Oxidative bromination of monoterpene (thymol) using dioxidomolybdenum(VI) complexes of hydrazones of 8-formyl-7-hydroxy-4-methylcoumarin

Article abstract of DOI:10.1016/j.poly.2015.05.001

Condensation of equimolar amount of 8-formyl-7-hydroxy-4-methylcoumarin (fhmc) and hydrazides [benzoylhydrazide (bhz), isonicotinoylhydrazide (inh), nicotinoylhydrazide (nah) and furoylhydrazide (fah)] in methanol results in the formation of potential ONO

Full text of DOI:10.1016/j.poly.2015.05.001

Accepted Manuscript
Oxidative bromination of monoterpene (thymol) using dioxidomolybdenum
(VI) complexes of hydrazones of 8-formyl-7-hydroxy-4-methylcoumarin
Mannar R. Maurya, Sarita Dhaka, Fernando Avecilla
PII:
S0277-5387(15)00242-9
DOI:
http://dx.doi.org/10.1016/j.poly.2015.05.001
POLY 11299
Reference:
Polyhedron
To appear in:
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Accepted Date:
29 March 2015
1 May 2015
Please cite this article as: M.R. Maurya, S. Dhaka, F. Avecilla, Oxidative bromination of monoterpene (thymol)
using dioxidomolybdenum (VI) complexes of hydrazones of 8-formyl-7-hydroxy-4-methylcoumarin, Polyhedron
(2015), doi: http://dx.doi.org/10.1016/j.poly.2015.05.001
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Oxidative bromination of monoterpene (thymol) using dioxidomolybdenum
(VI) complexes of hydrazones of 8-formyl-7-hydroxy-4-methylcoumarin
Mannar R. Maurya a,*, Sarita Dhaka 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: Tridentate ONO Donor Schiff base ligands; Molybdenum(VI) complexes; NMR
spectroscopy; Oxidative bromination of thymol.
__________
* Corresponding author. Tel.: +91 1332 285327; fax: +91 1332 273560.
E-mail address: rkmanfcy@iitr.ac.in (M.R. Maurya).
2

ABSTRACT
Condensation of equimolar amount of 8-formyl-7-hydroxy-4-methylcoumarin (fhmc) and
hydrazides [benzoylhydrazide (bhz), isonicotinoylhydrazide (inh), nicotinoylhydrazide (nah) and
furoylhydrazide (fah)] in methanol results in the formation of potential ONO tridentate ligands
H2fhmc-bhz (I), H2fhmc-inh (II), H2fhmc-nah (III), and H2fhmc-fah (IV), respectively. Reaction
of these ligands with [MoVIO2(acac)2] in refluxing methanol gives dioxidomolybdenum(VI)
complexes, [MoVIO2(fhmc-bhz)(MeOH)] (1), [MoVIO2(fhmc-inh)(MeOH)] (2) [MoVIO2(fhmc-
nah)(MeOH)] (3) and [MoVIO2(fhmc-fah)(MeOH)] (4), respectively. The structures of the
obtained ligand and their respective metal complexes were elucidated by elemental analyses,
1
spectroscopic techniques (IR, electronic, H and 13C NMR) and thermogravmetric analyses.
Single crystal X-ray analysis of complex 1 confirms the chelation of the ligand to the metal ion
through two oxygen and a nitrogen atoms. These metal complexes have been tested against
oxidative bromination of monoterpene (thymol) to yield 2-bromothymol, 4-bromothymol and
2,4-dibromothymol using H2O2 as an oxidant therefore they act as functional models of
vanadium dependent haloperoxidases. The effects of various factors, such as amounts of catalyst,
oxidant, KBr, HClO4 and different solvents have been considered to optimize the reaction
conditions for the maximum brominated products.
1. Introduction
Terpenes are extremely miscellaneous division of natural products from which numerous
commercial compounds are derived. Their potential therapeutic properties in pharmaceutical
(like anticancer, anti-microbial and anti-inflammatory), uses in sanitary, cosmetic, agricultural
and food industries, and making flavor and fragrance related compounds further show their
importance [1]. Particularly monoterpenes represent a cheap and most abundant source of chiral
substances that can be transformed into valuable bioactive compounds [2]. The most common
thyme oil (thymus vulgaris) contains 20–54% thymol [3] which is an antiseptic and is the main
active constituent of various mouth washes and has also been found to be effective against
various fungi.
Coumarins are oxygen containing heterocyclic compounds, which show their significance
in the territory of natural products and synthetic organic chemistry [4]. Coumarin and its
3

derivatives have been a subject of several investigations due to their miscellaneous biological
activities [5,6], interesting photophysical, photochemical and metal binding properties. It has
been found that the coordination of coumarin moiety to metal preserves or even boosts its
biological activity [6,7]. They can also provoke modification in cell growth, development of
intracellular communication mechanisms. Many coumarins are selective coronary vasodilators
[8] and possess radioactive properties [9].
The hydrazones are relatively cheap and environmentally endurable ligands, which can
be synthesize through simple synthetic procedures [10-12]. Hydrazone moieties are the most
important pharmacophoric cores of several anti-inflammatory, antinociceptive, and antiplatelet
drugs [13]. Their cis-[MoVIO2]2+ complexes have shown interesting antibacterial [10] and
catalytic activities [11]. Catalytic activities of cis-[MoVIO2]2+ complexes of other ONO donor
ligands have also been well documented in the literature [12,14-16].
The aim of the present work is to synthesize and characterize new cis-[MoVIO2]2+
complexes with newly synthesized hydrazones of 8-formyl-7-hydroxy-4-methylcoumarin
(Scheme 1) and use them as catalyst precursor towards the oxidative bromination of
monoterpene (thymol). Brominated compounds are the most plentiful organohalides in
environment. This category of molecules get usages in various fields such as agrochemicals and
pharmaceuticals, Furthermore, brominated derivatives are regarded as significant synthetic
intermediates for numerous selective and efficient transformations [17, 18]. Only few examples
of thymol bromination reaction are reported in the literature [19-22].
<<Scheme 1>>
2. Experimental Section
2.1. Materials
Analytical reagent grade ammonium molybdate, ethyl benzoate, hydrazine hydrate,
isonicotinoylhydrazide (Loba Chemie, Mumbai, India), nicotinoylhydrazide (Acros organics,
New Jersey, USA), furoylhydrazide, (Aldrich Chemicals Co., U.S.A.), acetylacetone, thymol
(Himedia ), 30% aqueous H2O2, (Rankem, New Delhi, India), were used as obtained.
[MoVIO2(acac)2] [23] and 8-formyl-7-hydroxy-4-methylcoumarin [24] were prepared according
4

