Synthesis, characterization, reactivity and catalytic activity of dioxidomolybdenum(VI) complexes derived from tribasic ONS donor ligands

Article abstract of DOI:10.1016/j.poly.2014.05.068

The reaction of three Schiff bases, H₃dfmp(sbdt)₂ (I), H₃dfmp(smdt)₂ (II) and H₃dfmp(tsc) ₂ (III) [derived from the condensation of 2,6-diformyl-4-methylphenol (dfmp) with S-benzyldithiocarbazate (sbdt), S-methyldithiocarbazate (smdt) and thiosemicarbazide (tsc)], with [Mo^VIO₂(acac)₂] (Hacac = acetylacetone) on stirring in methanol results in the formation of the dioxidomolybdenum(VI) complexes [Mo^VIO₂{Hdfmp(sbdt) ₂}(H₂O)] (1), [Mo^VIO₂{Hdfmp(smdt) ₂}(H₂O)] (2) and [Mo^VIO₂{Hdfmp(tsc) ₂}(H₂O)] (3), respectively. All these complexes have been characterized in the solid state and in solution by elemental analyses, spectroscopic techniques (IR, electronic, ¹H and ¹³C NMR) and thermogravimetric analyses. The structures of complexes 1, 2 and 3, confirmed by a single crystal X-ray study, reveal that only one set of azomethine nitrogen, enthiolate sulfur and phenolate oxygen atoms of the ligands are coordinated to the molybdenum centre. All the complexes form an antiparallel dimer through different π-π interactions. These molybdenum complexes catalyze the oxidation of styrene and cyclohexene using 30% H ₂O₂ as an oxidant in the presence of sodium bicarbonate. Styrene, under optimized reaction conditions, gave two reaction products, namely phenylacetaldehyde as the major product and styrene oxide as the minor product. Oxidation of cyclohexene selectively gave 2-cyclohexene-1-ol.

Full text of DOI:10.1016/j.poly.2014.05.068

Accepted Manuscript
Synthesis, characterization, reactivity and catalytic activity of dioxidomolyb-
denum(VI) complexes derived from tribasic ONS donor ligands
Mannar R. Maurya, Sarita Dhaka, Fernando Avecilla
PII:
S0277-5387(14)00385-4
DOI:
http://dx.doi.org/10.1016/j.poly.2014.05.068
POLY 10783
Reference:
Polyhedron
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13 March 2014
14 May 2014
Please cite this article as: M.R. Maurya, S. Dhaka, F. Avecilla, Synthesis, characterization, reactivity and catalytic
activity of dioxidomolybdenum(VI) complexes derived from tribasic ONS donor ligands, Polyhedron (2014), doi:
http://dx.doi.org/10.1016/j.poly.2014.05.068
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Synthesis, characterization, reactivity and catalytic activity of
dioxidomolybdenum(VI) complexes derived from tribasic ONS donor ligands
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
Abstract
The reaction of three Schiff bases, H3dfmp(sbdt)2 (I), H3dfmp(smdt)2 (II) and H3dfmp(tsc)2 (III)
[derived from the condensation of 2,6-diformyl-4-methylphenol (dfmp) with S-
benzyldithiocarbazate (sbdt), S-methyldithiocarbazate (smdt) and thiosemicarbazide (tsc)], with
[MoVIO2(acac)2] (Hacac = acetylacetone) on stirring in methanol results in the formation of the
dioxidomolybdenum(VI)
complexes
[MoVIO2{Hdfmp(sbdt)2}(H2O)]
(1),
[MoVIO2{Hdfmp(smdt)2}(H2O)] (2) and [MoVIO2{Hdfmp(tsc)2}(H2O)] (3), respectively. All
these complexes have been characterized in the solid state and in solution by elemental analyses,
1
spectroscopic techniques (IR, electronic, H and 13C NMR) and thermogravmetric analyses. The
structures of complexes 1, 2 and 3, confirmed by a single crystal X-ray study, reveal that only
one set of azomethine nitrogen, enthiolate sulfur and phenolate oxygen atoms of the ligands are
coordinated to the molybdenum centre. All the complexes form an antiparallel dimer through
different π-π interactions. These molybdenum complexes catalyze the oxidation of styrene and
cyclohexene using 30 % H2O2 as an oxidant in the presence of sodium bicarbonate. Styrene,
under optimized reaction conditions, gave two reaction products, namely phenylacetaldehyde as
the major roduct and styrene oxide as the minor product. Oxidation of cyclohexene selectively
gave 2-cyclohexene -1-ol.
Keywords: Mo complexes; catalysts, NMR spectroscopy; Crystal structure; Oxidation of
styrene; Oxidation of cyclohexene.
1

