Catalytic oxidation of secondary alcohols by molybdenum complexes derived from 4-acyl pyrazolone in presence and absence of an N-based additive: Conventional versus microwave assisted method

Article abstract of DOI:10.1016/j.ica.2015.09.013

Four dioxidomolybdenum(VI) complexes, [Mo^VIO₂(bp-bhz)(MeOH)] (1), [Mo^VIO₂(bp-inh)(MeOH)] (2), [Mo^VIO₂(bp-nah)(MeOH)] (3) and [Mo^VIO₂(bp-fah)(MeOH)] (4) have been isola

Full text of DOI:10.1016/j.ica.2015.09.013

Accepted Manuscript
Catalytic oxidation of secondary alcohols by molybdenum complexes derived
from 4-acyl pyrazolone in presence and absence of an N-based additive: Con-
ventional versus microwave assisted method
Mannar R. Maurya, Neeraj Saini, Fernando Avecilla
PII:
S0020-1693(15)00451-X
http://dx.doi.org/10.1016/j.ica.2015.09.013
ICA 16688
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
4 June 2015
12 August 2015
8 September 2015
Please cite this article as: M.R. Maurya, N. Saini, F. Avecilla, Catalytic oxidation of secondary alcohols by
molybdenum complexes derived from 4-acyl pyrazolone in presence and absence of an N-based additive:
Conventional versus microwave assisted method, Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/
j.ica.2015.09.013
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Catalytic oxidation of secondary alcohols by molybdenum complexes derived
from 4-acyl pyrazolone in presence and absence of an N-based additive:
Conventional versus microwave assisted method
Mannar R. Maurya a,*, Neeraj Saini a, Fernando Avecilla b
a Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India.
bDepartamento de Química Fundamental, Universidade da Coruña, Campus de A Zapateira,
15071 A Coruña, Spain
Keywords: Molybdenum(VI) complexes; NMR spectroscopy; Crystal structure; Microwave
assisted reaction; Oxidation of secondary alcohols.
__________
* Corresponding author. Tel.: +91 1332 285327; fax: +91 1332 273560.
E-mail address: rkmanfcy@iitr.ac.in (M.R. Maurya).
1

ABSTRACT
Four dioxidomolybdenum(VI) complexes, [MoVIO2(bp-bhz)(MeOH)] (1), [MoVIO2(bp-
inh)(MeOH)] (2), [MoVIO2(bp-nah)(MeOH)] (3) and [MoVIO2(bp-fah)(MeOH)] (4) have been
isolated with ONO tridentate Schiff base ligands, H2bp-bhz (I), H2bp-inh (II), H2bp-nah (III)
and H2bp-fah (IV) [where ligands I to IV are derived from 4-benzoyl-3-methyl-1-phenyl-2-
pyrazoline-5-one (Hbp) and isonicotinoyl hydrazide (Hinh), benzoyl hydrazide (Hbhz),
nicotinoyl hydrazide (Hnah) or furoyl hydrazide (Hfah), respectively] and characterized.
Structures of complexes [MoVIO2(bp-bhz)(MeOH)] (1) and [MoVIO2(bp-nah)(DMSO)]˖0.5 H2O
(3a) have been confirmed by single crystal X-ray study. Using conventional and microwave-
assisted methods, these complexes have been tested as catalysts for the oxidation of secondary
alcohols (1-phenyl ethanol, 2-propanol and 2-butanol) in the presence of 30% H2O2 as an
oxidant. Various parameters such as amounts of catalyst, oxidant, solvent and temperature of the
reaction mixture have been taken into consideration for the maximum conversion of substrates.
Under the atmospheric optimized reaction conditions, secondary alcohols give high yield of the
corresponding ketones selectively. Addition of an N-based additive reduces the reaction time
considerably. Microwave irradiation effect on the reaction time and yield has been taken into
consideration to conclude that microwave method is more efficient compared to conventional
liquid phase method.
1. Introduction
The oxidation of secondary alcohols to the corresponding carbonyl compounds is of
crucial importance in synthetic organic chemistry because the target molecules can be obtained
directly in one-pot [1]. The corresponding ketones are employed as high-value intermediates and
components for pharmaceutical, agrochemical, and perfumery industries [2,3]. Most of the
previously reported oxidation reactions involve the use of harmful oxidants, such as CrO3,
KMnO4, and/or halogenated solvents that produce toxic, environmentally and economically
unacceptable by-products. In order to eradicate such harmful wastes, researches on efficient
catalytic procedures that involve green oxidants, e.g. dioxygen or peroxides [H2O2, tert-
butylhydroperoxide (TBHP) etc.] have drawn attention [4,5].
2

