂{Hdfmp-(tsc)₂}] (9) are prepared similarly in the presence of CsOH. All these compounds are characterized by various spectroscopic techniques like FT-IR, UV-visible, and ¹H and ⁵¹V NMR and thermal studies. IR spectral data confirm the coordination of ligands through the azomethine nitrogen, the sulphur and the phenolic oxygen atoms to the metal. These complexes show excellent catalytic activity and selectivity for the oxidation of benzyl alcohol and ethylbenzene in the presence of H₂O₂ as an oxidant. Various parameters such as the amount of catalyst and oxidant, reaction time, reaction temperature and solvent were taken into consideration to optimize these catalytic oxidations. Compound 7 was also remarkably efficient and selective in the catalytic oxidation of primary and secondary alcohols to the corresponding aldehyde/ketone, as well as of several aromatic compounds such as toluene, benzene, cumene and tetralin.

Full text of DOI:10.1039/c9nj01486a

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New thiosemicarbazide and dithiocarbazate based oxidovanadium(IV) and
dioxidovanadium(V) complexes. Reactivity and catalytic potential
Mannar R. Maurya*,a, Bithika Sarkara, Amit Kumarb,c, Nádia Ribeiro b, Aistè Miliute b and J. C.
Pessoa*,b
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aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India,
Email: rkmanfcy@iitr.ernet.in
bCentro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior
Técnico, Universidade Técnica de Lisboa, Av Rovisco Pais, 1049-001 Lisboa, Portugal. Email:
joao.pessoa@ist.utl.pt
cCenter for Nano & Material Science, Jain (Deemed-to-be-University), Jakkasandra Post,
Bangalore 562112, Karnataka, India.
Abstract
The Schiff bases {H3dfmp-(smdt)2} (I), {H3dfmp-(sbdt)2} (II) and {H3dfmp-(tsc)2}(III)
are synthesized by reaction of 2,6-diformyl-4-methylphenol (H3dfmp) and
S-
methyldithiocarbazate (smdt), S-benzyldithiocarbazate (sbdt) and thiosemicarbazide (tsc),
respectively. Addition of [VIVO(acac)2] to solutions of these compounds in methanol leads to the
formation of the oxidovanadium(IV) complexes [VIVO{Hdfmp-(smdt)2(CH3OH)}] (1),
[VIVO{Hdfmp-(sbdt)2(CH3OH)}] (2) and [VIVO{Hdfmp-(tsc)2(CH3OH)}] (3). All these VIVO-
compounds can be oxidized to the corresponding dioxidovanadium(V) (VVO2) complexes in
methanolic solution upon aerial oxidation in the presence of KOH. The isolated compounds are
K[VVO2{Hdfmp-(smdt)2}] (4), K[VVO2{Hdfmp-(sbdt)2}] (5) and K[VVO2{Hdfmp-(tsc)2}] (6).
The Cs+ salt of these complexes i.e. Cs[VVO2{Hdfmp-(smdt)2}] (7), Cs[VVO2{Hdfmp-(sbdt)2}]
(8) and Cs[VVO2{Hdfmp-(tsc)2}] (9) are prepared similarly in the presence of CsOH. All these
1
compounds are characterized by various spectroscopic techniques like FT-IR, UV-Visible, H
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and V NMR and thermal studies. IR spectral data confirm the coordination of ligands through
the azomethine nitrogen, sulphur and the phenolic oxygen atoms to the metal. These complexes
show excellent catalytic activity and selectivity for the oxidation of benzyl alcohol and
ethylbenzene in the presence of H2O2 as an oxidant. Various parameters such as amount of
catalyst and oxidant, reaction time, reaction temperature and solvent were taken into
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consideration to optimize these catalytic oxidations. Compound 7 was also remarkably efficient
and selective in the catalytic oxidation of primary and secondary alcohols to the corresponding
aldehyde/ketone, as well as of several aromatic compounds such as toluene, benzene, cumene
and tetralin.
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Keywords: Oxidovanadium(IV) complexes; dioxidovanadium(V) complexes; polydentate
ligands; spectroscopic studies; catalytic oxidations.
Introduction
Within the areas of vanadium chemistry and biological chemistry, in addition to
therapeutic applications,1-12 one of the most active areas of research has been the study of the
catalytic potential of vanadium complexes,13-17 namely catalytic oxidations.18-28
Polynuclear metal complexes with phenol-based polynucleating ligands have been reviewed by
Ambrosi et al.29 Such a class of ligands has attracted attention of researchers because they are
able to simultaneously bind two or more metal ions. Transition metal ions such as Mn(III, IV),
Ni(II), Co(II), Cu(II) Zn(II) etc. generally form phenoxido bridged binuclear complexes with
ligands derived from 2,6-diformyl-4-methylphenol. Such oxidophenoxidovanadium(IV/V)
complexes of general formula [V2O3L] have only been isolated with the tribasic symmetrical
ONN(O)NNO-coordinating
heptadentate
ligand
2,6-bis[{{(2-hydroxybenzyl)(N′,N′-
(dimethylamino)ethyl)}amino}methyl]-4-methylphenol and its derivatives.30 Reducing the
functionalities to dibasic symmetrical ON(O)NO pentadentate resulted in the formation of
mainly monodentate vanadium complexes leaving two coordination sites free from
coordination.31 However, the additional suitable coordination site present on each pendent arm
led to the formation of polynuclear complex.24,31 Some of us also reported the characterization of
vanadium complexes of 2,6-diformyl-4-methylphenol derived polydentate ligands mainly in
solution31 as well as VIVO-mononuclear complexes.24
Continuing our efforts in this field, we have now designed dibasic symmetrical SN(O)NS
pentadentate ligands i.e. thiohydrazones of 2,6-diformyl-4-methylphenol (Scheme 1) and
prepared a series of monodentate oxidovanadium(IV)- (VIVO-), oxidovanadium(V)- (VVO-) and
dioxidovanadium(V)- (VVO2-) complexes. Their characterization, reactivity and catalytic
performance are reported.
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Direct oxidation of the abundant and cheap hydrocarbons and alcohols to the
corresponding aldehydes and ketones are processes of major important transformations in
organic synthesis, since these oxidation products are very important intermediates for the
generation of the useful primary or special chemicals, as well as highly valued fine chemicals,
agrochemicals, pharmaceutical and compounds for perfumes industry.
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Acetophenone is an important intermediate which is extensively used for the synthesis of
some perfumes, pharmaceuticals, resins, alcohols, esters, aldehydes and tear gas, and as a solvent
for cellulose ethers. Therefore, there has been significant interest in the development of
sustainable and efficient processes for the production of acetophenone from ethylbenzene. The
oxidation process of ethylbenzene to acetophenone with H2O2 as the oxidant has been reported
extensively.16,21 In addition to dioxygen, aqueous hydrogen peroxide (H2O2) is the ultimate green
oxidant since it produces water as the sole by-product, and it is easy to be dealt with after
reactions. The latest studies have therefore been paying attention on the catalytic oxidation of
ethylbenzene with H2O2 as the terminal oxidant.16
In this work we explore the performance of a set of VIVO- and VVO2-complexes as
catalyst precursors in the oxidation of benzyl alcohol and ethylbenzene. Additionally, one of the
complexes synthesized, Cs[VVO2{Hdfmp-(smdt)2}] 7, was applied for the catalytic oxidation of
(1) several primary and secondary alcohols to the corresponding aldehyde/ketone as well as of
(2) several different simple arenes.