* To whom correspondence should be addressed. E-mail: rkmanfcy@iitr.ac.in (M.R. Maurya),
Tel: +91 1332 285327. Fax: +91 1332 273560.
1. Introduction
Molybdenum-containing enzymes hold key positions both in biochemical cycles of carbon,
nitrogen and sulfur and in the metabolism of every organism [1]. Oxygen atom transfer (OAT)
between competent centers as a fundamental reaction in chemistry and biology has been
extensively investigated in the past decades [2,3]. Molybdenum enzymes catalyze a number of
such reactions. These oxotransferases containing oxido-(dioxido) molybdenum units are
additionally coordinated by sulfur atoms [4]. In most biological systems, molybdenum
containing enzymes, such as sulfite oxidase, dimethyl sulfoxide reductase and nitrate reductase,
catalyze oxygen atom transfer reactions at the mononuclear oxidomolybdenum active centers
[5,6], wherein XO/X functions act as donor/acceptors and the oxidation state of the molybdenum
atom changes by two units (Eq. 1).
MoVIO2Ln + X
MoIVOLn + XO
(1)
Molybdenum complexes are known for their catalytic applications in the oxidation [7-23]
and oxidative bromination of organic substrates [24-27], and the oxidation of sulfides [28-31].
The catalytic efficiency of these catalysts, e.g. the oxidation of organic compounds, has been
found to be improved using the co-catalyst NaHCO3 in the presence of H2O2 as a terminal
oxidant [32-35]. It has been presumed that this is due to the efficient formation of an
oxidoperoxidomolybdenum(VI) intermediate during the catalytic reaction.
Synthetic strategies for cis-[MoVIO2]2+ complexes of O and N donor ligands have been
extensively reported in the literature [25-27,36] in comparison to O, N and S donor ligands
[37,38]. Ligands derived from 2,6-diformyl-4-methylphenol with S-benzyldithiocarbazate, S-
methyldithiocarbazate and thiosemicarbazide are tribasic pentadenate O, N and S donor ligands
(Scheme 1), but non-participation of the second azomethine and thiol groups of the second Schiff
2

base moiety may results in tridentate behavior of these ligands. However, the non-participating
arm may be involved in intermolecular π-bonding/ hydrogen bonding, leading to a
supramolecular arrangement. Therefore, we report herein the synthesis and characterization of
new cis-[MoVIO2]2+ complexes of these ligands. Their catalytic potentials, in the presence of the
co-catalyst NaHCO3 are demonstrated by studying the oxidation of styrene and cyclohexene.
<<Scheme 1>>
2. Experimental section
2.1. Materials
Ammonium molybdate (Loba Chemie, India), styrene (Acros Organics, U.S.A.),
cyclohexene, methyl iodide (Himedia, India), acetylacetone (Hacac) (Aldrich, U.S.A.), 30%
aqueous H2O2 (Renkem, India) and benzyl chloride (S.D. fine chemicals, India) were used as
obtained. All other chemicals and solvents used were of AR grade. [MoVIO2(acac)2] [39], 2,6-
diformyl-4-methylphenol [40], S-benzyldithiocarbazate [41] and S-methyldithiocarbazate [42]
were prepared according to the methods reported in the literature.
2.2. Instrumentation and characterization procedures
Elemental analyses of the ligands and complexes were obtained by an Elementar model
Vario-EL-III. Thermogravimetric analyses of the complexes were obtained under an oxygen
atmosphere using a TG Stanton Redcroft STA 780 instrument. IR spectra were recorded as KBr
pellets on a Nicolet 1100 FT-IR spectrometer after mixing the sample with KBr. Electronic
spectra of ligand and complexes were recorded in DMSO or methanol using a Shimadzu 1601
1
UV-vis spectrophotometer. H NMR spectra of the 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 an 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 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.
3