Pyrazolone derivatives have widely been applied in the extraction and separation of
1
various metal ions from aqueous solvent, in laser materials, as H NMR shift reagents, in
chromatographic study and in the petrochemical industry [6-11]. Transition metal complexes of
ONO donor ligands have drawn much attention due to their efficient catalytic activities under
ambient temperature conditions [12-14]. However, molybdenum chemistry with pyrazolone-
based Schiff base and related ligands is less explored [15,16], though these ligands are known to
show appealing complex formation properties [17-19].
Continuing our research on molybdenum chemistry [20-25] and keeping above
considerations in mind, herein we report the synthesis, characterization and reactivity of
dioxidomolybdenum(VI) complexes of pyrazolone derived ligands (Scheme 1) and evaluate their
catalytic efficiency towards the oxidation of secondary alcohols. Conventional as well as
microwave assisted method has been applied for these catalytic reactions.
<<Scheme 1>>
2. Experimental
2.1. Materials
Ammonium molybdate tetrahydrate, isonicotinoyl hydrazide (Hinh), nicotinoyl hydrazide
(Hnah) (Loba Chemie, India), 2-furoyl hydrazide (Hfah), acetylacetone (Hacac),
phenylhydrazine and 1-phenylethanol (Aldrich, U.S.A.) were used as supplied.
Ethylacetoacetate, diethylether, 30% aqueous H2O2, and glacial acetic acid were procured from
Rankem, India. 2-Propanol and 2-butanol were purchased from Qualigens Pvt. Limited, India.
Benzohyrazide (Hbhz) was prepared by the reaction of a twofold excess of hydrazine hydrate
with ethyl benzoate. [MoVIO2(acac)2] [26], 3-methyl-1-phenyl-5-pyrazolone [27] and 4-benzoyl-
3-methyl-1-phenyl-2-pyrazoline-5-one [28] were synthesized following methods reported in
literature. Other chemicals required for the synthesis of various ligands, complexes and catalytic
reactions were purchased from certified companies and used as supplied.
2.2. Instrumentation and Characterization Procedures
Elemental analyses (C, H and N) of the ligands and complexes were carried out on
Elementar Analyser Vario-El-III. Thermo-gravimetric analyses (TGA) of the complexes were
3

carried out using TG Stanton Red croft STA 780. Infrared spectra were recorded as KBr pellets
on a Nicolet 1100 FT-IR spectrometer after grinding the sample with KBr. Electronic absorption
spectra of ligands and complexes were recorded on a Shimadzu single beam spectrophotometer
in methanol. 1H and 13C NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer
with the common parameter settings in DMSO-d6. The δ values are quoted relative to TMS as
internal standard. Anton Paar Monowave 300 Microwave synthesis reactor equipped with IR
sensing probe for controlling the reaction temperature was used to carry out all catalytic
reactions. Non-thermal effects were not studied as the instrument does not permit to work below
o
40 C. A Shimadzu 2010 plus gas-chromatograph fitted with Rtx-1 capillary column (30 m ×
0.25 mm × 0.25 µm) and a FID detector was used to analyze oxidation product(s).
2.3. Preparations
2.3.1. H2bp-bhz (I), H2bp-inh (II), H2bp-nah (III) and H2bp-fah (IV)
These ligands were prepared by following an identical procedure. Preparation of a
representative ligand is presented here. A solution of 4-benzoyl-3-methyl-1-phenyl-2-pyrazoline-
5-one (1.385 g, 0.005 mol) in 50 mL methanol was added to benzoylhydrazide (0.680 g, 0.005
mol) dissolved in 50 mL of methanol with stirring and the obtained reaction mixture was
refluxed on a water bath for 6 h. After keeping the reaction flask at room temperature for 8 h, the
separated pale yellow solid was collected by filtration, washed with methanol and dried in a
vacuum desiccator over silica gel.
H2bp-bhz (I): Yield: 1.78 g (89%). Anal. Calc. for C24H20N4O2 (396.44): C, 72.71; H,
5.08; N, 14.13. Found: C, 72.22; H, 4.86; N, 13.99%.
H2bp-inh (II): Yield: 1.71 g (85%). Anal. Calc. for C23H19N5O2 (397.43): C, 69.51; H,
4.82; N, 17.62. Found: C, 69.82; H, 4.61; N, 17.54%.
H2bp-nah (III): Yield: 1.80 g (90%). Anal. Calc. for C23H19N5O2 (397.43): C, 69.51; H,
4.82; N, 17.62. Found: C, 69.33; H, 4.63; N, 17.51%.
4

H2bp-fah (IV): Yield: 1.76 g (91%). Anal. Calc. for C22H18N4O3 (386.14): C, 68.38; H,
4.70; N, 14.50. Found: C, 68.07; H, 4.73; N, 13.91%.
2.3.2. [MoVIO2(bp-bhz)(MeOH)] (1), [MoVIO2(bp-inh)(MeOH)] (2), [MoVIO2(bp-nah)(MeOH)]
(3) and [MoVIO2(bp-fah)(MeOH)] (4)
Ligand H2bp-bhz (I) (1.98 g, 0.005 mol) was dissolved in 50 mL of methanol. A
methanolic solution (50 mL) of [MoVIO2(acac)2] (1.63 g, 0.005 mol) was added to the above
solution and the reaction mixture was refluxed on a water bath for 4 h. The obtained colored
solution was concentrated to half of its volume and then kept at room temperature for 12 h. The
separated crystalline yellow solid was filtered, washed with cold methanol and dried in vacuum
desiccator over silica gel.
Complexes 2, 3 and 4 were prepared analogously to 1 considering ligands H2bp-inh (II),
H2bp-nah (III) and H2bp-fah (IV), respectively.
[MoVIO2(bp-bhz)(MeOH)] (1). Yield: 2.47 g (89%). Anal. Calc. for C25H22MoN4O5
(554.41): C, 54.16; H, 4.00; N, 10.11. Found: C, 54.20; H, 3.84; N, 10.05%. X-ray diffraction
quality crystals for 1 were obtained by slow evaporation of its methanolic solution in air.
[MoVIO2(bp-inh)(MeOH)] (2). Yield: 2.25 g (81%). Anal. Calc. for C24H21MoN5O5
(555.39): C, 51.90; H, 3.81; N, 12.61. Found: C, 51.58; H, 3.70; N, 12.49%.
[MoVIO2(bp-nah)(MeOH)] (3). Yield: 2.37 g (85%). Anal. Calc. for C24H21MoN5O5
(555.39): C, 51.90; H, 3.81; N, 12.61. Found: C, 51.66; H, 3.77; N, 12.42%. X-ray diffraction
quality crystals for [MoVIO2(bp-nah)(DMSO)]·0.5 H2O (3a) were obtained from its DMSO
solution at low temperature.
[MoVIO2(bp-fah)(MeOH)] (4). Yield: 2.15 g (79%). Anal. Calc. for C23H22MoN4O6
(544.37): C, 50.75; H, 3.70; N, 10.29. Found: C, 50.51; H, 3.53; N, 10.11%.
5