Experimental Section
Materials and methods
Acetyl acetone (Aldrich Chemicals Co., U.S.A), thiosemicarbazide, 30 % aqueous H2O2
(Rankem, India) and benzyl alcohol were used as supplied. Other chemicals and solvents were of
analytical
reagent
grade.
[VIVO(acac)2],32
2,6-diformyl-4-methylphenol,33
S-
benzyldithiocarbazate34 and S-methyldithiocarbazate35 were prepared according to the methods
reported in the literature.
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Instrumentation and Characterization Procedures
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Elemental analyses of the compounds were carried out either on an Elementar model
Vario-El-III or in a FISONS EA 1108 CHNS-O apparatus. IR spectra were recorded as KBr
pellets either on a Nicolet NEXUS Aligent 1100 series or a JASCO FT/IR 4100FT-IR
spectrometer. Electronic spectra were measured in methanol either with an UV-1601 PC or with
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a Perkin Elmer lambda 35 UV-Vis spectrophotometer. H NMR spectra were obtained on a
Bruker 500 MHz, 13C and 51V NMR spectra on a Bruker Avance III 400 MHz spectrometer with
the common parameter settings. NMR spectra were usually recorded in MeOD-d4, and chemical
shift V values (δV) are referenced relative to neat VVOCl3 as external standard. EPR spectra
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were recorded with a Bruker ESP 300E X-band spectrometer and the spin Hamiltonian
parameters were obtained by simulation of the spectra with the computer program of
Rockenbauer and Korecz.36 Thermogravimetric analyses of the complexes were carried out
under oxygen atmosphere using a TG Stanton Redcroft STA 780 instrument.
A Shimadzu 2010 plus gas-chromatograph fitted with a Rtx-1 capillary column (30 m ×
0.25 mm × 0.25 μm) and a FID detector was used to analyze the reaction products and their
quantifications were made on the basis of the relative peak area of the respective product and
using the appropriate standards. The identity of the products was confirmed using a GC-MS
model Perkin-Elmer, Clarus 500 and comparing the fragments of each product with the library
available.
The HPLC experiments were done on a Jasco System using an 880-51 (2-line degasser),
an 887-PU (Intelligent Prep. Pump) and an 870-UV (Intelligent UV/Vis detector) with software
BorWin. The solutions were analyzed through a LiChrosphere 100 RP-18 (5 μm) column using a
MeOH/H2O= 65/35 eluent, with a flow rate of 0.9 mL/min and detection at λ= 300 nm.
Preparation of Ligands
H3dfmp-(smdt)2 (I) and H3dfmp-(sbdt)2 (II). The ligands H3dfmp-(smdt)2 (I) and H3dfmp-
(sbdt)2 (II) were prepared by a common procedure; the preparation of H3dfmp-(smdt)2 is
described here. A solution of 2,6-diformyl-4-methylphenol (1.64 g, 10 mmol) dissolved in
methanol (30 mL) was added to a solution of S-methyldithiocarbazate (3.96 g, 20 mmol)
dissolved in methanol (10 mL). The obtained reaction mixture was refluxed on a water bath for 4
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h. During this period a yellow solid slowly separated out. The separated yellow solid of I was
filtered, washed with methanol followed by petroleum ether and dried over silica gel under
vacuum.
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Data for H3dfmp-(smdt)2 (I). Yield: 3.49 g (93.8%). Anal. Calc. for C13H16N4S4O (372.6): C,
41.91; H, 4.33; N, 15.04; S, 34.43%. Found: C, 41.52; H, 4.16; N, 15.18; S, 34.26%. ESI-MS(-):
370.9 u (100%) (C13H15N4S4O, exact mass = 371.01 u), 371.88 (25%).
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Data for H3dfmp-(sbdt)2 (II). Yield: 4.93 g (94.0 %). Anal. Calc. for C25H24N4S4O (524.7): C,
57.22; H, 4.61; N, 10.69; S, 24.44%. Found: C, 56.92; H, 4.56; N, 10.58; S, 24.36%.
H3dfmp-(tsc)2 (III). Ligand III was prepared from 2,6-diformyl-4-methylphenol (1.64 g, 10
mmol) and thiosemicarbazide (1.828 g, 20 mmol) by the method outlined for I. Yield: 2.99 g
(96.3%). Anal. Calc. for C11H14N6S2O (310.4): C, 42.57; H, 4.55; N, 27.08; S, 20.66%. Found:
C, 42.72; H, 4.46; N, 27.19; S, 20.56%.
Fig. 1 Structural formulae of the ligand precursors described in this work.
Preparation of the vanadium Complexes
[VIVO{Hdfmp-(smdt)2}(CH3OH)] (1). A solution of H3dfmp-(smdt)2 (0.372 g, 1 mmol) was
prepared in absolute methanol (20 mL) by heating and filtered. A solution of [VIVO(acac)2]
(0.265 g, 1 mmol) in methanol (15 mL) was added to the above solution with stirring; a
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greenish-yellow solid started to precipitate immediately. After 4 h of stirring the solid was
filtered, washed with methanol followed by petroleum ether (b.p. 60 oC) and dried in a desiccator
over silica gel. Yield: 3.76 g (80.2 %). Anal. Calc. for C14H19N4S4O3V (469.52): C 35.81; H
3.86; N 11.93; S 27.32 %. Found: C, 35.9; H, 3.9; N, 12.2; S 27.4 %. ESI-MS(-) 468.8 u (calc.
C14H18N4O3S4V exact mass = 468.97 u).
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[VIVO{Hdfmp-(sbdt)2}(CH3OH)] (2) and [VIVO{Hdfmp-(tsc)2}(CH3OH)] (3). Brown
complexes 2 and 3 were prepared using H3dfmp-(sbdt)2 (II) and H3dfmp-(tsc)2 (III),
respectively, and following the method outlined for 1.
[VIVO{Hdfmp-(sbdt)2}(CH3OH)] (2). Yield: 0.491 g (80.1 %). Anal. Calc. for C26H25N4S4O3V
(620.70): C, 50.31; H, 4.06; N, 9.03; S, 20.66%. Found: C, 50.4; H, 4.0; N, 8.9; S, 21.0%.
[VIVO{Hdfmp-(tsc)2}(CH3OH)] (3). Yield: 0.322 g (78.4 %). Colour: brown. Anal. Calc. for
C12H15N6S2O3V (406.36): C, 35.47; H, 3.72; N, 20.68, S, 15.78%. Found: C, 35.9; H, 3.9; N,
20.8; S, 15.8%.
K[VVO2{Hdfmp-(smdt)2}] (4). A solution of KOH (0.112 g, 2 mmol) in methanol (15 mL) was
added slowly to a solution of [VIVO{Hdfmp-(smdt)2}(CH3OH)] (0.469 g, 1 mmol) in methanol
(150 mL) while stirring. The obtained orange solution was allowed to oxidize aerially at room
temperature. After 2 days the color of the solution changed to yellow. After reducing the solvent
volume to ca. 10 mL the mixture was kept for 12 h at room temperature. The separated yellow
solid was filtered off, washed with methanol and dried in a desiccator over silica gel. Yield:
0.261 g (52.8 %). Anal. Calc. for C13H14KN4S4O3V (492.57): C, 31.70; H, 2.86; N, 11.37; S,
26.04%. Found: C, 31.17; H, 3.1; N, 11.2; S, 26.6%. ESI-MS(-): 452.8 u (C13H14N4O3S4V exact
mass = 452.94).