to methods reported in the literature. Benzoylhydrazide was prepared by the reaction of a
twofold excess of hydrazine hydrate with ethyl benzoate in ethanol.
2.2. Instrumentation and characterization Procedures
Elemental analysis of the compounds was carried out on an Elementar model Vario-El-
III. Thermogravimetric analysis was carried out under an oxygen atmosphere using a TG Stanton
Redcroft STA 780 instrument. IR spectra were recorded as KBr pellets on a Nicolet NEXUS
Aligent 1100 series FT-IR spectrometer. Electronic spectra of ligand and complexes were
13
recorded in DMSO using Shimadzu 2450 UV-VIS spectrophotometer.1H and C NMR spectra
of complexes 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. 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 catalytic reaction products. The identity of
the products was confirmed using a GC-MS model Perkin-Elmer, Clarus 500 by comparing the
fragments of each product with the library available. The percent conversion of substrate and
selectivity of products were calculated from GC data using the formulae presented elsewhere.
[25]
2.3. Preparations
2.3.1. Preparations of H2fhmc-bhz (I), H2fhmc-inh (II) H2fhmc-nah (III) and H2fhmc-fah (IV)
All ligands were prepared using a general procedure. A representative preparation is
presented here. A solution of 8-formyl-7-hydroxy-4-methylcoumarin (2.04 g, 10 mmol) in
methanol (20 ml) was mixed to a filtered solution of benzoylhydrazide (1.36 g, 10 mmol) in
methanol (20 ml) with stirring and the obtained reaction mixture was refluxed on a water bath for
4 h. The resulted solid was filtered after cooling, washed with methanol and dried over silica gel
under vacuum.
H2fhmc-bhz (I): Yield: 2.50 g (77.6%). Anal. Calc. for C18H14N2O4 (322.30): C, 67.07;
H, 4.38; N, 8.69 %. Found: C, 67.19; H, 4.32; N, 8.72%.
H2fhmc-inh (II): Yield: 2.45 g (75.8%). Anal. Calc. for C17H13N3O4 (323.30): C, 63.16;
H, 4.05; N, 13.00 %. Found: C, 63.08; H, 4.15; N, 13.12%.
5

H2fhmc-nah (III): Yield: 2.47 g (76.4%). Anal. Calc. for C17H13N3O4 (323.30): C, 63.16;
H, 4.05; N, 13.00 %. Found: C, 63.10; H, 4.11; N, 13.09%.
H2fhmc-fah (IV): Yield: 2.39 g (76.6%). Anal. Calc. for C16H12N2O5 (312.28): C, 61.54;
H, 3.87; N, 8.97 %. Found: C, 61.60; H, 3.85; N, 9.00%.
2.3.2. Preparation of [MoVIO2(fhmc-bhz)(MeOH)] (1)
A solution of [MoVIO2(acac)2] (0.66 g, 2 mmol) dissolved in 5 ml of methanol
was added to a stirred solution of H2fhmc-bhz (0.64 g, 2 mmol) in methanol (10 ml) and the
obtained reaction mixture was stirred at room temperature whereupon an yellow solid started to
form. After 3 h of stirring, the separated solid was filtered, washed with cold methanol followed
by petroleum ether and dried in a vacuum desiccator over silica gel. Yield: 0.910 g (94.7%).
Anal. Calc. for C19H16N2O7Mo (480.30): C, 47.51; H, 3.36; N, 5.83%. Found: C, 47.45; H, 3.28;
N, 5.89%. X-ray diffraction quality crystals for [MoVIO2(Hfhmc-bhz)(DMSO)] (1a) were grown
by slow evaporation of a solution of 1 in DMSO.
2.3.3. Preparations of [MoVIO2(fhmc-inh)(MeOH)] (2), [MoVIO2(fhmc-nah)(MeOH)] (3) and
[MoVIO2(fhmc-fah)(MeOH)] (4)
These complexes were prepared similarly by the reaction of 2 mmol of [MoVIO2(acac)2]
and 2 mmol of ligand, H2fhmc-inh (II) H2fhmc-nah (III) or H2fhmc-fah (IV).
[MoVIO2(fhmc-inh)(MeOH)] (2): Yield: 0.920 g (95.6%). Anal. Calc.for C18H15N3O7Mo
(481.29): C, 44.92; H, 3.14; N, 8.73 %. Found: C, 44.85; H, 3.18; N, 8.80 %.
[MoVIO2(fhmc-nah)(MeOH)] (3): Yield: 0.936 g (97.3%). Anal. Calc.for C18H15N3O7Mo
(481.29): C, 44.92; H, 3.14; N, 8.73 %. Found: C, 44.90; H, 3.10; N, 8.78 %.
[MoVIO2(fhmc-fah)(MeOH)] (4): Yield: 0.900 g (95.7%) Anal. Calc.for C17H14N2O8Mo
(470.26): C, 43.42; H, 3.00; N, 5.96%. Found: C, 43.48; H, 2.96; N, 5.89%.
6