2.4. X-Ray crystal structure determination
Three-dimensional X-ray data were collected on a Bruker Kappa Apex CCD
diffractometer at room temperature for 1 and 3a by the φ-ω scan method. Reflections were
measured from a hemisphere of data collected from frames, each of them covering 0.3º in ω. A
total of 71972 reflections for 1 and 21911 for 3a measured, all were corrected for Lorentz and
polarization effects and for absorption by multi-scan methods based on symmetry-equivalent and
repeated reflections. Of them, 6424 and 5600 independent reflections, respectively, exceeded the
significance level (F/σF) > 4.0. After data collection, in each case a multi-scan absorption
correction (SADABS) [29] was applied, and structures were solved by direct methods and
refined by full matrix least-squares on F2 data using SHELX suite of programs [30]. In 1,
hydrogen atoms were included in the calculated position and refined in the riding mode, except
for O(5) and O(10), which were located in difference Fourier map and fixed to oxygen atoms. In
3a, hydrogen atoms were located in difference Fourier map and left to refine freely, except for
C(1S), C(2S), C(8), C(21), C(22) and O(1W), which were included in the calculated positions
and refined in the riding mode. Refinements were done with allowance for thermal anisotropy of
all non-hydrogen atoms. To remove the spurious bonds between water molecule and its
overlapping symmetry equivalent we use PART -1 instruction. A population parameter was
refined with a value of 0.49837 for O(1W), and fixed to 0.5 to simplify. A final difference
Fourier map showed no residual density in the crystals: 0.463 and -0.696 e.Å–3 for 1 and 0.535
2
and -0.514 e.Å–3 for 3a. A weighting scheme w = 1/[σ2(Fo ) + (0.050900 P)2 + 3.195900 P] for 1
2
and w = 1/[σ2(Fo ) + (0.053800 P)2 + 0.378000 P] for 3a, where P = (|Fo|2 + 2|Fc|2)/3, were used
in the latter stages of refinement. Further details of the crystal structure determination are given
in Table 1.
<<Table 1>>
2.5. Catalytic activity studies-oxidation of secondary alcohols with aqueous H2O2
2.5.1. Conventional liquid phase method
Oxidation of secondary alcohols was carried out in a 50 mL round-bottom flask equipped
with a reflux condenser. Under typical conditions, 1-phenylethanol, 2- propanol or 2- butanol (10
mmol), aqueous 30% H2O2 (3.40 g, 30 mmol), additive (0.56 mmol) (or no additive) and catalyst
6

(0.001 g) were dissolved in acetonitrile (10 mL) and the obtained solution was stirred using
magnetic stirrer for a particular time depending upon the substrate. The desired reaction
o
temperature (typically 80 C) was maintained using an oil bath. The reaction was monitored by
withdrawing small aliquots of the reaction mixture at definite time interval, extracting with
hexane and analyzing quantitatively by gas chromatograph. The identities of the products were
confirmed by GC–MS.
2.5.2. Microwave-assisted method
The oxidation of secondary alcohols was carried out in a sealed Pyrex tube under focused
microwave irradiation under the reaction conditions already fixed as mentioned above in
CH3CN. The reaction mixture was irradiated (25 W) along with stirring for 30–90 min
(depending up on the substrate) at 80 oC. After cooling the reaction mixture to room temperature,
it was extracted with hexane and analyzed by GC and GC–MS.
3. Results and discussion
3.1. Synthesis and characterization of complexes
The reaction between equimolar amounts of [MoVIO2(acac)2] and H2bp-bhz (I), H2bp-inh
(II), H2bp-nah (III) or H2bp-fah (IV) in refluxing methanol leads to the formation of
dioxidomolybdenum(VI) complexes [MoVIO2(bp-bhz)(MeOH)] (1), [MoVIO2(bp-inh)(MeOH)]
(2), [MoVIO2(bp-nah)(MeOH)] (3) and [MoVIO2(bp-fah)(MeOH)] (4), respectively (Eq. 1
considering I as representative example).
[MoVIO2(acac)2] + H2bp-bhz + MeOH → [MoVIO2(bp-bhz)(MeOH)] + 2Hacac (1)
All complexes exist in their monomer form. Scheme 2 provides their possible structures
1
13
which are based on the spectroscopic (IR, UV/Vis, H and C NMR) data, elemental analyses
and X-ray diffraction studies of [MoVIO2(bp-bhz)(MeOH)] (1) and [MoVIO2(bp-
nah)(DMSO)]·0.5H2O (3a). These complexes are soluble in methanol, ethanol, DMF and
DMSO.
<<Scheme 2>>
7