K[VVO2{Hdfmp-(sbdt)2}] (5) and K[VVO2{Hdfmp-(tsc)2}] (6). Complexes 5 and 6 were
prepared by the procedure outlined for 4.
K[VVO2{Hdfmp-(sbdt)2}] (5). Yield: 0.373 g (58.4 %). Colour: yellow. Anal. Calc. for
C25H24N4S4O3KV (644.8): C, 46.54; H, 3.44; N, 8.69; S, 19.89%. Found: C, 46.4; H, 3.0; N,
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K[VVO2{Hdfmp-(tsc)2}] (6). Yield: 0.219 g (51.4 %). Colour: yellow. Anal. Calc. for
C11H11N6S2O3KV (430.4): C, 30.70; H, 2.81; N, 19.53; S, 14.90%. Found: C, 30.4; H, 3.0; N,
19.0; S, 15.10%.
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Cs[VVO2{Hdfmp-(smdt)2}] (7). A solution of CsOH (0.330 g, 2 mmol) in methanol (15 mL)
was added while stirring to a solution of [VIVO{Hdfmp-(smdt)2}(CH3OH)] (0.469 g, 1 mmol) in
methanol (150 mL) slowly. The obtained orange solution was left at room temperature for slow
aerial oxidation. After 2 days the color of the solution slowly changed to yellow. After reducing
the volume to ca. 10 mL the mixture was kept at room temperature for 12 h where yellow solid
slowly separated out. This was filtered off, washed with methanol and dried in a desiccator over
silica gel. Yield 0.351 g (61.01 %). Anal. Calc. for C13H14N4S4O3CsV (586.38): C, 26.63; H,
2.41; N, 9.55; S, 21.87%. Found: C, 26.33; H, 2.57; N, 9.0; S, 22.3 %. ESI-MS(-): 453.0
(C13H14N4O3S4V exact mass = 452.94).
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Cs[VVO2{(Hdfmp-(sbdt)2}] (8) and Cs[VVO2{(Hdfmp-(tsc)2}] (9). These complexes were
prepared similarly as mentioned for 6 using [VIVO{dfmp-(sbdt)2}(CH3OH)] (2) and
[VIVO{dfmp-(tsc)2}(CH3OH)] (3), respectively.
Cs[VVO2{(Hdfmp-(sbdt)2}] (8). Yield: 0.428 g (58.4 %). Colour: yellow. Anal. Calc. for
C25H24N4S4O3CsV (738.57): C, 40.66; H, 3.00; N, 7.59; S, 17.37%. Found: C, 40.4; H, 3.08; N,
6.9; S, 17.2%.
Cs[VVO2{Hdfmp-(tsc)2}] (9). Yield: 0.431 g (68.4 %). Colour: brown. Anal. Calc. for
C11H11N6S2O3CsV (524.23): C, 25.20; H, 2.31; N, 16.03; S, 12.23%. Found: C, 25.5; H, 2.73; N,
15.9; S, 11.9%.
Catalytic oxidation of alcohols
The catalytic oxidation of benzyl alcohol, 4-isopropylbenzyl alcohol, 1-phenylethanol,
cyclohexanol, isopropyl alcohol, 2-butanol and 3-methyl-1-butanol was carried out using a
similar procedure using VVO2-complexes as catalyst precursors. Here experimental details are
given for a representative substrate, benzyl alcohol, using Cs[VVO2{Hdfmp-(smdt)2] (7). In a
typical reaction, benzyl alcohol (1.08 g, 0.010 mol) and 30% aqueous H2O2 (2.3 g, 0.020 mol)
were mixed in 5 mL of CH3CN followed by addition of 0.002 g catalyst precursor. The reaction
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mixture was slowly stirred at 80 C for 6 h. The obtained oxidation products, i.e. benzaldehyde,
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benzoic acid and benzylbenzoate, were analyzed quantitatively using gas chromatography and by
comparing their retention times with those of authentic samples; their identity was also
confirmed by GC-MS. The catalytic reactions with the remaining substrates were carried out
under the ‘optimized’ reaction conditions (details given in results and discussion section).
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Catalytic oxidation of ethylbenzene
The catalytic oxidation of ethylbenzene was carried out in a 25 mL round bottom flask
equipped with a magnetic stirrer and immersed in a thermostated oil bath. In a typical
experiment, the catalyst precursor, a VVO2-complex (0.003 g) was added to ethylbenzene (1.08
g, 0.010 mol) and 30% aqueous H2O2( (2.3 g, 0.020 mol) in CH3CN (5 mL). The course of the
reaction was monitored by GC-MS. The oxidation products were identified by comparing their
retention times with those of authentic samples; their identity was also confirmed by GC-MS.
Control reactions were carried out in the absence of catalyst, under the same conditions as the
catalytic runs. No relevant amounts of products were detected.
Oxidation of other hydrocarbons e.g. tetraline, toluene, benzene and cumene was carried out
similarly under the ‘optimized’ reaction conditions (details are given in results and discussion
section).
Tests of sulfoxidation of ligand precursors
These tests were done with both H3dfmp-(smdt)2 (I) and Cs[VVO2{Hdfmp-(smdt)2}] (7)
using a 10%, v/v, H2O2 solution (2.94 M), prepared by a dilution of a commercial 30% v/v
solution with methanol (MeOH).
In a vial 1.1 mg (2.95×10−6 mol) of (I) in 2 mL of a 30% MeOH in acetonitrile solution,
followed by the addition of 8 μL of the oxidant solution (2.35×10−5 mol). The mixture was left to
stir at 60 °C and an HPLC of a 1:5 dilution of 100 μL in MeOH was performed every 2 h to
follow the reaction up to 6 h. In another vial 2.15 mg (3.67×10−6 mol) of the complex were
dissolved in 5 mL of CH3CN followed by the addition of 15 μL of the H2O2 solution (4.41×10−5
mol). The mixture was left to stir at 60°C and an HPLC of a 1:2.5 dilution of 200 μL in MeOH
was carried out every 2 h to follow the reaction up to 6 h.
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The HPLC were performed on a Jasco System using an 880-51 (2-line degasser), an 887-PU
(Intelligent Prep. Pump) and an 870-UV (Intelligent UV/Vis detector) with software BorWin.
The solutions were analyzed through a LiChrosphere 100 RP-18 (5 μm) column using a
MeOH/H2O= 65/35 eluent; flow rate: 0.9 mL/min; detecting at λ= 300 nm. In the
chromatograms the H3dfmp-(smdt)2 corresponds to a broad peak at ~133 min. The samples
corresponding to the peaks that showed up were analyzed by ESI-MS (positive and negative
modes.
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Results and Discussion
[VIVO(acac)2] reacts with the ligand precursors H3dfmp-(smdt)2 (I), H3dfmp-(sbdt)2 (II)
and H3dfmp-(tsc)2 (III) in 2:1 molar ratios in refluxing methanol to give the oxidovanadium(IV)
(VIVO) complexes [VIVO{Hdfmp-(smdt)2}(CH3OH)] (1), [VIVO{Hdfmp-(sbdt)2}(CH3OH)] (2)
and [VIVO{Hdfmp-(tsc)2}(CH3OH)] (3), respectively. Oxidation of these compounds in the
presence of KOH gave the corresponding VVO2-species K[VVO2{Hdfmp-(smdt)2}] (4),
K[VVO2{Hdfmp-(sbdt)2}] (5) and K[VVO2{Hdfmp-(tsc)2}] (6), or in presence of CsOH gave
Cs[VVO2{Hdfmp-(smdt)2}] (7), Cs[VVO2{Hdfmp-(sbdt)2}] (8) and Cs[VVO2{Hdfmp-(tsc)2}]
(9). Eqns. (1) to (3) summarize the synthetic procedures considering H3dfmp-(smdt)2 (I) as a
representative ligand.