The percent conversion of substrate and the selectivity of the products were calculated from GC
data using the formulae presented elsewhere [17].
2.3. X-Ray crystal structure determination
Three-dimensional X-ray data were collected on a Bruker Kappa Apex CCD
diffractometer at low temperature for 1a and 2a, and at room temperature for 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. Of the 117594 for 1a, 51408 for 2a and 41327 for 3a reflections
measured, all were corrected for Lorentz and polarization effects and for absorption by multi-
scan methods based on symmetry-equivalent and repeated reflections, 7162, 6643 and 5942,
respectively, independent reflections exceeded the significance level (F/σF) > 4.0.
Complex scattering factors were taken from the program package SHELXTL [43]. The
structures were solved by direct methods and refined by full matrix least-squares on F2.
Hydrogen atoms were located in the difference Fourier map and left to refine freely in 1a, except
those bonded to C(1M), C(2M), C(3M) and C(16). The hydrogen atom of O(3M) was located in
the difference Fourier map and fixed to the oxygen atom in 1a. In the other structures, the
hydrogen atoms were included in calculated positions and refined in the riding mode, except
those bonded to C(3), N(4), C(5), N(8), C(11), C(16), C(18), C(21) and C(24) in 2a, and O(1M),
C(2), N(3), C(4), N(5), N(6), C(6) and C(9) in 3a, which were located in the difference Fourier
map and left to refine freely. Refinements were done with allowance for thermal anisotropy of all
non-hydrogen atoms. The crystal of 2a presents a disorder on the second dmso coordinated
molecule present in the asymmetric unit. This disorder has been refined and two atomic sites for
one sulfur atom have been observed and refined with anisotropic atomic displacement
parameters in each case. Unfortunately, the hydrogen atoms for this disordered dmso molecule
have not been located. The site occupancy factor was 0.70240 for S(10A). Further details of the
crystal structure determination are given in Table 1. A final difference Fourier map showed no
residual density outside 0.865 and -0.794 e Å–3 for 1a, 2.401 and -1.268 e Å–3 for 2a and 1.733
2
and -1.362 e Å–3 for 3a. A weighting scheme w = 1/[σ2(Fo ) + (0.053000 P)2 + 0.922900 P] for
2
2
1a, w = 1/[σ2(Fo ) + (0.078300 P)2 + 8.261801 P] for 2a and w = 1/[σ2(Fo ) + (0.019700 P)2 +
3.996000 P] for 3a, where P = (|Fo|2 + 2|Fc|2)/3, were used in the latter stages of refinement.
4

<<Table 1>>
2.4. Preparation
2.4.1. Preparation of H3dfmp(sbdt)2 (I), H3dfmp(smdt)2 (II) and H3dfmp(tsc)2 (III)
The ligands H3dfmp(sbdt)2 (I), H3dfmp(smdt)2 (II) and H3dfmp(tsc)2 (III) were prepared
by a general procedure; the preparation of H3dfmp(sbdt)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-benzyldithiocarbazate (3.966 g, 20 mmol) dissolved in methanol (10 ml). The
obtained reaction mixture was refluxed on a water bath for 4 h. During this period a yellow solid
slowly separated out. This was filtered, washed with methanol followed by petroleum ether and
dried over silica gel under vacuum.
Data for H3dfmp(sbdt)2 (I): Yield: 4.93 g (94%). Anal. Calc. for C25H24N4S4O (524): C,
57.24; H, 4.62; N, 10.69; S, 24.40. Found: C, 56.92; H, 4.56; N, 10.58; S, 24.36%.
Data for H3dfmp(smdt)2 (II): Yield: 3.49 g (93.8%). Anal. Calc. for C13H16N4S4O (372):
C, 41.93; H, 4.33; N, 15.06; S, 34.38. Found: C, 41.52; H, 4.16; N, 15.18; S, 34.26%.
Data for H3dfmp(tsc)2 (III): Yield: 2.99 g (96.5%). Anal. Calc. for C11H14N6S2O (310):
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%.
2.4.2. Preparation of [MoVIO2{Hdfmp(sbdt)2}(H2O)] (1)
A solution of [MoVIO2(acac)2] (0.66 g, 2 mmol) dissolved in 10 ml of methanol was
added to a stirred solution of H3dfmp(sbdt)2 (1.048 g, 2 mmol) in methanol (20 ml) and the
obtained reaction mixture was stirred at room temperature, whereupon an yellow solid started to
form. After 2 h of stirring, the separated solid was filtered, washed with methanol and dried in a
vacuum desiccator over silica gel. Yield: 1.15 g (85.8%). Anal. Calc. for C25H24N4S4O4Mo
(668.7): C, 44.91; H, 3.62; N, 8.38; S, 19.18. Found: C, 44.63; H, 3.56; N, 8.54; S, 19.26%.
Crystals of [MoVIO2{Hdfmp(sbdt)2}(MeOH)]·2MeOH (1a·2MeOH) suitable for X-ray analysis
were grown by slow evaporation of a solution of 1 in methanol.
5