3.2. Thermal studies
The thermal stability of [MoO2]2+ complexes has been studied under an oxygen
atmosphere. These complexes lose weight roughly equal to one methanol molecule in the
temperature range 128–220 °C, indicating the presence of coordinated methanol and good
thermal stability of complexes. Further increasing the temperature, the anhydrous complexes
decompose exothermically in two/ three overlapping steps and form MoO3 at ca. 540 °C. Table
S1 (see Supporting Information) presents details of the corresponding decomposition
temperature for the weight loss of methanol and the temperature at which the formation of MoO3
occurs in complexes.
3.3. Structure descriptions
ORTEP diagram of compounds [MoVIO2(bp-bhz)(MeOH)] (1) and [MoVIO2(bp-
nah)(DMSO)]·0.5 H2O (3a) are presented in Figs. 1 and 2, respectively. Selected bond distances
and angles are given in Table 2.
In complex 1, two molecules are present in the asymmetric unit. The Opyrazolyl, Nazo and
Ohydrazide atoms of the ligand, which acts as tridentate, and two oxido groups coordinate to the
molybdenum centre. The equatorial plane is formed for these three atoms of the ligand and one
of the terminal oxygen atoms. One MeOH molecule completes the coordination sphere of a
distorted octahedral geometry. The O=Mo=O angle is 105.16(16)º and Mo=O distances are
1.682(3) and 1.691(3) Å. The Mo–O–C angle and Mo–O distance for Mo–MeOH are 125.3(4)°
and 2.365(4) Å for Mo(1) and 131.0(3)º and 2.325(3) Å for Mo(2), respectively, similar to other
examples in literature [31].
The structure of 3a is similar, but only one complex is present in the asymmetric unit and
half water molecule. The ligand acts as tridentate bonding Opyrazolone, Nazo and Ohydrazide atoms.
The equatorial plane is formed for these three atoms of the ligand and one of the terminal oxygen
atoms. The other terminal oxygen and the ODMSO atoms occupy the axial sites of a distorted
octahedral geometry. The O=Mo=O angle is 105.57(10)º and Mo=O distances are 1.6816(19)
and 1.6950(17) Å. The Mo–O–S angle and Mo–O distance for Mo–DMSO are 127.43(10)° and
2.2606(19) Å, respectively, similar to other examples in literature. Intermolecular hydrogen
bonds are present in the crystal packing of the two compounds (Figs. 3 and 4). Table 3 presents
hydrogen bonds interactions.
8

<<Figures 1 to 4>>
<<Tables 2 and 3>>
3.4. IR spectroscopic studies
Table 4 presents a partial list of IR spectral data of [MoVIO2]2+ complexes. According to
previously reported values for pyrazolone derived Schiff base ligand systems in literature
[15,16,26], the characteristic peaks appearing at 3130, 1644, 1594 and 1539 cm–1 in H2bp-bhz
(I), 3061, 1611, 1587 and 1531 cm–1 in H2bp-inh (II), 3090, 1647, 1594 and 1530 cm–1 in H2bp-
nh (III), and 3130, 1632, 1593 and 1542 cm–1 in H2bp-fah (IV) were assigned due to υ(NH),
υ(C=Ohydrazide/pyazolone) and υ(C=N) stretches, respectively. The presence of υ(C=Ohydrazide/pyrazolone
)
and υ(N-H) are indicative of their ketonic nature in the solid state. The enolization of ketonic
group and replacement of H atom by the metal ion is supported by the shift of band to lower
wavenumber by 27-53 cm-1 (hydrazide)/ 25-33 cm-1 (pyrazolone) in complexes. A distinct band
appearing in the region 1252-1292 cm–1 is assigned to the υ(C-Oenolic) mode. Consequently, a
ligand band appearing at 944-1011 cm–1 due to υ(N-N) shifts to higher wave number by 8-20
cm–1 in complexes. The shift of the ν(N-N) band is expected because of reduced repulsion
between the lone pairs of adjacent nitrogen atoms. A sharp band appearing at 1514-1519 cm–1 in
complexes compared to 1530-1542 cm–1 in the ligands is indicative of the coordination of
azomethine nitrogen atom to the metal ion. All ligands exhibit a medium-intensity ν(OH) band
covering the region 2920-2900 cm–1 due to the intramolecular hydrogen bonds. In metal
complexes, this band is shifted to ca. 3400-3440 cm–1, indicating the breaking of this bond as
well as plausible coordination of methanol.
The dioxidomolybdenum(VI) complexes are also well characterized by two prominent
peaks at 938-952 and 915-920 cm–1 which are assigned to υasym(O=Mo=O) and υsym(O=Mo=O)
modes, respectively due to cis-[MoO2] structure [32].
<<Table 4 >>
3.5. UV-vis spectral study
UV-vis spectral data of ligands and their corresponding [MoVIO2]2+ complexes are
presented in Table 5. Figs. S1 and S2 (see Supporting Information section) include spectra of
ligands and complexes. All ligands exhibit very similar three characteristic absorption bands at
ca. 204-205, 250-253 and 346-365 nm. Based on the extinction co-efficient values, these
9