[VIVO(acac)2] + H3dfmp-(smdt)2 + CH3OH → [VIVO{Hdfmp-(smdt)2}(CH3OH)] + 2Hacac
[VIVO{Hdfmp-(smdt)2}(CH3OH)] + 2KOH + ½O2 → K[VVO2{Hdfmp-(smdt)2}] + CH3OH
(1)
(2)
[VIVO{dfmp-(smdt)2}(CH3OH)] + 2CsOH + ½O2 → Cs[VVO2{dfmp-(smdt)2}] + CH3OH (3)
Fig. 2 provides an overview of the complexes described in this work. The structural formulae of
each of these complexes is based on elemental analyses and spectroscopic (IR, UV-vis, EPR, 1H,
13C and 51V NMR) data.
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Fig. 2 Proposed structural formulae of VIVO- and VVO2-complexes described in this work.
Thermal analysis
Thermal behavior of complexes 1-9 was studied under an oxygen atmosphere. The
samples of 1-3 lose mass above 105 °C roughly corresponding to one solvent molecule,
indicating the presence of weekly bound or adsorbed MeOH. Upon further increasing the
temperature the solvent free complexes 1-3 decompose exothermically in one step with the
formation of a residue of V2O5, while 4-9 decompose in two/ three overlapping steps forming
MVO3 (M = K or Cs) as final residues. The final weight of each residue agrees with that
expected form the initial amount of complexes used (see Table ESI-1).
IR spectral study
Selected IR spectral data of the compounds are presented in Table ESI-2. The IR spectra of
the ligands exhibit (N-H) and (C=S) at 2980–3098 cm−1 and 1023–1038 cm−1, respectively.
All complexes retain these two bands with some deviations along with the appearance of a new
band in the region 1230-1280 cm−1. The later one suggests the thioenolization of one of the S-
containing moieties and replacement of H by the metal ion, while presence of former bands
suggests that the ligands have at least one non-metal bound thionic group. Similarly, the (C=N)
(azomethine) stretch, appearing at 1609 – 1616 cm−1 in ligands, splits into two: one remains at
nearly the same position to that of respective ligand, while the other one shows up at lower wave
number (1554-1560 cm−1). This is also in the line of the coordination of N-atom of only one
azomethine group.
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9
The VIVO-complexes exhibit a sharp band at 991-998 cm–1 due to (V=O) stretch37 while
VVO2-complexes exhibit two such sharp bands in the 908 – 921 and 890–897 cm–1 region
assigned to asym(O=V=O) and sym(O=V=O) modes, respectively, typical of cis-VVO2-
complexes.38
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Electronic spectral study
Compounds H3dfmp-(smdt)2, H3dfmp-(sbdt)2 and H3dfmp-(tsc)2 exhibit absorption bands at
ca. 206, 255, 328 and 360 nm which are assigned to φ → φ*, π → π1*, π → π2*, and n →π*
transitions, respectively. Similar slightly shifted bands are also observed in the corresponding
complexes (Table ESI-3). In addition, new bands of medium intensity appear at ca. 410-450 nm,
which is assigned to ligand to metal charge transfer (LMCT) bands (Fig. 3).
Upon dissolution the VIVO-complexes have tendency to hydrolyze and oxidize (vide
infra). Three bands at 751, 740 and 760 nm are observed at higher concentration of complexes 1,
2 and 3, respectively, which are assigned to d–d transitions. However, it is probable that these
bands might be due to the presence of more than one isomer of each VIVO-complex in solution.
For the VVO2-complexes no such bands were detected.
Fig. 3 (a) UV-Vis spectra of the VIVO-complexes in DMSO (each ca. 2.5±0.2×10–4 M. (b) UV-
vis spectra of the VVO2-complexes in MeOH 4 (2.6×10–4 M), 5 (1.9×10–4 M) and 6 (2.8×10–4 M).
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1H NMR spectral study
1
The H NMR spectra of the ligands and complexes were recorded to confirm their
structure as well as coordinating modes of the ligands. The relevant spectral data are collected in
Table 1. The broad signal appearing at δ = 9.56-10.90 ppm in the ligands due to the phenolic OH
group is not seen in the spectra of the complexes, suggesting the coordination of the phenolate
O-atom to the metal (Fig. ESI-1). The appearance of a new resonance significantly down field,
equivalent to one proton in the complexes along with the original signal of the ligands equivalent
to one proton due to the azomethine moiety demonstrates the coordination of the only one of the
azomethine N-atom to the metal. Similarly, the resonance due to the hydrogen of -NH protons
(equivalent to two protons) remains at nearly same position (but corresponding to only one
proton) supports the enolization of only one of the NH groups and consequent replacement of H
by the metal ion. Aromatic protons appear in the expected regions in spectra of the ligands and
complexes with slight shifts in their positions. Signals due to the protons of the methyl appear
well within the expected range.
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9
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1
Table 1 H NMR spectral data of ligand precursors and VVO2-complexes in DMSO–d6
Compounds
–CH=N–
Aromatic H
-OH
–NH
-CH3
-NH2
8.50 (d, 2H)
7.70 (s, 1H), 7.78 (s, 1H)
10.85
(s, H)
13.50 (s, 2H)
2.49 (s,3H), 2.5 (s,6H)
I
8.86 (s, 1H)
8.84 (s, 1H)
7.73 (s,1H), 7.76 (s, 1H)
7.73 (s,1H), 7.76 (s, 1H)
13.40 (s, 1H)
13.40 (s, 1H)
2.41 (s, 3H), 2.6 (s,
3H), 2.65 (s ,3H)
4
7
8.86 (s, 1H)
8.84 (s, 1H)
2.41 (s, 3H), 2.6 (s,
3H), 2.65 (s ,3H)
8.50 (d, 2H)
7.57 (s, 5H), 7.51 (s, 5H)
7.70 (s, 1H), 7.78 (s,1H)
10.7
(s, 1H)
13. 35 (s, 2H) 2.4 (s,3H), 2.7 (m, 6H)
II
5
8.81 (s,1H)
8.85 (s,1H)
7.51 (s, 5H), 7.6 (s, 5H)
7.70 (s, 1H), 7.78 (s,1H)
12.8 (s, 1H)
12.8 (s, 1H)
11.52, 11.46
11.7 (s, 1H)
11.7 (s, 1H)
4.65 (s, 2H), 4.74(s,
2H), 2.4 (s, 3H)
8.81 (s, 1H)
8.85 (s,1H)
7.51 (s, 5H), 7.6 (s, 5H),
7.70 (s, 1H), 7.78 (s,1H)
4.65 (s, 2H), 4.74 (s,
2H), 2.4 (s, 3H)
8
8.36 (d, 2H)
7.64 (s, 1H), 7.59 (s, 1H)
7.65 (s, 1H), 7.49 (s, 1H)
7.65 (s, 1H), 7.49 (s, 1H)
9.56
(s, 1H)
2.25 (s, 3H)
2.45 (s, 3H)
2.45 (s, 3H)
8.18, 8.08
III
6
8.45 (s, 1H)
8.52 (s,1H)
8.29 (s, 2H)
8.08 (s, 2H)
8.45 (s, 1H)
8.52 (s,1H
8.29 (s, 2H)
8.08 (s, 2H)
9
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13C NMR spectral study
13
The C NMR spectra of the ligands are recorded (Table ESI-4) and the peaks are
assigned based on the intensity patterns of the chemical shift and on the coordination-induced
shifts (Δδ) of the signals for carbon atoms in the vicinity of the coordinating atoms. These
assignments also provide the useful information for the elucidation of the structures of the
ligands and complexes. Compounds I, II and III display a lower number of resonances that the
corresponding number of carbon atoms. This happened due to the presence of a centre of
symmetry in the ligands. For example, complex K[VVO2{Hdfmp-(smdt)2}] (4) (Fig. 4), shows 13
signals corresponding to the 13 carbon atoms present in the system.