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 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. 14540
reflections measured were corrected for Lorentz and polarization effects and for absorption by
multi-scan methods based on symmetry-equivalent and repeated reflections. Of them, 3113
independent reflections exceeded the significance level (F/σF) > 4.0. After data collection,
in each case an empirical absorption correction (SADABS) [26] was applied, and the structure
was solved by direct methods and refined by full matrix least-squares on F2 data using SHELX
suite of programs [27]. Hydrogen atoms were included in calculation position and refined in the
riding mode, except for C(2), C(9), C(10), C(11) and C(14), which were located in difference
Fourier map and left to refine freely. Refinements were done with allowance for thermal
anisotropy of all non-hydrogen atoms. A final difference Fourier map showed no residual density
2
in the crystal: 0.496 and -0.727 e.Å–3. A weighting scheme w = 1/[σ2(Fo ) + (0.066000 P)2 +
0.015900 P], was used in the latter stages of refinement. Further details of the crystal structure
determination are given in Table 1.
<<Table 1>>
2.5. Catalytic Oxidative bromination of thymol
Thymol (1.50 g, 10 mmol), 30 % aqueous H2O2 (3.39g, 30 mmol), 70% HClO4 (5.72 g,
40 mmol) and KBr (3.57 g, 30 mmol ) were taken in water (20 mL) at room temperature. After
adding catalyst (0.002 g) to the above reaction mixture, it was stirred and the obtained oxidized
products were analyzed quantitatively by gas chromatography. The obtained main products were
1
confirmed by H NMR spectroscopy as well as GC–MS after their separations and their
quantifications were made on the basis of the relative peak area of the respective product.
3. Results and discussion
3.1. Synthesis and characterization of complexes
The reaction between equimolar amounts of [MoVIO2(acac)2] and H2fhmc-bhz (I),
H2fhmc-inh (II), H2fhmc-nah (III) or H2fhmc-fah (IV) in methanol results in the formation of
cis-[MoVIO2]2+ complexes, [MoVIO2(fhmc-bhz)(MeOH)] (1), [MoVIO2(fhmc-inh)(MeOH)] (2)
7

[MoVIO2(fhmc-nah)(MeOH)] (3) and [MoVIO2(fhmc-fah)(MeOH)] (4), respectively. The sixth
coordination site is occupied by methanol in these complexes. [Eq. (1) taking I as a
representative example].
[MoVIO2(acac)2] + H2fhmc-bhz + MeOH → [MoVIO2(fhmc-bhz)(MeOH)] + 2Hacac
(1)
Idealized structures of the complexes are shown in Scheme 2 which are based on the
1
spectroscopic (IR, UV/Vis, H and 13C NMR) data, elemental analyses and X-ray diffraction
study of [MoVIO2(fhmc-bhz)(DMSO)] (1a). All these complexes are soluble in DMF and DMSO
and partially soluble in methanol and acetonitrile.
<<Scheme 2>>
3.2. Thermal studies
Thermal stability of monomeric complexes 1, 2, 3 and 4 has been studied under an
oxygen atmosphere. These complexes lose weight roughly equal to one methanol molecule in the
temperature range 100-170 °C, indicating the presence of a coordinated methanol. On further
increasing the temperature the solvent free complexes decompose exothermically in two/ three
overlapping steps and form MoO3 as final product. The observed MoO3 contents of 29.2 % at ca.
o
o
o
o
615 C for 1, 29.0 % at 440 C for 2, 28.9 % at 440 C for 3 and 29.7 % at 490 C for 4 match
closely with the calculated values of 30.0, 29.9, 29.9 and 30.6 %, respectively.
3.3. Structure descriptions
ORTEP diagram for the complex [MoVIO2(Hfhmc-bhz)(DMSO)] (1) is shown in Fig. 1.
Selected bond distances and angles are given in Table 2. In compound 1, one complex is present
in the asymmetric unit. The Ohydroxy of methyl coumarin group, Nazo and Obenzoylhydrazide atoms of
the ligand, which acts as ONO tridentate, and two oxido groups coordinate to molybdenum
centre. The equatorial plane is formed for these three atoms of the ligand and one of the terminal
oxygen atoms. The molybdenum atom is displaced toward the apical oxido ligand from the
equatorial plane by 0.3003 Å, with respect to the plane O3-O4-O5-N1 (mean deviation from
plane 0.0402(15) Å) One DMSO molecule completes the coordination sphere of a distorted
octahedral geometry. The O=Mo=O angle is 105.90(16)º and Mo=O distances are 1.696(3) Å
and 1.695(3) Å. The Mo–O–S angle and Mo–O distance for Mo–DMSO are 126.37(18)°
8