characteristic peaks are designated as ϕ →ϕ*, π→π* and n→π* transitions, respectively. In
complexes, these prominent bands appear at nearly same positions. In addition, a new band of
medium intensity appears at ca. 400 nm, which is assigned to a ligand to metal charge transfer
(LMCT) band originating from partial electron movement from the filled p-orbital of the ligand
to the vacant d-orbital of metal ion of proper symmetry. As MoVI-complexes have 4d0
configuration, d → d transition is not expected.
<<Table 5>>
3.6. 1H and 13C NMR studies
The coordinating modes of ligands H2bp-bhz (I), H2bp-inh (II), H2bp-nah (III) and H2bp-
1
fah (IV) were confirmed by comparing their H NMR spectral patterns with the corresponding
complexes. The relevant spectral data are collected in Table 6 and the representative spectra are
presented in Fig. 5. The 1H NMR spectra of the ligands exhibit signals at δ =11.00 in I, 11.46 in
II, 11.20 in III and at 10.96 ppm in IV due to proton associated with –NH group, indicating their
existence in the ketonic form. The absence of this signal in [MoVIO2]2+ complexes are in
agreement with the enolization and subsequent replacement of H by the metal ion. The 4-
benzoyl-3-methyl-1-phenyl-2-pyrazoline-5-one exists in the pyrazolol form in their free ligands
with a signal at δ = 6.58-7.24 ppm. The absence of this signal in the complexes indicates the
coordination of pyrazolic oxygen. The methyl and alcoholic protons of the coordinated methanol
1
in complexes appear at δ = 3.21-3.23 and 4.07-4.11 (br) ppm, respectively. Thus, H NMR data
are consistent with the ONO dibasic tridentate binding mode of ligands and are in agreement
with the conclusions drawn from IR data.
<<Table 6>> <<Figure 5>>
Comparison of the 13C NMR spectra of the ligands and corresponding complexes also
provides useful information for the elucidation of the structures of the complexes. The
representative 13C NMR spectra are presented in Fig. 6 and complete data are provided in Table
7. Assignments of the peaks are based on the intensity patterns of the chemical shifts of carbon
atoms, the coordination-induced shifts [δ∆ = δ (complex) – δ (free ligand)] of the signals for
carbon atoms in the vicinity of the coordinating atoms [33] and Chemdarw® program. Ligands I,
13
II, III and IV display 20, 19, 19 and 19 C NMR signals corresponding to 24, 23, 23 and 22
10

carbon atoms, respectively. All expected signals are also present in complexes. A large
coordination induced shift of the signals for the carbon atoms associated with pyrazolol oxygen
(C1), azomethine nitrogen (C4) and enolic oxygen (C18) (see Table 7) confirms the coordination
of these functionalities to the molybdenum. Signals for C2 and C3 i.e. close to the above carbons
are only slightly affected. The signal due to methyl carbon (C19) of methanol was observed at
ca. 49 ppm in all complexes.
<<Table 7>> <<Figure 6>>
3.7. Catalytic activity
Oxidation of alcohols catalyzed by molybdenum (VI) complexes has been reported using
TBHP as well as H2O2 as an oxidant [20,34]. Catalytic oxidation of 1-phenylethanol, 2-propanol
and 2-butanol was carried out successfully using synthesized [MoVIO2]2+ complexes as catalyst
precursors in the presence of H2O2 as an oxidant. Both, conventional and microwave assisted
methods have been applied and independent of method used, the above secondary alcohols gave
corresponding ketone selectively with a remarkable yield and turn over frequency.
<<Scheme 3>>
3.7.1. Conventional method for the oxidation of secondary alcohols
Considering oxidation of 1-phenylethanol as a representative reaction and [MoVIO2(bp-
bhz)(MeOH)] (1) as a representative catalyst precursor, parameters like, amounts of catalyst,
oxidant and solvent, temperature of the reaction and reaction time have been optimized for the
maximum oxidation of substrate. Thus, for 10 mmol (1.22 g) of 1-phenylethanol three different
amounts of catalyst precursor (i.e. 0.001, 0.002 and 0.003 g) and aqueous 30% H2O2 (20, 30 and
40 mmol) were taken in three different volumes of acetonitrile (5, 10 and 15 mL) and the
o
reaction was carried out at 80 C. Table 8 summarizes all the conditions and the conversion
obtained under a particular set of conditions while Fig. 7 presents details of analytical results. It
is clear from the table that the optimized reaction conditions for 10 mmol of 1-phenylethanol are:
o
oxidant (3.40 g, 30 mmol), CH3CN (10 mL) and reaction temperature (80 C) (entry no. 6 of
Table 8). The reaction requires 20 h to attain equilibrium and gave a maximum of 91%
conversion.
11

Under similar reaction conditions, other catalysts were also tested and results are
summarized in Table 9. The catalytic potential of other catalysts are equally good with 85-93%
conversion. Oxidation of 2-propanol and 2-butanol was also carried with all four catalysts
precursors and results presented in Table 9 confirm their equally good catalytic potential. Blank
reactions conducted under the above mentioned conditions gave 28% conversion of 1-
phenylethanol, 38% of 2-propanol and 33% of 2-butanol. Thus, the results obtained in the
presence of catalysts are not only selective but satisfactory along with high turnover frequency.
Oxidation of alcohols catalyzed by molybdenum (VI) complexes has been studied using TBHP
as well as H2O2 as oxidant and our results compare well with the literature data [23,24,34].
<<Tables 8 and 9>> <<Figure 7>>
We have also studied the catalytic potential of these complexes in the presence of N-
based additive NEt3 (0.56 mmol) and the reaction equilibrium using catalyst 1 was achieved in 8
h with 91% conversion of 1-phenylethanol against 20 h with same conversion, in 2 h with 90%
of conversion of 2-propanol against 6 h with 87% and in 3 h with 92% of conversion of 2-
butanol against 8 h with 88% conversion. Data with other catalysts are also presented in Table 9.
Thus, a considerable reduction in the reaction time was observed on addition of N-based
additive. The N-base additive helps in removing a proton from peroxide moiety which possibly
facilitates the formation of peroxido intermediate complex [5]. These studies inference that
additive plays an important role in reducing catalytic reaction time with equally good or better
conversion. Either in an aerobic or anaerobic conditions, catalysts show almost negligible
conversion in the absence of oxidant.
3.7.2. Microwave assisted oxidation of sec-alcohols
The microwave assisted oxidation of secondary alcohols, 1-phenylethanol, 2-proponal
and 2-butanol, under a low microwave irradiation powers catalyzed by [MoO2]2+ complexes 1 to
4 was also carried out in the presence and the absence of N-based additive. For the latter case,
three screening times (1.0, 1.5 and 2.0 h) and three microwave irradiation powers (20, 25 and 30
W) were selected considering catalyst 1 as a representative for the oxidation of 10 mmol of 1-
phenylethanol. Other conditions were considered as optimized above [i.e. catalyst (0.001 g,
1.8×10–3 mmol), 30% H2O2 (3.40 g, 30 mmol), CH3CN (10 mL) and conversion of 1-
12