2.4.3. Preparation of [MoVIO2{Hdfmp(smdt)2}(H2O)] (2)
Complex 2 was prepared from [MoVIO2(acac)2] (0.66 g, 2 mmol) and H3dfmp(smdt)2
(0.744, 2 mmol) in methanol by the method outlined for 1. The separated yellow solid was
filtered, washed with cold methanol and dried in a vacuum desiccator over silica gel. Yield:
0.890 g (86.4%). Anal. Calc. for C13H16N4S4O4Mo (516.4): C, 30.23; H, 3.12; N, 10.85; S, 24.83.
Found: C, 30.28; H, 3.36; N, 10.50; S, 24.96%. Crystals of [MoVIO2{Hdfmp(smdt)2}(dmso)]
·dmso (2a·dmso) suitable for X-ray analysis were grown by slow evaporation of a solution of 2
in dmso.
2.4.4. Preparation of [MoVIO2{Hdfmp(tsc)2}(H2O)] (3)
Complex 3 was prepared from [MoVIO2(acac)2] (0.66 g, 2 mmol) and H3dfmp(tsc)2
(0.620 g, 2 mmol) in methanol following the method described for 1. After 2 h of stirring, the
separated yellow solid was filtered, washed with cold methanol and dried in a vacuum desiccator
over silica gel. Yield: 0.772 g (85.0%). Anal. Calc. for C11H14N6S2O4Mo (454.33): C, 29.08; H,
3.11; N, 18.50; S, 14.11. Found: C, 29.19; H, 3.28; N, 18.48; S, 14.26%. Crystals of
[MoVIO2{Hdfmp(tsc)2}(MeOH)]·MeOH (3a·MeOH) suitable for X-ray analysis were grown by
slow evaporation of a solution of 3 in methanol.
2.5. Catalytic activity studies
2.5.1. Oxidation of styrene
In a typical reaction, styrene (0.52 g, 5 mmol), aqueous 30% H2O2 (2.27 g, 20 mmol),
NaHCO3 (0.168 g, 2 mmol) and catalyst (0.0005 g) were dissolved in acetonitrile (5 mL) and the
temperature of the reaction mixture was set to 60 °C. The reaction was monitored by
withdrawing small aliquots of the reaction mixture at every 30 min and analyzing them
quantitatively by gas chromatography. The identities of the products were confirmed by GC–MS.
The effects of various parameters, such as amount of oxidant, amount of sodium bicarbonate,
catalyst, temperature and solvent, were studied to see their effect on the conversion and
selectivity of the reaction products.
2.5.2. Oxidation of cyclohexene
6

Aqueous 30% H2O2 (2.27 g, 20 mmol), cyclohexene (0.41 g, 5 mmol), NaHCO3 (0.168 g,
2 mmol) and catalyst (0.0005 g) were mixed in CH3CN (5 mL) and the reaction mixture was
heated at 60 °C with continuous stirring in an oil bath for ca. 2.5 h. The reaction was monitored
by withdrawing small aliquots of the reaction mixture at every 30 min and analyzing them
quantitatively by gas chromatography. The identities of the products were confirmed as
mentioned above. The effects of various parameters, such as amounts of oxidant, amount of
sodium bicarbonate, catalyst, temperature and solvent, were studied to see their effect on the
conversion and selectivity of the reaction products.
3. Results and discussion
3.1. Synthesis and characterization of the complexes
The dioxidomolybdenum(VI) precursor [MoVIO2(acac)2] reacts with the ligands
H3dfmp(sbdt)2 (I), H3dfmp(smdt)2 (II) and H3dfmp(tsc)2 (III) (c.f. Scheme 1) in methanol to
give
the
dioxidomolybdenum(VI)
complexes
[MoVIO2{Hdfmp(sbdt)2}(H2O)(1),
[MoVIO2{Hdfmp(smdt)2}(H2O)] (2) and [MoVIO2{Hdfmp(tsc)2}(H2O)] (3), respectively [Eq. (2)
taking I as a representative example].
[MoVIO2(acac)2] + H3dfmp(sbdt)2 + H2O → [MoVIO2{Hdfmp(sbdt)2}(H2O)] + 2Hacac
(2)
All these complexes are soluble in methanol, ethanol and dichloromethane. In these
complexes the sixth coordination site is occupied by water, however, complexes 1, 2 and 3 could
be isolated as MeOH, dmso or MeOH coordinated compounds, respectively on crystallizing
them in the respective solvent. Scheme 2 provides the idealized structures of the complexes,
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{Hdfmp(sbdt)2}(MeOH)]·2MeOH (1a·2MeOH),
[MoVIO2{Hdfmp(smdt)2}(dmso)]·dmso (2a·dmso) and [MoVIO2{Hdfmp(tsc)2}(MeOH)]·MeOH
(3a·MeOH).
<<Scheme 2>>
7