(average of two angles of disordered sulphur atoms) and 2.286(3) Å, respectively, similar to
other examples in literature [28,29].
The presence of intermolecular forces between the rings of methyl coumarin group and
phenyl ring of benzoylhydrazide group determine the crystal packing. The π-π stacking
interactions between the rings establish the linked between frameworks and determine the
structure in layers (see Fig. 2). The distances between π cloud around carbon atom, C6, and the
centroids of phenyl rings are: dC6-c1 = 3.510 Å [(C6A), c1 (C13B-C14B-C15B-C16B-C17B-
C18B) and repeated for other carbon-centroids].
<<Table 2>> <<Figures 1 and 2>>
3.4. IR spectral studies
Table 3 reveals a partial list of the IR spectral data. The IR spectra of the ligands H2fhmc-
bhz (I), H2fhmc-inh (II), H2fhmc-nah (III) and H2fhmc-fah (IV) show four characteristic bands
at 3053-3060, 1705-1716, 1673-1678, and 1586-1588 cm–1 due to υ(NH), υ(C=O)pyrone/hydrazide
and υ(C=N) stretches, respectively [30], which show their ketonic behavior in the solid state.
Appearance of a new band at 1179–1200 cm–1 due to the ν(C-O) stretch and the absence of a
υ(NH) band in all the complexes suggests the enolization of the ketonic group and replacement
of H by the metal ion. A band at ca. 1703-1717 cm–1 due to the ketonic group of the pyrone
moiety is still present in all the complexes. A very sharp band appearing at 1543-1551 cm-1 in
complexes confirms the coordination of the azomethine nitrogen atom. This lower wavenumber
band occurs due to the donation of electron density from the nitrogen atom to an empty d-orbital
of the metal ion. A band appearing at 1066–1074 cm-1 due to υ(N–N) in ligands is also affected
by the coordination of the nitrogen atom and shifts to a higher wavenumber in complexes. The
presence of several medium intensity bands between 2500 and 2800 cm–1 in the ligands as well
as in complexes suggest the existence of C–H stretching bands due to -CH2. The
[MoO2]2+complexes are well characterized by two prominent peaks at 936-945 and 906-920
cm–1, which are assigned to υasym(O=Mo=O) and υsym(O=Mo=O) modes, respectively due to the
cis-[MoO2] structure [31].
<<Table 3>>
9

3.5. UV-Vis spectral studies
Table 4 presents absorption maxima of ligands and complexes with their extinction
coefficients. The UV-Vis spectra of ligands and [MoVIO2]2+ complexes were recorded in DMSO.
All ligands exhibit three very similar spectral bands at 380-388, 314-317 and 267-272 nm. Based
on their extinction coefficient values, these bands explicate as n→π* (first two bands) and π→π*
transitions, respectively. All these bands are also present in complexes with slight variations. In
addition, all complexes exhibit a medium intensity band at 425 – 431 nm due to the ligand to
metal charge transfer (LMCT) transition from the phenolate oxygen atom to an empty d-orbital
of the molybdenum. As [MoVIO2]2+ complexes have 4d0 configuration, d→d band is not
expected.
<<Table 4>>
3.6. 1H and 13C NMR studies
1
The H NMR spectral data of ligands and complexes are collected in Table 5. The
1
spectral data confirm their coordinating modes. The H NMR spectra of a representative ligand
H2fhmc-nah (III) and its complex is shown in Fig. 3. The ligands show two broad signals at δ =
12.61-12.74 ppm and 12.41 – 12.57 ppm owing to the phenolic –OH and –NH, respectively.
These signals disappear in the spectra of complexes due to coordination of the phenolate oxygen
and enolate oxygen after replacing H by the metal ion. A signal due to azomethine proton
(equivalent to 1 H) in ligands illustrates a notable downfield shifts in complexes with a
coordination-induced shifts [∆δ= [δ(complex) – δ(free ligand)] of 0.06 –0.14 ppm, confirming
the coordination of the azomethine nitrogen atom. The aromatic and aliphatic protons appear in
the expected regions in spectra of ligands as well as of complexes, with slight shifts in their
positions.
<<Table 5>> <<Figure 3>>
Table 6 provides 13C NMR spectral data of ligands and their corresponding
dioxidomolybdenum (VI) complexes whereas Fig. 4 presents spectra of a representative ligand
H2fhmc-nah (III) and its complex (3). The peaks are assigned again on the basis of the
coordination-induced shifts (∆δ) of the signals for carbon atoms in the vicinity of the
coordinating atoms [32].These assignments also provide useful information for the elucidation of
the structures of the complexes. Ligands I, II, III and IV display 16, 15, 17 and 16 signals
corresponding to the 18, 17, 17 and 16 carbon atoms, respectively. A total of 17, 16, 18 and 17
10

signals were observed for complexes 1, 2, 3 and 4, respectively after coordination of ligands to
the molybdenum. A large coordination induced shift of the signals for the carbon atoms
associated with the phenolic oxygen (C1), the azomethine nitrogen (C11) and the enolic oxygen
(C12) confirms the coordination of these functionalities to the molybdenum ion. In addition,
these complexes show one signal each for coordinated MeOH at ca. 48.80 – 49.10 ppm.
<<Table 6>> <<Figure 4>>
3.7. Catalytic activity studies
Scheme 3 presents the oxidative bromination of thymol, using 1, 2, 3 and 4 as catalyst
precursors in the presence of KBr, 70% aqueous HClO4 and 30 % aqueous H2O2 under an
aqueous system. The bromination reaction takes place on the most activated site as a result of
electrophilic aromatic substitution in the phenolic ring. Particularly, the bromination reaction led
to the formation of two isomers, 4-bromothymol and 2-bromothymol; 4-bromothymol being the
major product due to steric hindrance at the ortho position. Further bromination of the para
isomer gave 2,4- dibromothymol in excellent yield.
Consequently, the reaction has been investigated by changing different parameters that may
affect the rate of thymol bromination and the selectivity of products.
<<Scheme 3>>
Reaction conditions were optimized for considering different parameters like amounts
of catalyst, 30% aqueous H2O2, KBr and HClO4 using [MoVIO2(fhmc-bhz)(MeOH)] (1) as a
representative catalyst precursor. Thus, for 10 mmol of thymol (1.50 g), three different
amounts of catalyst (0.001, 0.002 and 0.003 g), 30 % aqueous H2O2 (10, 20 and 30 mmol), KBr
(10, 20 and 30 mmol) and 70% HClO4 (10, 20, 30 and 40 mmol, added in four equal portions to
the reaction mixture, first portion at t = 0 and other three portions after every 30 min intervals)
were taken in 20 mL water and the reaction was carried out at room temperature. The HClO4 was
found to be crucial to bring out catalytic bromination and its amount has great influence on the
conversion and selectivity of products. The complexes slowly decompose during the reaction;
decomposition is slowed down, if HClO4 is successively added in four equal portions during 2 h
of reaction time.
11