phenylethanol was monitored at temp. (80 oC)]. Table S2 presents details of conversion obtained
under a particular set of conditions. A maximum of 75% conversion at a microwave irradiation
power of 25 W can be considered as optimized conditions from Table S2 wherein reaction
completes in 1.5 h. This reaction time could further be reduced to 0.5 h along with better
conversion (80%) when N-based additive (0.057 g, 0.56 mmol) was added to the reaction
mixture. Catalytic activities of other three catalysts for both categories (i.e. with and without N-
based additive) under same reaction conditions are detailed in Table 9. It is clear from the data
presented in table that the catalytic efficiency shown by microwave assisted oxidation method is
better than the conventional oxidation method.
3.8. Reactivity of dioxidomolybdenum(VI) complexes with H2O2 and possible reaction
intermediate
As observed earlier, [MoVIO2]2+ complexes react with H2O2 to give the corresponding
[MoVIO(O2)]2+ complexes [25]. The generation of such species has also been established in
methanol by UV-vis spectroscopy. In a typical reaction, 20 mL of 4.9×10–5 M solution of
[MoVIO2(bp-bhz)(MeOH)] (1) was treated with one drop portion of 30% aqueous H2O2 (0.056 g,
0.49 mmol) dissolved in 10 mL of methanol and the observed spectroscopic changes are
presented in Fig. 8. Thus, the intensity of the 207 nm band increases considerably while the 247
nm band broadens along with increase in intensity upon addition of H2O2; Fig 8 (a). The band at
357 nm registers small shift towards higher wavelength with slight increase in intensity. Using
higher concentration of complex in methanol, the LMCT band appearing at 410 nm also shows
slight shifting to higher wave length [Fig. 8(b)] along with partial reduction in intensity.
Appearance of isosbestic point at 386 nm and adjustment of bands in the presence of H2O2
suggest the interaction of H2O2 with complex and transformation into peroxido species. Very
similar spectral changes have also been observed during interaction of H2O2 with complexes 2, 3
and 4 in methanol; Fig. S3. Thus, reaction of H2O2 with complex is likely to proceed through
peroxido intermediate which may transfer oxygen to the substrate during catalytic oxidation.
Addition of NEt3 (0.057 g, 0.56 mmol) to 20 mL of a methanolic solution of 3 (6.7×10–5 M) and
recording spectral changes after every 2 min interval for ca. 15 min shows the interaction of Net3
with the complex (Fig. S4). However, addition of 2 drops of 30% H2O2 (0.056 g, 0.50 mmol)
dissolved in 10 mL of methanol to the above solution quickly generates a new spectrum with a
13

charge transfer band at ca. 400 nm possibly due to the generation of the peroxido species of
complex 3. This change remains nearly constant upon further addition of H2O2 solution (Fig. S4)
and thus suggests no decomposition of complex in the presence of additive and the stability of
the generated peroxido species.
<<Figure 8>>
4. Conclusions
Using potential tridentate ligands H2bp-bhz (I), H2bp-inh (II), H2bp-nah (III) and H2bp-
fah (IV), four [MoVIO2]2+ complexes, [MoVIO2(bp-bhz)(MeOH)] (1), [MoVIO2(bp-inh)(MeOH)]
(2), [MoVIO2(bp-nah)(MeOH)] (3) and [MoVIO2(bp-fah)(MeOH)] (4) have been prepared and
characterized. Structures of complexes 1 and 3a {[MoVIO2(bp-nah)(DMSO)]˖0.5 H2O} have
further been confirmed by single crystal X-ray study. Using conventional as well as microwave
irradiation method, [MoVIO2]2+ complexes are found to be very efficient homogenous catalyst
precursors for the oxidation of volatile organic compounds, 1-phenylethanol, 2-propanol and 2-
butanol in acetonitrile, to the corresponding ketone selectively. Addition of N-based additive
(NEt3) assists catalytic systems in reducing time and simultaneously increasing the conversion.
The green oxidant H2O2 is employed for the generation of active intermediate species during the
catalytic process. Microwave irradiation technique emerges out as a highly time efficient system
over conventional oxidation method.
Appendix A
CCDC 1051725 for 1 and 1051726 for 3a contain the supplementary crystallographic
data for the structures reported in this paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the
online version, at DOI: $$$$$.
14