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Fig. 4 13C NMR spectra of H3dfmp-(smdt)2 (I) and of K[VVO2{Hdfmp-(smdt)2}] (4) recorded in
DMSO–d6. The (*) in figure indicates signal due to DMSO itself.
EPR study
The EPR spectra of “frozen” solutions (77 K) in DMSO of compounds 1 – 3 are depicted in
(Fig. 5). The lines show broadening due to both incomplete rotational averaging of the g and A
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tensors and probable partial precipitation of the complexes upon freezing the solutions. The
spectra were simulated and the spin Hamiltonian parameters obtained by simulation of the
spectra are included in Table 2.
7
8
9
est
The values of |Az| can be estimated using the additivity relationship [Az = Az,i (i = to
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4)], (Az,i = contribution of each of the four donor atoms coordinated equatorially), proposed by
Wüthrich and Chasteen with estimated accuracy of ±3×10–4 cm–1. The |Az| values obtained for 1
– 3 (Table 2) agree with the |Azest| values calculated from the partial contributions of the
equatorial donor groups20, 39-42 relevant in the present case {Ophenolate (38.9×10−4 cm−1), Nimine
(38.1 to 43.7×10−4 cm−1), ODMSO (41.9×10−4 cm−1), Sthiolate (31.9×10−4 cm−1), H2O (45.7×10−4
cm−1)} either assuming a NSO2 (species I) with H2O coordination, or a NSO2 (species II) with
DMSO coordination, as equatorial binding sets. In both cases Sthiolate is considered as an
equatorial donor atom.43
Table 2 Spin Hamiltonian parameters obtained by simulation of the experimental 1st derivative
EPR spectra recorded for DMSO solutions of complexes 1 – 3 at 77 K.
Complexes in DMSO (4 mM)
gz
|Az|
gx , gy
|Ax| , |Ay|
1
1
(×10−4 cm− )
(×10−4 cm− )
[VIVO{Hdfmp-(smdt)2}] (1)
Species I
1.951
1.946
167.5
1.978
47.1
Species II
[VIVO{Hdfmp-(sbdt)2}] (2) Species I
Species II
155.4
1.979
47.5
1.951
1.948
1.956
159.3
155.5
159.8
1.976
1.975
1.978
44.3
44.4
44.7
[VIVO{Hdfmp-(tsc)2}] (3)
Species I
Species II
1.942
156.8
1.975
44.6
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Fig. 5 First derivative EPR spectra of frozen (77 K) solutions (ca. 4 mM) (a) [VIVO{Hdfmp-
(sbdt)2}] (2) (b) [VIVO{Hdfmp-(smdt)2}] (1) in DMSO and (c) [VIVO{Hdfmp-(tsc)2}] (3) in
DMSO.
51V NMR study
51
The VVO2-complexes were further characterized in solution by recording their V NMR
spectra in DMSO-d6. The 51V chemical shift values (V) are reported in Table 3. The resonances
are somewhat broadened due to quadrupolar interactions; the line widths at half height are
around 200 Hz, which may be considered comparatively narrow in 51V NMR spectroscopy.42,44,45
Complexes 4 – 9 display one strong resonance in the –450.0 to –468.5 ppm region, values in
agreement with other reported examples where S-atoms participate in coordination in addition to
the O and N donor atoms.42,46
Table 3 Summary of the 51V NMR data (V values) of the VVO2-complexes studied in this work
in DMSO-d6.
Chemical
shifts (V/ppm)
Chemical
shifts (V/ppm)
Compounds
Compounds
K[VVO2{Hdfmp-(smdt)2}] (4)
K[VVO2{Hdfmp-(sbdt)2}] (5)
K[VVO2{Hdfmp-(tsc)2}] (6)
–464.1
–464.3
–450.0
Cs[VVO2{Hdfmp-(smdt)2}] (7)
Cs[VVO2{Hdfmp-(sbdt)2}] (8)
Cs[VVO2{Hdfmp-(tsc)2}] (9)
–464.0
–459.8
–460.9
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Several experiments were carried out to detect the possible formation of VV-species in
solutions containing Hdfmp-(smdt)2 (I) and VVO2-complexes. Fig. ESI-2 depicts 51V NMR
spectra of a solution of [VIVO{Hdfmp-(smdt)2}(MeOH)] (1) in MeOD a few minutes after
addition of KOH, and with time up to 72 h. Initially a resonance at ca. -405 ppm is detected; after
2 h and till 72 h, only the resonance with V = –453 ppm, corresponding to the formation of
[VVO2{Hdfmp-(smdt)2}]− is detected. In similar experiments but adding CsOH in MeOD, instead
of KOH, Fig. 6, only the resonance with V = –453 ppm was detected.
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Cs[VVO2{Hdfmp(smdt)2}] – 72 h
Cs[VVO2{Hdfmp(smdt)2}] – 48 h
Cs[VVO2{Hdfmp(smdt)2}] – 24 h
Cs[VVO2{Hdfmp(smdt)2}] – 2 h
Cs[VVO2{Hdfmp(smdt)2}] – ~0 h
Fig. 6 51V NMR spectra of solutions of Cs[VVO2{Hdfmp-(smdt)2}] (7) in MeOD. The spectra
were measured upon addition of a solution of CsOH in MeOD, and after 2, 24, 48 and 72 h, to
[VIVO{Hdfmp(smdt)2}(MeOH)] (1) (4 mM) in MeOD.
Experiments were also carried out to detect the possible formation of [VVO(O2)(Hdfmp-
(smdt)2] species in solutions containing either 4 or 7 in MeOD upon additions of an aqueous
solution of H2O2. In these solutions only the resonances corresponding to [VVO2{Hdfmp-
(smdt)2}]− were detected, even upon addition of relatively high excesses of H2O2. Upon addition
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of an excess of H2O2, and observation of the resonance at -453 ppm, after ca. 24 h these peaks
either disappeared (case of 4) (Fig. ESI-3), or decreased significantly its intensity (case of 7)
(Fig. ESI-4). Thus, after 24 h either the VV was reduced to a VIV-containing species or some type
7
8
9
51
of not clearly visible solid formed, not allowing obtaining the V NMR resonances. Globally, if
any VVO-(O2)-dfmp-(smdt)2} species form in solution, they could not be detected in these V
51
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NMR measurements.
Experiments were also carried out adding tert-butylhydroperoxide (t-BuOOH) to
solutions of Cs[VVO2{Hdfmp-(smdt)2}] in 10% CD3OD:CH3OH (Fig ESI-5A). Also in this case
only the resonance at -453 ppm was detected, at least up to a molar ratio t-BuOOH:7 of 10.