Table 7 presents all the conditions considered and the corresponding percentage of
oxidative bromination of thymol along with the selectivity of different reaction products. It is
clear from the data shown in table that the conversions and the selectivity of products differ on
varying the reagents. However, the best appropriate reaction conditions (entry no. 10 of Table 7)
for the maximum oxidative bromination of thymol with 94% conversion are: catalyst (0.002 g),
H2O2 (3.39 g, 30 mmol), KBr (3.57 g, 30 mmol) and HClO4 (5.72 g, 40 mmol) in 20 mL water.
Under these conditions the selectivity of different major products follows the order: 2,4-
dibromothymol (84.6%) > 2-bromothymol (8.4%) > 4-bromothymol (7%). Using 0.002 g of
catalyst and considering the substrate to different reagents in the molar ratio presented in entry
no 15 of Table 7 gave highest selectivity (85.3%) of 4-bromothymol amongst all the conditions
applied. Increasing the amounts of KBr or acid results in the formation of more 2,4-dibromo
thymol product at the expense of 4-bromo product. Under the optimized conditions for the
maximum conversion of thymol, other catalysts i.e. 2, 3 and 4 exhibit similar catalytic activity
along with similar selectivity of different products (entry no 11, 12 and 13). Blank reaction i.e
without catalyst, under same conditions (entry no. 16 of Table 7) resulted in 54% conversion.
Thus, these complexes enhance conversion of thymol and alter the products yield.
<<Table 7>>
The oxidative bromination of thymol and the selectivity of different reaction products for
catalyst 1 under the optimized reaction conditions (see Table 8) have also been studied in
different solvents. Though, conversion of thymol is almost same in all solvents, the selectivity of
products varies. In H2O, H2O/CH2Cl2 and H2O/CHCl3, all three products were obtained and the
selectivity followed the order: 2,4-dibromothymol > 2-bromothymol> 4-bromothymol, whereas
H2O/MeOH and H2O/MeCN systems gave only two products; 2,4–dibromothymol being a major
one and 2-bromothymol a minor. With all three products in H2O/hexane, the selectivity follows
the order: 2,4–dibromothymol (77.7%) > 4-bromothymol (17.6%) >2-bromothymol (4.7%).
<<Table 8>>
3.8. Reactivity of dioxidomolybdenum(VI) complexes and possible mechanism for catalytic
oxidation of substrate.
As reported earlier, dioxidomolybdenum(VI) complexes react with H2O2 to give the
corresponding [MoVIO(O2)]2+ complexes [33]. Such species has been generated in DMSO and
12

monitored by UV-vis spectroscopy. In a typical reaction, 20 mL of 4.0 × 10–5 M solution of
[MoVIO2(fhmc-bhz)(MeOH)] was treated with a drop wise solution prepared by dissolving 30%
aqueous H2O2 (0.904 g, 8.0 mmol) in 5 mL of DMSO and the observed spectroscopic changes
are depicted in Fig. 5 (a). The intensity of the bands at 322 and 261 nm considerably increases
and finally disappears. The broad charge transfer band appearing at 425 nm slowly decreases in
intensity [Fig. 5(a)] along with broadening and finally vanishes. Simultaneously, a band at ca.
378 nm also decreases in intensity and disappears. These changes in spectra occur due to the
interaction of H2O2 with the complex and plausible formation of [MoVIO(O2)(fhmc-bhz)] in
DMSO. Similar spectral changes have also been noted for other complexes; Figs. 5(b–d).
In the model vanadium complex system studied in bi-phasic system for oxidative
bromination, initially the intermediate oxidoperixidovanadium(V) derivative forms in the presence
of H2O2 which oxidizes Br– to HOBr/Br2. This intermediate enters into organic phase where the
bromination of appropriate organic substrates takes place [22,34]. During this process acid probably
promotes the protonation of the peroxide intermediate. A similar mechanism may also be proposed
here.
<<Figure 5>>
4. Conclusions
The
dioxidomolybdenum(VI)
complexes
[MoVIO2(fhmc-bhz)(MeOH)]
(1),
[MoVIO2(fhmc-inh)( MeOH)] (2) [MoVIO2(fhmc-nah)(MeOH)] (3) and [MoVIO2(fhmc-
fah)(MeOH)] (4) have been prepared from potential dibasic tridentate ONO type ligands
H2fhmc-bhz (I), H2fhmc-inh (II), H2fhmc-nah (III) and H2fhmc-fah (IV), respectively and
cgharacterized. These complexes are good catalyst precursors for the oxidative bromination of
thymol in the presence of green oxidant 30 % H2O2, HClO4 and bromide ion, therefore acting as
functional models of vanadium dependent haloperoxidases. [MoVIO2]2+ complexes upon
treatment with H2O2 instantly generates [MoVIO(O2)]2+ species which ultimately oxidizes Br– to
HOBr/ Br2 in the presence of acid. This species brominates the thymol and give corresponding
brominating products in high yield of ca. 94-99% under optimized reaction conditions. The
conversion of thymol is generally same in different solvents but the number of products and their
selectivity are solvent dependent.
13