3.2. Thermal studies
The thermal stability of the monomeric complexes 1, 2 and 3 has been studied under an
oxygen atmosphere. These complexes lose weight roughly equal to one water molecule in the
temperature range 100-160 °C, indicating the presence of coordinated water. The anhydrous
complexes on further increasing the temperature decompose exothermically in two/three
overlapping steps and form MoO3 at ca. 650 oC for 1 (Found: 21.5; Calc.: 21.5%), ca. 520 oC for
o
2 (Found: 26.3; Calc.: 27.9%) and at ca. 480 C for 3 (Found: 30.8; Calc.: 31.7%) as the final
product.
3.3. Structure descriptions
<<Figures 1 to 6>> <<Tables 2 and 3>>
ORTEP diagrams of [MoVIO2{Hdfmp(sbdt)2}(MeOH)]·2MeOH (1a·2MeOH),
[MoVIO2{Hdfmp(smdt)2}(dmso)]·dmso (2a·dmso) and [MoVIO2{Hdfmp(tsc)2}(MeOH)]·MeOH
(3a·MeOH) are shown in Figs. 1, 2 and 3, respectively. Figs. 4, 5 and 6 present the
intermolecular π-π interactions. Selected bond distances and angles are given in Table 2. These
complexes adopt a six-coordinated structure with a distorted octahedral geometry. In the
complexes, the phenolic oxygen, enthiolate sulfur and the azomethine nitrogen atoms of the
ligands coordinate to the molybdenum centre, while the other set of sulfur and azomethine
nitrogen atoms of the ligands do not participate in coordination. One methanol molecule in
1a·2MeOH, one dmso molecule in 2a·dmso and one methanol molecule in 3a·MeOH, and two
oxido groups complete the coordination sphere. Two methanol molecules in 1a·2MeOH, another
dmso molecule in 2a·dmso and one methanol molecule in 3a·MeOH are present in the crystal
packing. One of the coordinated dmso molecules presents a disorder on the sulfur atom. Two
atomic sites have been observed for the first dmso molecule [the site occupancy factor for
S(10A) is 0.70240].
The asymmetric unit of complex 2a·dmso contains two [MoVIO2{Hdfmp(smdt)2}(dmso)]
complexes and two dmso molecules. In complex 2a·dmso,lateral chains of the two complexes
interact by π-π interactions. In the π-π interactions the major contributions to the interaction
8

energy come from electrostatic and van der Waals components [44], but geometrical
requirements are necessary for the attractive electrostatic interactions to dominate over the
repulsive ones. Steric requirements, derived from the coordination sphere around Mo, prevent π-
π interactions between the phenol rings. In the solid state, the complexes form an antiparallel
dimer through π-π interactions between C=S bonds and phenol rings and between the C=N
bonds of two complexes (see Fig. 5). The distances between the centroids are: dc1-c2 = 3.863 Å
[c1 (C8-N3), c2 (C24-N7)], dc3-c4 = 3.991 Å [c3 (C9-S3), c4 (C14-C15-C16-C17-C18-C19)] and
dc5-c6 = 3.933 Å [c5 (C25-S7), c6 (C1-C2-C3-C4-C5-C6). In 1a·2MeOH, weak π-π interactions
appear only between C=N bonds, dc1-c2 = 3.737 Å [c1 (C9J-N2J), c2 (C9A-N2A)], but the
antiparallel placement is retained (Fig. 4). In compound 3a·MeOH, similar π-π interactions are
present in the crystal packing, but in this case the interactions occur between C=N bonds and
phenol rings in an antiparallel placement. The distances between the centroids are: dc1-c2 = 3.329
Å [c1 (C2F-N4F), c2 (C3A-C4A-C5A-C6A-C7A-C8A)] and dc3-c4 = 3.329 Å [c3 (C2A-N4A), c4
(C3F-C4F-C5F-C6F-C7F-C8F)] (see Fig. 6). Intermolecular hydrogen bonds are present in the
crystal packing in the three structures (see Table 3).
3.4. IR spectral studies
A partial list of the IR spectral data of the ligands and the complexes is presented in
Table 4. The IR spectra of the ligands exhibit a sharp band due to the ν(C=N) (azomethine)
stretch at 1609-1616 cm–1, though there are two azomethine groups present in the ligands. All the
complexes retain this band's position, with slight deviations, along with the appearance of a new
band at a considerably lower wavenumber, indicating the coordination of only one of the
azomethine nitrogen atoms to the molybdenum. The appearance of a strong band between 1030
and 1038 cm–1 due to ν(C=S) indicates the thione (thiocarbonyl) nature of the uncoordinated
ligands, which is further supported by the absence of an IR band at ca. 2500 cm–1 for ν(SH) and
the presence of a band at ca. 3100 cm–1 for ν(NH) of the hydrazide moiety [45]. Again all the
complexes retain the positions of these bands, with slight deviations, but the appearance of a new
band at 679-697 cm–1 due to the ν(C-S) stretch suggests thioenolisation of one of the C=S groups
and coordination of the enethiolate sulfur to molybdenum. A new band at 965-998 cm–1 arising
9