Therefore, if any VVO-(t-BuOOH)-dfmp-(smdt)2} species form in solution, they could not be
detected in these 51V NMR measurements.
Assays were also carried out adding HCl to solutions of Cs[VVO2{Hdfmp-(smdt)2}] in
MeOH (Fig ESI-5B). Up to a ratio of added HCl : 7 of 3:1, only the resonance at -453 ppm was
detected. For the ratios 5:1 and 10:1, the solution turned yellow and only a resonance at -402
ppm was recorded, this probably being due to the formation of [VVO(OH){Hdfmp-(smdt)2}] at
lower pH values. Similar observations of resonances at downfield V values upon addition of
acid have been reported and were also explained as due to the formation of
oxidohydroxidovanadium(IV) complexes.27, 47-50
Reactivity of complexes
Reactivity of oxidovanadium(IV) and dioxidovanadium(V) complexes with H2O2
The
oxidovanadium(IV)
complexes,
[VIVO{Hdfmp-(smdt)2}(CH3OH)]
(1),
[VIVO{Hdfmp-(sbdt)2}(CH3OH)] (2) and [VIVO{Hdfmp-(tsc)2}(CH3OH)] (3) in DMSO depict
changes upon addition of H2O2 which were monitored by electronic absorption spectroscopy.
The changes observed in the case of [VIVO{Hdfmp-(smdt)2(CH3OH)}] are depicted in Fig. 7.
The progressive addition of a dilute solution of H2O2 in DMSO, prepared by adding the required
amount of a 30% aqueous H2O2, to a solution of [VIVO{Hdfmp-(smdt)2}] in DMSO resulted in
the flattening of the d-d band ( at ca. 751 nm). Disappearance of this band indicates the oxidation
of the vanadium(IV) center to a higher oxidation state. The intensity of charge transfer band at
422 nm decreased and finally disappeared.
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Fig. 7 Titration of 25 mL of a 5.310–4 M solution of [VIVO{Hdfmp-(smdt)2(CH3OH)}] (1) in
DMSO with H2O2. The UV-Vis absorption spectra were recorded upon stepwise additions of one
drop portions of a H2O2 solution (2.3×10–2 M in DMSO), prepared by adding the required
amount of a 30% aqueous H2O2. The inset shows an expanded spectral region (700 – 840 nm);
these spectra were recorded upon stepwise additions of one drop portions of a H2O2 solution
(4.2×10–2 M in DMSO) to 25 mL of a 4.510–3 M solution in DMSO.
Complex [VIVO{Hdfmp-(sbdt)2(CH3OH)}] depicted different trends. Here, the charge
transfer band appearing at ~413 nm slowly shifts to ~425 nm without changing its intensity.
Bands of other regions remain in nearly the same positions as in the original complex (Fig. 8).
Notwithstanding, the progressive addition of a dilute H2O2 solution in DMSO to a solution of
[VIVO2{Hdfmp-(sbdt)2}] in DMSO resulted in the flattening of the d – d band ( at ca. 740 nm),
showing that the VIV-center oxidizes to VV upon addition of H2O2.
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Fig. 8 Titration of 25 mL of a 8.310–4 M solution in DMSO of [VIVO{Hdfmp-
(sbdt)2(CH3OH)}] (2) with H2O2.The UV-Vis spectra were recorded upon stepwise additions of
one drop portions of a H2O2 solution (2.1×10–2 M in DMSO). The inset shows the expanded
spectral region (700 – 850 nm); these were recorded upon stepwise additions of one drop
portions of a H2O2 solution (3.8×10–2 M in DMSO), also prepared by adding the required amount
of a 30% aqueous H2O2, to 25 mL of a 7.210–3 M solution of the complex in DMSO.
In the experiment with [VIVO{Hdfmp-(smdt)2(CH3OH)}] (1), it is clear that upon adding
H2O2 the process VIV → VV took place, but there is no obvious indication that a VVO-O2-
{Hdfmp-(smdt)2(CH3OH)} species might have formed. The appearance of the shoulder band at
ca. 340-350 nm, at some stage of addition of H2O2 might be an indication for the formation of
the diperoxido complex VVO(O2)2,51 but not of VVO(O2) species. For higher amounts of H2O2
probably there are no more VVO-Hdfmp-(smdt)2 species in solution. In contrast, with
[VIVO{Hdfmp-(sbdt)2(CH3OH)}] (2), upon additions of H2O2 the appearance of the band at ca.
425 nm (and at ca. 310 nm) suggests the formation of VVO-O2-Hdfmp-(sbdt)2 species.52
Upon additions of three drops of a dilute H2O2 solution in DMSO to a solution of
[VIVO{Hdfmp-(tsc)2(CH3OH)}] (3) in DMSO, the intensity of the charge transfer band
appearing at ~428 nm slowly decreases along with the appearance of a weak shoulder at ca. 385
nm (Fig. ESI-6). Simultaneously, the 370 nm band gained intensity, while the 318 nm band
increased its intensity along with slight shift to ~312 nm. The d-d band appearing at 760 nm
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slowly flattened and finally almost disappeared. This experiment demonstrates that upon addition
of H2O2, with time the VIV-center oxidizes to VV upon addition of H2O2. The maintenance of
clear bands at ca. 370 and 312 nm53 suggests that VVO-O2-dfmp(tsc)2 species form in these
conditions.
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Adding a dilute H2O2 solution in DMSO to solutions of the VVO2-complexes, instead of
the corresponding VVO-compounds, some trends observed were somewhat different. In the case
of Cs[VVO2{Hdfmp-(smdt)2}] (7), the intensity of the charge transfer band at 421 nm slowly
decreased and flattened, while a new band appeared at 375 nm. The intensity of the ~306 nm
band decreased and finally almost disappeared (Fig. 9). This suggests that VVO-O2-dfmp(smdt)2
species formed, with max at ~375 and ~310 nm.52
Fig. 9 Titration of 25 mL of a 4.310–4 M solution in CH3OH of Cs[VVO2{Hdfmp-(smdt)2}].
The absorption spectra were recorded upon stepwise additions of one drop portions of a H2O2
solution (2.2×10–2 M in DMSO, prepared by adding the required amount of a 30% aqueous
H2O2).
In the experiments with both H3dfmp-(smdt)2 (I) and Cs[VVO2{Hdfmp-(smdt)2}] (7)
adding a ~10-fold excess of H2O2 solution leaving the mixtures stirring at 60 ºC for 6 h, and
controlling the evolution of the solutions by HPLC, extensive hydrolysis of the VVO2-complex
was noticed, but no clear peaks corresponding to sulfoxidation products of I were obtained.
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Complexes Cs[VVO2{Hdfmp-(sbdt)2}] (8) (Fig. ESI-7) and Cs[VVO2{Hdfmp-(tsc)2}] (9) ) (Fig.
ESI-8) exhibit rather similar trends upon additions of dilute H2O2 solutions. The intensity of CT
bands appearing at ~423 nm (for 8) and at 414 nm (for 9) gradually decreases, and the band at
~364 nm shifts to 359 nm in the case of compound 8, while the band at ~364 nm of 9 increases
intensity upon the addition of the H2O2 solution. Again these trends, namely the observation of
bands at these max values suggest the formation of either VVO-O2-dfmp(sbdt)2 or VVO-O2-
dfmp(tsc)2 species, as was reported for many other peroxidovanadate complexes.52
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Reactivity of oxidovanadium(IV) complexes with HCl
The behavior of a methanolic solution of [VIVO{Hdfmp-(smdt)2(CH3OH)}] (1) upon
addition of HCl was studied by drop wise addition of a methanolic solution of HCl to 25 mL of a
solution of 1. The changes observed are presented in Fig. 10(a). The intensity of band appearing
at 433 nm decreased. Simultaneously, the weak band appearing at 401 nm sharpens with slight
gain in intensity. The band at 256 nm loses its intensity while the band at ~325 nm shifts to ~349
nm with gain in intensity. In the case of [VIVO{Hdfmp-(sbdt)2(CH3OH)}] (2) the results are
rather similar (Fig. 10(b)), the band at 414 nm disappears gradually. Simultaneously, band at 314
nm gains intensity shifting to ~329 nm. The solution became dark at the end the HCl additions.