Acknowledgments
S.D. acknowledges the University Grant Commission, New Delhi, India for a Senior
Research fellowship.
Appendix A. Supplementary data
CCDC 1056678 contains the supplementary crystallographic data for the structure
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: $$$$$.
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15

Schemes
Scheme 1. Structure of the ligands designated by I, II, III and IV used in this work.
Scheme 2. Proposed structure of cis-[MoVIO2]2+complexes.
16

Scheme 3. Products of the oxidative bromination of thymol. 2-Brth = 2-bromothymol, 4-Brth =
4-bromothymol, 2,4-dBrth = 2,4-dibromothymol.
17

Tables
Table 1
Crystal Data and Structure Refinement for the complexes [MoVIO2(Hfhmc-bhz)(DMSO)] (1).
1
Formula
C20 H18 Mo N2 O7 S
526.36
Formula weight
T, K
293
Wavelength, Å
0.71073
Triclinic
P1
Crystal system
Space group
a/Å
8.7732(3)
8.9318(3)
13.2399(5)
86.631(2)
88.075(2)
86.938(2)
1033.73(6)
2
b/Å
c/Å
α/º
β/º
γ/º
V/Å3
Z
F000
532
Dcalc/g cm-3
µ/mm-1
1.691
0.782
1.54 to 25.57
0.0401
θ/ (º)
Rint
Crystal size/ mm3
Goodness-of-fit on F2
R1[I>2σ(I)] a
wR2 (all data) b
0.22 x 0.21 x 0.18
1.105
0.0386
0.1138
Largest differences peak and hole (eÅ-3) 0.496 and -0.727
2 2
aR1 = ΣFo - Fc/ΣFo. bwR2 = {Σ[w(Fo2 -Fc2)2]/Σ[w(Fo ) ]}1/2
18

Table 2
Bond lengths [Å] and angles [°] for [MoVIO2(Hfhmc-bhz)(DMSO)] (1).
Bond lengths
1
Mo(1)-O(3)
Mo(1)-O(4)
Mo(1)-O(5)
Mo(1)-O(6)
Mo(1)-O(1S)
Mo(1)-N(1)
N(1)-C(11)
N(1)-N(2)
1.937(3)
1.999(3)
1.696(3)
1.695(3)
2.286(3)
2.243(3)
1.294(5)
1.396(4)
1.313(5)
N(2)-C(12)
Angles
1
O(6)-Mo(1)-O(5)
O(6)-Mo(1)-O(3)
O(5)-Mo(1)-O(3)
O(6)-Mo(1)-O(4)
O(5)-Mo(1)-O(4)
O(3)-Mo(1)-O(4)
O(6)-Mo(1)-N(1)
O(5)-Mo(1)-N(1)
O(3)-Mo(1)-N(1)
O(4)-Mo(1)-N(1)
O(6)-Mo(1)-O(1S)
O(5)-Mo(1)-O(1S)
O(3)-Mo(1)-O(1S)
O(4)-Mo(1)-O(1S)
N(1)-Mo(1)-O(1S)
S(1A)-O(1S)-Mo(1)
105.90(16)
97.81(13)
103.21(13)
95.43(13)
97.65(13)
151.10(12)
94.25(14)
158.32(13)
81.39(11)
72.08(11)
167.83(13)
86.06(13)
81.23(11)
80.47(10)
73.60(11)
125.34(17)
19

S(1B)-O(1S)-Mo(1)
127.4(2)
Table 3
IR spectral data [in cm–1] of compounds.
Compounds
ν(NH)
3060
3058
3056
3053
ν(C=O)pyrone/hydrazide
1705, 1673
ν(C=N)
1586
1588
1586
1587
1551
ν(C-O)
ν(O=Mo=O)a
H2fhmc-bhz (I)
H2fhmc-inh(II)
1710, 1678
1713, 1676
1716, 1674
1703, 1624
H2fhmc-nah (III)
H2fhmc-fah (IV)
[MoVIO2(fhmc-bhz)(MeOH)] (1)
1182
1179
1200
1180
938, 906
945, 915
936, 906
938, 920
[MoVIO2(fhmc-inh)(MeOH)] (2)
[MoVIO2(fhmc-nah)(MeOH)] (3)
[MoVIO2(fhmc-fah)(MeOH)] (4)
1707, 1621
1711, 1616
1717, 1611
1547
1551
1543
aAsymmetric and symmetric ν(O=Mo=O) values.
Table 4
UV-vis spectral data of ligands and complexes recorded in methanol.
λmax /nm (ε/ M–1 cm–1)
Compound
H2fhmc-bhz (I)
272 (1.8 × 104), 317 (3.1 × 104), 380 (7.6 × 103)
H2fhmc-inh(II)
268 (8.8 × 103), 317 (2.3 × 104), 382 (3.2 × 103)
267 (7.8 × 103), 314 (2.4 × 104), 381 (2.1 × 103)
270 (9.5 × 103), 317 (2.6 × 104), 388 (5.7 × 103)
H2fhmc-nah (III)
H2fhmc-fah (IV)
[MoVIO2(fhmc-bhz)(MeOH)] (1)
[MoVIO2(fhmc-inh)(MeOH)] (2)
[MoVIO2(fhmc-nah)(MeOH)] (3)
[MoVIO2(fhmc-fah)(MeOH)] (4)
261 (1.8 × 104), 322 (2.8 × 104), 378 (1.0 × 104), 425 (4.3 × 103)
259 (1.2 × 104), 318 (2.3 × 104), 382 (6.1 × 103), 430 (2.4 × 103)
261 (1.3 × 104), 318 (2.9 × 104), 380 (6.0 × 103), 431 (2.7 × 103)
267 (1.4 × 104), 317 (3.3 × 104), 379 (7.2 × 103), 430 (3.0 × 103)
20