Acknowledgements
N.S. acknowledges MHRD fellowship and financial support through Indian Institute of
Technology Roorkee, Roorkee, India. M.R.M. thanks the Department of Science and
Technology (DST), Government of India, New Delhi (SR/S1/IC-32/2010) for financial support
in procuring gas chromatograph.
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16

Schemes
R
R
Ligands
Ligands
H2bp-bhz (I)
H2bp-nah (III)
N
O
H2bp-inh (II)
H2bp-fah (IV)
N
Scheme 1. Ligands derived from 4-benzoyl-3-methyl-1-phenyl-2-pyrazoline-5-one used in this
study.
17

R
Complexes
[MoVIO2(bp-bhz)(MeOH)] (1)
[MoVIO2(bp-inh)(MeOH)] (2)
[MoVIO2(bp-nah)(MeOH)] (3)
N
N
[MoVIO2(bp-fah)(MeOH)] (4)
O
Scheme 2. Proposed structures of [MoO2]2+ complexes.
Scheme 3. Oxidation of secondary alcohols.
18

Figures
Fig. 1. ORTEP plot of the asymmetric unit in the compound [MoVIO2(bp-bhz)(MeOH)] (1). All
the non-hydrogen atoms are presented by their 50% probability ellipsoids. Hydrogen atoms are
omitted for clarity.
Fig. 2. ORTEP plot of the asymmetric unit in the compound [MoVIO2(bp-nah)(DMSO)]·0.5 H2O
(3a). All the non-hydrogen atoms are presented by their 50% probability ellipsoids. Hydrogen
atoms are omitted for clarity.
19

Fig. 3. Crystal packing of the compound [MoVIO2(bp-bhz)(MeOH)] (1). Intermolecular hydrogen
bonds between N(5) and N(6) of aromatic hydrazide groups and O(5) and O(10) of coordinated
methanol groups in blue dashed lines.
20

Fig. 4. Crystal packing of the compound [MoVIO2(bp-nah)(DMSO)]·0.5H2O (3a). Intermolecular
hydrogen bonds between O(1W) of water molecules and O(1) of coordinated oxo groups and
N(5) of pyridine groups in blue dashed lines.
21

Fig. 5. 1H NMR spectra of H2bp-fah (IV), [MoVIO2(bp-fah)(MeOH)] (4)
Fig. 6. 13C NMR spectra of H2bp-inh (II) and [MoVIO2(bp-inh)(MeOH)] (2).
22

23

Fig. 7. (a) Effect of variation of amount of catalyst [MoVIO2(bp-bhz)(MeOH)] (1) on the
oxidation of 1-phenylethanol. Reaction conditions: 1-phenylethanol (1.22 g, 0.010 mol), 30%
H2O2 (3.40 g, 0.030 mol), acetonitrile (10 mL) at 80 oC for 20 h. (b) Effect of amount of oxidant
i.e. H2O2 on the oxidation of 1-phenylethanol. Reaction conditions: 1-phenylethanol (1.22 g,
0.010 mol), catalyst 1 (0.001 g) at 80 oC for 20 h. (c) Effect of variation of amount of solvent on
the rate of the oxidation of 1-phenylethanol. Reaction conditions: 1-phenylethanol (1.22 g, 0.010
o
mol), 30% H2O2 (3.40 g, 0.030 mol), catalyst 1 (0.001 g), at 80 C for 20 h. (d) Effect of
different temperature on the oxidation of 1-phenylethanol. Reaction conditions: (1.22 g, 0.010
mol), 30% H2O2 (3.40 g, 0.030 mol), catalysts 1 (0.001 g) and temp. (60, 70 and 80 oC) for 20 h.
(e) Effect of different homogenous catalysts on the oxidation of 1-phenylethanol. Reaction
conditions: 1-phenylethanol (1.22 g, 0.010 mol), 30% H2O2 (3.40 g, 0.030 mol), catalysts 1, 2, 3
o
or 4, (0.001 g) at 80 C for 20 h. (f) Effect of different solvents on the oxidation of 1-
phenylethanol. Reaction conditions: 1-phenylethanol (1.22 g, 0.010 mol), 30% H2O2 (3.40 g,
o
0.030 mol), catalysts 1 (0.001 g) at 80 C for 20 h. (g) Comparison of conversion of 1-
phenylethanol in the presence and absence of catalyst. Reaction conditions: 1-phenylethanol
(1.22 g, 0.010 mol), 30% H2O2 (3.40 g, 0.030 mol) at 80 oC for 20 h.
24

Fig. 8. (a) Spectral changes recorded after successive addition of one drop portion of 30% H2O2
(0.056 g, 0.49 mmol) dissolved in 10 mL of MeOH (final concentration of H2O2; 4.9×10–2 M) to
20 mL of (4.9×10–5 M) solution of [MoVIO2(bp-bhz)(MeOH)] (1). (b) Similar spectral changes
with 30% H2O2 (0.060g, 0.53 mmol) dissolved in 10 mL of methanol (final concentration of
H2O2; 5.3×10–2 M) to 20 mL of methanolic solution (8.6×10–5 M) of complex 1.
25