Such changes have been interpreted as partial hydrolysis of the complexes, oxidation of V(IV) to
V(V) and formation of oxidohydroxidovanadium(V) species. The formation of VVO(OH)L
complexes upon addition of acid has been previously reported.18,19,47,48,53 The experiments with
the other complexes yielded similar results.
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b
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Fig. 10 (a) Titration of methanolic solution of [VIVO{Hdfmp-(smdt)2(CH3OH)}] (1) with HCl
solution. The spectra were recorded upon stepwise addition of two drop portions of HCl solution
(prepared by dissolving two drops of 12.5 M HCl in 10 mL of MeOH) to 25 mL of 2.310–4 M
solution of (1) in CH3OH. (b) Titration of methanolic solution of [VIVO{Hdfmp-
(sbdt)2(CH3OH)}] (2) with HCl solution. The spectra were recorded upon stepwise addition of
two drop portions of HCl solution (prepared by dissolving two drops of 12.5 M HCl in 10 mL of
MeOH) to 25 mL of 3.410–4 M solution of 2 in CH3OH.
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Catalytic oxidations
Oxidation of benzyl alcohol
The oxidation of alcohols is one of the important reactions of organic molecules. These
reactions may involve quite different terminal oxidants and metal catalysts are capable of
promoting this reaction. Vanadium compounds have been used as catalyst precursors and the
subject was recently reviewed.23 We tested the efficacy of the presently synthesized complexes
for the catalytic oxidation of primary and secondary alcohols using aqueous H2O2 as oxidant and
taking benzyl alcohol as a reference compound to establish the best experimental conditions.
Generally, the oxidation of benzyl alcohol gives mainly three products: benzaldehyde, benzoic
acid and benzyl benzoate, which were also observed previously.54
OH
Catalyst, H2O2
CH3CN, 80 oC
OH
O
O
O
O
+
+
benzyl benzoate
benzaldehyde benzoic acid
benzyl alcohol
Scheme 1 Oxidation of benzyl alcohol catalyzed by oxidovanadium(V) complexes.
In order to achieve the maximum oxidation products of benzyl alcohol the effects of the
following parameters were studied in detail using Cs[VVO2{Hdfmp-(smdt)2}] (7) as a
representative catalyst:
i. Amount of catalyst
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ii. Amount of H2O2
iii. Amount of solvent
iv. Temperature.
7
8
9
The effect of different amounts of catalyst in the oxidation of benzyl alcohol was
checked. Three different amounts of catalyst were taken in the mixture of benzyl alcohol (1.08 g,
10 mmol), 30% H2O2 (2.27 g, 0.020 mmol) and 5 mL of acetonitrile and the reaction was carried
out at 80 ºC. As shown in Fig. 11(a), increasing the amount of catalyst from 0.002 g to 0.003 g,
the overall % conversion increases from 60 to 72 %. Upon further increase of the catalyst
amount to 0.004 g, the overall conversion remained constant (73%). The order of the selectivity
of products for 0.002 g of catalyst after 2 h of reaction was: benzaldehyde (89%) > benzyl
benzoate (9%) > benzoic acid (1%). In the case of 0.003 g of catalyst, the selectivity of products
followed the order: benzaldehyde (90%) > benzoic acid (6%)> benzyl benzoate (2%), whereas
for 0.004 g of catalyst the selectivity order was: benzaldehyde (91%) > benzoic acid (6%) >
benzyl benzoate (1%). Therefore, an amount of 0.003 g of catalyst was considered as optimum
for optimizing the remaining conditions.
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The effect of amount of oxidant (30% aqueous H2O2) for the oxidation of benzyl alcohol
is illustrated in Fig. 11(b). Under above conditions, a oxidant:substrate ratio of 1:1 gave 48%
conversion with 88% selectivity of benzaldehyde, 9% of benzoic acid and 2% of benzyl
benzoate. Increasing this ratio to 2:1 improved the conversion to 90%, while conversion
remained almost constant on further increasing this ratio to 3:1. With higher amounts of oxidant
the conversion did not increase much possibly due to the presence of larger amounts of water.
Therefore, a substrate:H2O2 ratio of 1:2 was considered suitable to optimize the other conditions.
Similarly, the effect of solvent was studied considering 5, 7 and 10 mL of acetonitrile.
Under the above conditions, 5 mL acetonitrile gave 70% conversion with selectivity of
benzaldehyde (90%), benzoic acid (7%) and benzyl benzoate (2%), while increasing the solvent
volume to 7 mL did not improve the overall conversion of benzyl alcohol. Upon further
increment of solvent to 10 mL the conversion remained almost same (Fig. 11(c)). Therefore, 5
mL was considered as optimized under these reaction conditions.
Under the optimized reaction conditions i.e. benzyl alcohol (1.08 g, 0.010 mol), catalyst (0.003
g), 30% H2O2 (2.27 g, 0.020 mol) and acetonitrile (5 mL), the reaction was carried out at three
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different temperatures: 60, 70 and 80 oC. A maximum of 42% conversion was obtained at 60 oC.
This conversion improved to 60% at 70 oC and to 73% at 80 oC (Fig. 11(d)). Therefore, we chose
80 ºC as the optimized reaction temperature. Table ESI-5 summarizes some of the data of the
above described experiments.
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b
a
c
d
Fig. 11 Catalytic oxidation of benzyl alcohol using Cs[VVO2{Hdfmp-(smdt)2}] (7) as catalyst
precursor. (a) Effect of different amounts of catalyst precursor; other reaction conditions: benzyl
alcohol (1.08 g, 10 mmol), 30% H2O2 (2.27 g, 0.020 mol), acetonitrile (5 mL) and reaction temp.
o
80 C. (b) Effect of different substrate:oxidant ratios; other reaction conditions: benzyl alcohol
(1.08 g, 0.010 mol), catalyst (0.003 g), acetonitrile (5 mL) and reaction temp. 80 oC. (c) Effect of
amount of solvent (acetonitrile); other reaction conditions: benzyl alcohol (1.08 g, 0.010 mol),
o
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temperature; other reaction conditions: benzyl alcohol (1.08 g, 0.010 mol), catalyst precursor
(0.003 g), 30% H2O2 (2.27 g, 0.020 mol) and acetonitrile (5 mL).
7
8
9
Under the above optimized reaction conditions a blank reaction was carried out without
addition of catalyst which yielded the percent conversion of only 6% (Fig. ESI-9).
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Table ESI-5 presents oxidation of benzyl alcohol under different reactions. Taking the
data from entry no 2, considered as the best suited for the reaction, where 72% conversion of
benzyl alcohol is achieved. The formation of three products starts with the consumption of
benzyl alcohol (Fig. 12). The selectivity for the formation of benzaldehyde is by far the highest
amongst all products. After 2 h of the reaction the selectivity of benzaldehyde reaches 90% and
then decreases to 80% at the end of 8 h, possibly due to overoxidation: formation of small
amounts of benzoic acid, which reaches 7% at the end of 8 h (Fig. 12). Part of the benzoic acid
reacts with benzyl alcohol to give benzyl benzoate and its overall conversion reaches 13 %.