Table 5
1H NMR chemical shifts a [δ in ppm] of ligands and complexes recorded in DMSO-d6.
Compoun -CH=N-
ds
Aromatic H
-OH
–NH
12.41
-
-CH3
2.35
2.42
2.36
2.43
2.37
9.06
6.18 (d, 1H), 6.88 (t, 1H), 7.54 (t, 2H), 7.61 12.74
(t, 2H), 7.94 (t, 2H)
I
9.14
6.34 (s, 1H), 6.97 (d, 1H), 7.51 (t, 2H), 7.59 -
(t, 1H), 7.89 (d, 1H), 8.03 (d, 2H)
1
∆δ
II
(0.08)
9.07
6.22 (s, 1H), 6.91 (d, 1H), 7.66 (d, 1H), 7.85 12.63
(d, 2H), 8.81 (s, 2H)
12.57
-
9.21
6.34 (s, 1H), 6.99 (d, 1H), 7.93 (s, 3H), 8.77 -
(s, 2H)
2
∆δ
III
(0.14)
9.11
6.22 (s, 1H), 6.92 (d, 1H), 7.66 (s, 1H), 7.68 12.61
(d, 1H), 8.28 (d, 1H), 8.79 (s, 1H), 9.07 (s,
1H)
12.55
9.18
6.35 (s, 1H), 6.99 (d, 1H), 7.57 (s, 1H), 7.91 -
(d, 1H), 8.35 (d, 2H), 8.78 (s,1H)
-
2.50
2.40
2.40
3
∆δ
IV
(0.07)
9.07
6.26 (s, 1H), 6.73 (s, 1H), 6.75 (d, 1H), 7.35 12.66
(s, 1H), 7.70 (d, 1H), 8.00 (s,1H)
12.51
-
9.13
6.34 (s, 1H), 6.73 (s, 1H), 6.97 (d, 1H), 7.27 -
(d, 1H), 7.88 (d, 1H), 7.98 (s, 1H)
4
(0.06)
∆δ
a Letters given in parentheses indicate the signal structure: s = singlet, d = doublet.
21

Table 6
13C NMR spectral data [δ in ppm] of ligand and complexes.
Compound C1
C2- C9
C10
C11
C12
C13
-
R
159.71
113.90, 152.90, 106.11,128.48,
112.25, 161.56, 111.20,154.28
113.89, 132.81, 106.23, 128.65,
112.00, 154.21, 108.65, 149.30
113.96, 151.04, 105.82, 128.91
112.33, 161.46, 111.29, 154.32
115.92, 136.24, 108.79, 127.12,
113.28, 154.90, 112.43, 149.56
113.94, 153.24, 105.88, 128.41,
112.34, 161.54, 111.23, 154.34
115.93, 149.35, 108.55, 131.60,
113.96, 159.28, 112.05, 153.06
114.10, 146.28, 111.24, 114.21
112.90, 159.77, 112.39, 153.00
117.38, 147.79, 113,27, 131.35,
115.94, 159.37, 114.01, 149.03
18.80
143.97
162.90
132.83,
132.53
129.95,
131.36
128.16,
115.93,
129.16,
129.36,
I
154.12
(–5.59)
159.66
18.84
18.83
19.30
18.77
18.81
18.91
18.89
162.40
169.65 48.80
1
(∆δ)
II
(18.43) (6.75)
145.21
161.59
-
139.58, 121.90, 153.04
128.50, 122.76, 132.43
152.75
(–6.91)
159.64
162.13
168.19 48.90
2
(∆δ)
III
(16.92) (6.60)
144.70
161.65
135.89, 128.81, 124.24, 149.12,
153.02
154.11
(–5.53)
154.44
162.40
168.06 49.05
126.20,
150.14
128.17,
116.18
147.06,
144.58
136.15,
113.87,
108.71,
124.53,
106.09,
112.16,
3
(∆δ)
IV
(17.70) (6.41)
144.20
161.50
-
152.18
(–2.26)
162.23
168.35 49.10
4
(∆δ)
(18.03) (6.85)
22