Tables
Table 1. Crystal Data and Structure Refinement for the complexes [MoVIO2(bp-bhz)(MeOH)]
(1) and [MoVIO2(bp-nah)(DMSO)]·0.5H2O (3a)
1
C25 H22 Mo N4 O5
554.41
3a
C25 H24 Mo N5 O5.50
610.49
Formula
S
Formula weight
T, K
293
293
Wavelength, Å
0.71073
0.71073
Triclinic
P1
Crystal system
Monoclinic
P21/n
Space group
a/Å
15.776(3)
16.304(3)
20.032(4)
8.9373(4)
11.0413(5)
14.4105(6)
102.767(2)
106.854(2)
91.800(2)
1320.33(10)
2
b/Å
c/Å
α/º
β/º
110.915(7)
γ/º
V/Å3
4812.9(14)
8
Z
F000
2256
621
Dcalc/g cm-3
µ/mm-1
1.530
1.534
0.589
0.623
1.42 to 28.22
0.0648
1.90 to 28.30
0.0275
θ/ (º)
Rint
Crystal size/ mm3
Goodness-of-fit on F2
R1[I>2σ(I)] a
wR2 (all data) b
0.27 × 0.25 × 0.21 0.26 × 0.24 × 0.15
1.042
1.083
0.0491
0.0323
0.1348
0.1009
Largest differences peak and hole (eÅ-3) 0.463 and -0.696
0.535 and -0.514
26

2 2
aR1 = ΣFo - Fc/ΣFo. bwR2 = {Σ[w(Fo2 -Fc2)2]/Σ[w(Fo ) ]}1/2
Table 2. Bond lengths [Å] and angles [°] for [MoVIO2(bp-bhz)(MeOH)] (1) and [MoVIO2(bp-
nah)(DMSO)]·0.5H2O (3a).
Bond lengths
1
3a
Mo(1)-O(1)
Mo(1)-O(2)
Mo(1)-O(3)
Mo(1)-O(4)
Mo(1)-O(5)
Mo(1)-N(3)
Mo(2)-O(6)
Mo(2)-O(7)
Mo(2)-O(8)
Mo(2)-O(9)
Mo(2)-O(10)
Mo(2)-N(7)
N(3)-C(11)
N(3)-N(4)
1.682(3)
1.691(3)
1.977(3)
1.980(3)
2.365(4)
2.286(3)
1.689(3)
1.704(3)
1.992(2)
1.979(2)
2.325(3)
2.284(3)
1.314(5)
1.409(4)
1.295(5)
1.319(4)
1.403(4)
1.6816(19)
1.6947(17)
1.9844(17)
1.9929(17)
2.2603(19)
2.2657(17)
1.311(3)
1.396(2)
1.292(3)
N(4)-C(18)
N(7)-C(35)
N(7)-N(8)
27

N(8)-C(42)
Angles
1.299(5)
1
3a
O(1)-Mo(1)-O(2)
O(1)-Mo(1)-O(3)
O(2)-Mo(1)-O(3)
O(1)-Mo(1)-O(4)
O(2)-Mo(1)-O(4)
O(3)-Mo(1)-O(4)
O(1)-Mo(1)-N(3)
O(2)-Mo(1)-N(3)
O(3)-Mo(1)-N(3)
O(4)-Mo(1)-N(3)
O(1)-Mo(1)-O(5)
O(2)-Mo(1)-O(5)
O(3)-Mo(1)-O(5)
O(4)-Mo(1)-O(5)
N(3)-Mo(1)-O(5)
O(6)-Mo(2)-O(7)
O(6)-Mo(2)-O(9)
O(7)-Mo(2)-O(9)
O(6)-Mo(2)-O(8)
O(7)-Mo(2)-O(8)
O(9)-Mo(2)-O(8)
105.16(16)
97.61(14)
101.53(12)
100.36(14)
97.07(12)
149.69(12)
94.98(13)
158.64(14)
82.41(11)
71.92(11)
171.76(13)
83.07(14)
80.80(12)
77.92(12)
76.81(11)
105.77(15)
101.38(14)
95.52(11)
97.04(13)
102.37(11)
149.72(11)
105.57(10)
96.25(9)
103.59(8)
98.15(9)
96.26(8)
151.31(8)
89.27(8)
162.62(8)
83.27(6)
72.26(6)
169.41(8)
85.01(9)
80.91(7)
80.40(8)
80.28(7)
28

O(6)-Mo(2)-N(7)
O(7)-Mo(2)-N(7)
O(9)-Mo(2)-N(7)
O(8)-Mo(2)-N(7)
O(6)-Mo(2)-O(10)
O(7)-Mo(2)-O(10)
O(9)-Mo(2)-O(10)
O(8)-Mo(2)-O(10)
N(7)-Mo(2)-O(10)
93.75(13)
158.67(13)
71.80(10)
83.26(10)
168.51(12)
85.54(12)
79.09(11)
78.19(10)
75.39(10)
Table 3
Intermolecular hydrogen bonds for the complexes [MoVIO2(bp-bhz)(MeOH)] (1) and
[MoVIO2(bp-nah)(DMSO)]·0.5 H2O (3a).
D-H...A
Compound d(D-H)
d(H...A) d(D...A)
<(DHA)
166.6
156.8
168.3
166.6
O(5)-H(5S)...N(2)#1
O(1W)-H(1WA)...N(5)
O(10)-H(10S)...N(6)#2
O(1W)-H(1WB)...O(1)#3
1.05
0.85
0.98
0.85
1.81
1.91
1.71
2.10
2.846(4)
2.711(7)
2.676(4)
2.931(6)
1
3a
1
3a
Symmetry transformations used to generate equivalent atoms: #1 -x+3/2,y-1/2,-z+3/2
x+1/2,y+1/2,-z+3/2 #3 -x,-y,-z+1
#2 -
29