from thioenolization and the formation of an –N=N- group has also been observed. The presence
of several medium intensity bands between 2800 and 2500 cm–1 in the ligands as well as in the
complexes suggest the existence of C-H stretching due to -CH2. In addition, the complexes
display two sharp bands at 923-945 and 892-910 cm–1 due to the νasym(O=Mo=O) and
νsym(O=Mo=O) streching modes, respectively, due to the presence of the cis-[MoVIO2] moiety
[46].
<<Table 4>>
3.5. Electronic spectral studies
The electronic spectra of the ligands could only be recorded in DMSO due to their
insolubility in other solvents, while the spectra of the complexes were recorded in methanol
successfully. Table 5 presents the electronic spectral data of the ligands and the complexes. The
electronic spectra of H3dfmp(sbdt)2 (I), H3dfmp(smdt)2 (II) and H3dfmp(tsc)2 (III) exhibit two
sets of UV/vis absorption bands, possibly due to the presence of asymmetry in the two sets of
coordination sites. Based on their extinction coefficients, these bands are interpreted as n→π*
(365-367 and 401-405 nm) and π→π* (329 and 245 nm). The region below this did not show
any band in DMSO solvent. However, the electronic spectra of the metal complexes recorded in
methanol exhibit bands in the lower region as well. As one site of the each ligand (i.e.
azomethine N and thioenolic S) is not involved in coordination, as confirmed by X-ray crystal
studies e.g. for complexes 2 and 3 (vide supra), these complexes exhibit two sets of intraligand
electronic spectral transitions, i.e. due to free as well as coordinated portions of ligands, and thus
show complexity in the UV region (Table 5); the band due to ϕ →ϕ* (207-209 nm) also
becomes visible in the complexes. In addition, all the complexes exhibit a medium intensity band
at 430-435 nm due to a ligand to metal charge transfer (LMCT) transition from the phenolate
oxygen atom to an empty d-orbital of the molybdenum. As MoVI complexes have 4d0
configuration, no d→d bands are expected.
<<Table 5>>
3.6. 1H and 13C NMR studies
10

The coordinating modes of H3dfmp(sbdt)2 (I), H3dfmp(smdt)2 (II) and H3dfmp(tsc)2 (III)
1
were also confirmed by comparing their H NMR spectral patterns with those of the
corresponding complexes. The relevant spectral data are collected in Table 6. Fig. 7 reproduces
1
the H NMR spectra of a representative ligand (II) and its complex. All the ligands show two
signals at δ = 11.52-13.50 (br, 2H) and 9.5610.81 (br, 1H) ppm, corresponding to the NH and
phenolic OH protons, respectively. The absence of a signal due to a phenolic OH in the
complexes confirms coordination of the phenolic oxygen to the molybdenum centre. The
existence of an NH signal equivalent to 1H indicates that only one of the NH protons is involved
in tautomerization with the C=S moiety, which then coordinates with molybdenum after proton
replacement, while the other C=S moiety remains free. Similarly, the appearance of two signals
due to the azomethine protons (each equivalent to 1H) in the complexes with coordination-
induced shifts [∆δ = [δ(complex) – δ(free ligand)] of 0.08-0.51 ppm in the complexes
demonstrates the coordination of only one of the azomethine N atoms to molybdenum. Further,
the equal intensity for these two azomethine protons signals rules out the possibility of a
dynamic equilibrium between the free and coordinated nitrogen atoms in solution. Signals due to
aromatic protons and aliphatic protons appear in the expected regions in the spectra of the
ligands as well as the complexes, with slight shifts in their positions.
<<Figure 7>> <<Table 6>>
The 13C NMR spectra of a representative ligand H3dfmp(smdt)2 (II) and its complex (2)
are presented in Fig. 8, while Table 7 provides the entire spectral data. Assignments of the peaks
are based on the intensity patterns for the various chemical shifts and the coordination-induced
shifts (∆δ) of the signals for carbon atoms in the vicinity of the coordinating atoms [47]. These
assignments also provide useful information for the elucidation of the structures of the
complexes. Ligands I, II and III display 12, 8 and 7 signals corresponding to the 24, 16 and 14
carbon atoms, respectively, due to the presence of a centre of symmetry. A maximum of 21, 13
and 11 signals were observed for complexes 1, 2 and 3, respectively, due to the asymmetry
generated after coordination of the ligands to the molybdenum centre. All the ligands display a
single signal each for azomethine (C8 and C9) and thiolate (C10 and C11) carbon atom.
Appearance of two distinct signals each due to azomethine (C8 and C9) and thiolate carbon
11