Fig. 12 Profile showing the % consumption of benzyl alcohol along with the % selectivity of the
formation of products using Cs[VVO2{Hdfmp-(smdt)2}] (7) as catalyst precursor under the
optimized reaction conditions (entry no 2 of Table ESI-5).
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9
We also tested the catalytic potential of two other VVO2-complexes, Cs[VVO2{Hdfmp-
(sbdt)2}] (8) and Cs[VVO2{Hdfmp-(tsc)2}] (9) for the oxidation of benzyl alcohol under the
optimized reaction conditions mentioned above. Use of Cs[VVO2{Hdfmp-(sbdt)2}] gave 73%
conversion while Cs[VVO{Hdfmp-(tsc)2}] gave 70% conversion. Fig. ESI-10 compares
conversion profiles of all three catalyst precursors under optimized reaction conditions and Table
4 presents turn over frequency and selectivity data of products for these catalysts. It is clear that
there are not much differences between the performances of the three compounds.
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Table 4 Conversion of benzyl alcohol, TOF and product selectivity. Conditions: benzyl alcohol
(1.08 g, 0.010 mol), catalyst precursor (0.003 g), 30% aqueous H2O2 (2.27 g, 0.020 mol),
acetonitrile (5 mL) and temperature 80 oC.
1
TOF/ h−
S. No.
Conv.
(%)
Selectivity (%)
Benzaldehyde Benzoic Benzyl
Catalyst
acid
7
benzoate
1.
2
Cs[VVO2{Hdfmp-(smdt)2}] 72
Cs[VVO2{Hdfmp-(sbdt)2}] 73
240
304
206
80
78
80
13
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5
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14
2
Cs[VVO2{Hdfmp-(tsc)2}]
70
Under the above reaction conditions, the oxidation of other primary and secondary
alcohols was also tested. Table 5 presents the conversion of alcohols as well as the selectivity of
the key product i.e., the corresponding aldehyde/ketone. Amongst the secondary alcohols studied
2-butanol gave maximum conversion of 95% followed by 1-phenylethanol with 90% conversion.
Similarly amongst the primary alcohol tested, Cs[VVO2{Hdfmp-(smdt)2}] (7) showed the highest
conversion of 82% for 3-methyl-1-butanol (isoamyl alcohol). The selectivity of the products was
100% in all cases except for 4-isopropylbenzyl alcohol, where the formation of a small amount
4-isoporpylbenzoic acid was also obtained.
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Table 5 Conversion of several different alcohols to the corresponding aldehyde/ketone, their
TOF and product selectivity using Cs[VVO2{Hdfmp-(smdt)2}] (7) as catalyst precursor.a
7
8
9
1
Product
selectivity (%)
TOF/h−
Sr No
1
Substrate
Product
Conv.
(%)
OH
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83
441
4-isopropylbenzyl
alcohol
4-isopropylbenzaldehyde
O
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662
17
1-phenyl ethanol
OH
acetophenone
O
cyclohexanol
OH
cyclohexanone
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536
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586
isopropyl alcohol
OH
acetone
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OH
2-butanone
O
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3-methyl-1-butanol
3-methyl-1-butanal
a Reaction conditions: substrate (0.010 mol), catalyst (0.003 g), 30% aqueous H2O2 (2.27 g,
0.020 mol), acetonitrile (5 mL) and temperature 80 oC.
Oxidation of arenes
The oxidation of simple arenes is an important type of transformation in fine chemicals
industry. For example, phenol is an important molecular precursor of many high value aromatics
and resins. Thus the direct and selective oxidation of benzene and several other aromatic
compounds are reactions that require suitable catalytic processes. Vanadium compounds have
been used as catalyst precursors and the subject was also recently reviewed.23 We were interested
in probing the efficacy of the presently synthesized complexes for the catalytic oxidation, with
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aqueous H2O2, of ethylbenzene, tetraline, toluene, benzene and cumene, and we chose
ethylbenzene as a reference compound to establish the best experimental conditions.
7
8
9
The catalytic oxidation of ethylbenzene with aqueous H2O2 was studied in the presence of
Cs[VVO2{Hdfmp(smdt)2}] (7) as catalyst precursor. Generally, ethylbenzene upon oxidation of
gives acetophenone, along with a small amount of benzyl alcohol.22,25,55 However, in the present
systems by far acetophenone was characterized as the major oxidation product (Scheme 2)
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OH
O
H2O2, catalyst
+
CH3OH, 80 oC
ethylbenzene
benzyl alcohol acetophenone
Scheme 2 Oxidative products of ethylbenzene catalyzed by the present VVO2-based systems.
In order to achieve the maximum oxidative products of ethylbenzene the effects of
following parameters were studied in detail using Cs[VVO2{Hdfmp-(smdt)2}] (7) as a
representative catalyst:
i. Amount of catalyst
ii. Amount of H2O2
iii. Amount of solvent
iv. Temperature.
Three different amounts of catalyst precursors were added to the mixture containing
ethylbenzene (1.06 g, 0.010 mol), 30% aqueous H2O2 (2.27 g, 0.020 mol) and 5 mL of
acetonitrile and the reaction was carried out at 80 ºC. As shown in Fig. 13(a), increasing the
amount of catalyst from 0.001 g to 0.002 g or to 0.003 g increases the overall percentage
conversion of the substrate from 48 to 51, and 66%, respectively. The order of the selectivity of
products for 0.001 g of catalyst at the end of 20 h is: acetophenone (93%) > benzylalcohol (7%);
for 0.002 g and 0.003 g the selectivity remained the same. Therefore, an amount of 0.003 g of
catalyst was taken for optimizing rest of the conditions.
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The effect of amount of oxidant (30% aqueous H2O2) for the oxidation of ethylbenzene is
illustrated in Fig. 13(b). Under the above conditions, with an oxidant:substrate ratio of 1:1 gave
45% conversion with 92% selectivity of acetophenone and 7% of benzylalcohol. Increasing this
ratio to 2:1 improved the conversion to 65%. The conversion remained almost the same on
further increasing this ratio to 3:1. At higher oxidant:substrate ratio the conversion did not
increase, possibly due to dilution of the reaction mixture and/or hydrolysis of the catalyst, due to
the presence of larger amounts of water. Therefore, a H2O2:substrate ratio of 2:1 was considered
suitable to optimize other conditions. Similarly, the effect of solvent was checked considering 5,
10 and 15 mL of acetonitrile. Under the above conditions, 5 mL acetonitrile gave 63%
conversion with selectivity of acetophenone (90%) and benzylalcohol (7%), while increasing the
solvent volume to 10 to 15 mL did not improve the overall conversion or changed the selectivity
of the products (Fig 13(c)). Under the optimized reaction conditions i.e. ethylbenzene (1.06 g,
0.010 mol), catalyst (0.003 g), 30% H2O2 (2.27 g, 0.020 mol) and acetonitrile (5 mL), the
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o
reaction was carried out at three different temperatures i.e. 70, 80 and 90 C. A maximum of 50
o
o
% conversion was obtained at 70 C. This conversion improved to 65% at 80 C and to 71% at
90 oC, Fig 13(d).
b
a
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