Table 7
Conversion of thymol (1.50g, 10 mmol), TOF and product selectivity using [MoVIO2(fhmc-
nah)(MeOH)] (1) as a catalyst precursor in 2 h of reaction time under different reaction
conditions.
Entry KBr
H2O2
HClO4
Catalyst
Conv. TOF
[%]
14
2-Brth 4-Brth
2,4-dBrth
No.
[g (mmol)] [g (mmol)] [g (mmol)] [g (mmol)]
[h-1]
171
1.
1.19 (10)
1.19 (10)
1.19 (10)
1.19 (10)
1.19 (10)
2.38 (20)
3.57 (30)
3.57 (30)
3.57 (30)
3.57 (30)
3.57 (30)
3.57 (30)
3.57 (30)
2.38 (20)
2.38 (20)
3.57 (30)
1.13 (10)
1.13 (10)
1.13 (10)
2.27 (20)
3.39 (30)
3.39 (30)
3.39 (30)
3.39 (30)
3.39 (30)
3.39 (30)
3.39 (30)
3.39 (30)
3.39 (30)
2.27 (20)
2.27 (20)
3.39 (30)
1.43 (10)
1.43 (10)
1.43 (10)
1.43 (10)
1.43 (10)
1.43 (10)
1.43 (10)
2.86 (20)
4.29 (30)
5.72 (40)
5.72 (40)
5.72 (40)
5.72 (40)
2.86 (20)
4.29 (30)
5.72 (40)
0.001
15.3
14.7
16.2
12.7
13.3
8.5
84.7
83.3
83.8
85
0
(2.0 × 10–3)
2
0.002
24
27
37
45
62
71
79
86
94
99
98
97
60
73
54
293
330
451
549
750
872
956
1048
1146
1207
1195
1155
731
890
-
1
(4.1 × 10–3)
3
0.003
2
(6.2 × 10–3)
4
0.002
2.3
(4.1 × 10–3)
5
0.002
84.3
57
2.4
(4.1 × 10–3)
6
0.002
34.5
38.8
63.5
77.3
84.6
83.7
88.3
89.3
5.3
(4.1 × 10–3)
7
0.002
9.2
52
(4.1 × 10–3)
8
0.002
4.2
32.3
18.7
7
(4.1 × 10–3)
9
0.002
4.0
(4.1 × 10–3)
10
11a
12 b
0.002
8.4
(4.1 × 10–3)
0.002
8.3
8
(4.1 × 10–3)
0.002
6.3
5.4
4.2
82.2
85.3
26
(4.1 × 10–3)
c
13
14
15
16
0.002
6.5
(4.1 × 10–3)
0.002
12.5
12.6
20.7
(4.1 × 10–3)
0.002
2.1
(4.1 × 10–3)
blank
53.3
a[MoVIO2(fhmc-inh)(MeOH)] (2)
b[MoVIO2(fhmc-nah)(MeOH)] (3)
c[MoVIO2(fhmc-fah)(MeOH)] (4)
23

Table 8
Solvents effect on the selectivity of product.
Solvents
Conversion [%]
2-Brth
8.4
4-Brth
2,4-dBrth
84.6
H2O
94
99
99
99
99
97
7
H2O/CH2Cl2
H2O/CHCl3
H2O/MeOH
H2O/MeCN
H2O/Hexane
9.1
1.3
3.4
-
89.6
13.2
13.6
13.3
4.7
86.4
86.4
-
86.7
17.6
77.7
24

Figures
Fig. 1. ORTEP plot of [MoVIO2(Hfhmc-bhz)(DMSO)] (1). All the non-hydrogen atoms are
presented by their 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
Fig. 2. Crystal packing of [MoVIO2(Hfhmc-bhz)(DMSO)] (1). Layer structure linked by π-π-
stacking interactions (see Fig. 2).
25

[MoVIO2(fhmc-nah)(MeOH)] (3)
-CH3
H2fhmc-nah (III)
-CH=N-
-ArH
-OH, -NH
Fig. 3. 1H NMR spectra of H2fhmc-nah (III) and [MoVIO2(fhmc-nah)(MeOH)] (3).
26

Fig. 4. 13C NMR spectra of H2fhmc-nah (III) and [MoVIO2(fhmc-nah)(MeOH)] (3).
27

318
1.5
1.2
0.9
0.6
0.3
0.0
1.5
1.2
0.9
0.6
0.3
0.0
(b)
(a)
322
378
382
430
425
300
375
450
525
300
375
450
525
Wavelength/nm
Wavelength/nm
1.5
1.2
0.9
0.6
0.3
0.0
1.5
317
(d)
(c)
318
1.2
0.9
0.6
0.3
0.0
379
380
430
431
300
375
450
525
300
375
450
525
Wavelength/nm
Wavelength/nm
Fig. 5. Spectral changes observed during the titration of dioxidomolybdenum(VI) complexes. (a)
The spectra were recorded after successive addition of one drop portion of H2O2 [30% H2O2
(0.904 g, 8.0 mmol) dissolved in 5 mL of DMSO; final concentration of H2O2 = 1.59 M)] to 20
mL of 4.0× 10–5 M solution of [MoVIO2(fhmc-bhz)(MeOH)]. (b) The spectra were recorded after
successive addition of one drop portion of H2O2 [30% H2O2 (0.990 g, 8.8 mmol) dissolved in 5
mL of DMSO; final concentration of H2O2 = 1.74 M)] to 20 mL of 5 × 10–5 M solution of
[MoVIO2(fhmc-inh)(MeOH)]. (c) The spectra were recorded after successive addition of one drop
portion of H2O2 [30% H2O2 (0.825 g, 7.3 mmol) dissolved in 5 mL of DMSO; final
concentration of H2O2 = 1.45 M] to 20 of mL of 4 × 10–5 M solution of [MoVIO2(fhmc-
nah)(MeOH)]. (d) The spectra were recorded after successive addition of one drop portion of
H2O2 [30% H2O2 (0.938 g, 8.3 mmol) dissolved in 5 mL of DMSO; final concentration of H2O2
= 1.65 M] to 20 of mL of 3.5 × 10–5 M solution of [MoVIO2(fhmc-fah)(MeOH)].
28

Graphical abstract
Oxidative bromination of monoterpene (thymol) using dioxidomolybdenum
(VI) complexes of hydrazones of 8-formyl-7-hydroxy-4-methylcoumarin
Mannar R. Maurya, Sarita Dhaka, Fernando Avecilla
Oxidative bromination of thymol catalyzed by dioxidomolybdenum(VI) complexes are reported.
30

Oxidative bromination of monoterpene (thymol) using dioxidomolybdenum
(VI) complexes of hydrazones of 8-formyl-7-hydroxy-4-methylcoumarin
Mannar R. Maurya, Sarita Dhaka, Fernando Avecilla
H O , KBr,
2
2
H O
322
2
2
1.2
0.9
0.6
0.3
0.0
1.5
1.2
0.9
0.6
0.3
0.0
322
378
378
425
425
300
375
450
525
300
400
Wavelength/nm
500
Wavelength/nm
31