atoms (C10 and C11), with good coordination-induced shifts (∆δ) against the single signal each
for these carbons in the ligands suggest the involvement of only one set of the azomethine
nitrogen and thiolate sulfur atoms in coordination. Complexes 1 and 2 shows two signals each
for -CH2 and –CH3 carbons due to asymmetry in the structure against a single signal each for
these carbons in the corresponding ligand
<<Figure 8>> <<Table 7>>
3.7. Catalytic activity studies
3.7.1. Oxidation of styrene
The oxidation of styrene catalyzed by vanadium complexes has been extensively studied
where H2O2 has been used as an oxidant [48]. At least five oxidation products have been
obtained using the polymer-anchored catalysts PS-[VIVO(sal-ohyba)·DMF] [49] and PS-
K[VVO(O2)(L)] [L = 2-(2-pyridyl)benzimidazole and 2-(3-pyridyl)benzimidazole] [50].
Oxidation of styrene, catalyzed by the dioxidomolybdenum(VI) complexes using aqueous 30%
H2O2 as an oxidant did not proceed. However, addition of NaHCO3 activated the catalytic
oxidation process and gave two products namely, styrene oxide and phenyl acetaldehyde
(Scheme 3). Such NaHCO3 assisted oxidation of styrene has been reported in the literature [32-
35].
<<Scheme 3>>
We have selected [MoO2{Hdfmp(tsc)2}(H2O)] (3) as a representative catalyst and
optimized the reaction conditions for the maximum oxidation of styrene by studying five
different parameters in detail, viz. the effect of the amounts of oxidant (moles of H2O2 per mole
of styrene), catalyst (amount of catalyst per mole of styrene), NaHCO3 and solvent, and
temperature of the reaction mixture. The effect of oxidant was studied considering
styrene:aqueous 30% H2O2 molar ratios of 1:2, 1:3 and 1:4, where the mixture of styrene (0.520
g, 5 mmol), catalyst (0.0005 g), NaHCO3 (0.168 g, 2 mmol) and oxidant were taken in 5 ml of
CH3CN and the reaction was carried out at 60 °C. As illustrated in entries no. 1, 6 and 7 of Table
12

8, the percent conversion of styrene improved from 27% to 97% on increasing the
styrene:oxidant ratio from 1:2 to 1:4, through 82% for 1:3, suggesting that a 1:4 (styrene:H2O2)
molar ratio is sufficient enough to perform the reaction with good conversion. Similarly, for
three different amounts, viz. 0.0005, 0.0010 and 0.0015 g, of catalyst at a styrene to H2O2 molar
ratio of 1:4 under above reaction conditions, 0.0005 g catalyst gave 97% conversion, while 0.001
and 0.0015 g catalyst improved the conversion only slightly (98%) (entries no. 1, 2 and 3 of
Table 8). The amount of NaHCO3 plays a significant role on the conversion of styrene and the
best conversion was obtained with 2 mmol of NaHCO3 (entries no. 1, 4 and 5 in Table 8). In the
absence of NaHCO3 there was no conversion of substrate. Amongst the three different
temperatures of 40, 50 and 60 °C for the fixed operating condition of styrene (0.52 g, 5 mmol),
H2O2 (2.27 g, 20 mmol), [MoO2{Hdfmp(tsc)2}(H2O)] (3) (0.0005 g), NaHCO3 (0.168, 2 mmol)
and MeCN (5 ml), running the reaction at 60 °C gave a much better conversion (entries no. 1, 10
and 11 of Table 8). Only the time required to achieve the maximum conversion was reduced on
carrying out the reaction at 80 °C. Variation in the volume of CH3CN (5, 10 and 15 ml) was also
studied (entries no. 1, 8 and 9 of Table 8) and it was observed that 5 ml of CH3CN was sufficient
enough to get a good transformation of styrene while running the reaction at 60 °C under the
above reaction conditions. Thus, all the reaction conditions as concluded above (i.e. styrene (0.52
g, 5 mmol), H2O2 (2.27 g, 20 mmol), [MoO2{Hdfmp(tsc)2}(H2O)] (3) (0.0005 g), NaHCO3
o
(0.168, 2 mmol), MeCN (5 ml) and reaction temperature 60 C) were considered essential and
applied for the maximum transformation of styrene into a mixture of the oxidation products. Fig.
9 and Table 8 summarize all the conditions and conversions obtained under a particular set of
conditions.
<<Figure 9>>
The conversion of styrene and the selectivity of different reaction products using
[MoO2{Hdfmp(tsc)2}(H2O)] (3) as a catalyst under the optimized reaction conditions (entry no. 1
of Table 8) have been analyzed as a function of time and are presented in Fig. 10 and Table 9.
Thus, under the optimized reaction conditions, the selectivity of the two oxidation products
varies in the order: phenyl acetaldehyde (92%) > styrene oxide (8%). Complexes 1 and 2 also
follow same order of selectivity for the products, with only slightly different turn over
frequencies. A blank reaction (i.e. in the absence of catalyst while using H2O2 and NaHCO3)
13

under the above reaction conditions gave 32% conversion. The reaction was also carried out at
room temperature under the same conditions in the presence of NaHCO3 only and it gave 11%
conversion of styrene. All these suggest the enhancement of the catalytic ability of the
complexes using H2O2 in presence of NaHCO3.
<<Figure 10>> <<Tables 8 and 9>>
3.7.2. Oxidation of cyclohexene
The complex [MoVIO2{Hdfmp(tsc)2}(H2O)] (3) was also used as a representative catalyst
to study the oxidation of cyclohexene by H2O2, and NaHCO3 was found necessary to assist the
catalytic activity. As shown in Scheme 4, it gave 2-cyclohexene-1-ol selectively. In order to
achieve the optimum reaction conditions, three different cyclohexene to aqueous 30% H2O2
molar ratios, viz. 1:2, 1:4 and 1:6, were considered for a fixed amount of cyclohexene (0.41 g, 5
mmol), NaHCO3 (0.168 g, 2 mmol) and catalyst (0.0005 g) in 5 ml of MeCN, and the reaction
o
was carried out at 60 C. As presented in entries no. 1, 6 and 7 of Table 10 and Fig. 11 (a), a
maximum of 22% conversion was obtained at a cyclohexene to H2O2 molar ratio of 1:2 in 2.5 h
of reaction time. This conversion reached 96% on increasing this ratio to 1:4, while the 1:6 ratio
only showed a marginal improvement (98%) in the conversion. Under the operating conditions
as fixed above, i.e. cyclohexene (0.41 g, 10 mmol), 30% H2O2 (2.27 g, 20 mmol), NaHCO3
(0.168 g, 2 mmol) in 5 ml of MeCN, the effect of the catalyst considering three different
amounts, viz. 0.0005, 0.0010 and 0.0015 g, as a function of time was studied and the results are
illustrated in Fig. 11 (b) (also see entries no. 1, 2 and 3 of Table 10). It is clear from the plot that
0.0005 g catalyst was the best one to obtain a maximum of 96% conversion of cyclohexene, as
0.001 and 0.0015 g catalysts showed only marginal improvements in the conversion. We have
also optimized the amount of NaHCO3 (entries no. 1, 4 and 5), the amount of solvent
(acetonitrile, entries no. 1, 8 and 9) and temperature (entries no. 1, 10 and 11) of the reaction and
found that 2 mmol of NaHCO3, 5 ml of solvent and 60 °C reaction temperature were sufficient
enough to obtained 96% conversion under the above reaction conditions. Table 10 and Fig. 11
summarize all the conditions and conversions obtained under a particular set of conditions. No
conversion of substrate in the absence of NaHCO3 was obtained. A blank reaction (i.e. in the
14

absence of catalyst while using H2O2 and NaHCO3) under the above reaction conditions gave
36% conversion. Oxidation reaction of cyclohexene under same conditions at room temperature
gave only 22% conversion.
<<Scheme 4>> <<Figure 11>> <<Tables 10 and 11>>
3.8. Reactivity of the dioxidomolybdenum(VI) complexes and possible mechanism for the
catalytic oxidation of substrates.
As observed earlier, dioxidomolybdenum(VI) complexes react with H2O2 to give the
corresponding [MoVIO(O2)]2+ complexes [51]. The generation of such species has been also
established in methanol by electronic absorption spectroscopy. In a typical reaction, 20 mL of 2
× 10–5 M solution of [MoVIO2{Hdfmp(smdt)2}(H2O)] (2) was treated with a one drop portion of
30% aqueous H2O2 (0.825 g, 7.3 mmol) dissolved in 5 mL of methanol, and the resultant
spectroscopic changes are presented in Fig. 12 (a). Thus the 325, 375 and 399 nm bands shift to
322, 380 and 394 nm, respectively, along with only a marginal change in the intensity, while the
208 and 280 nm bands (not shown in the figure) increase their intensity considerably and finally
disappear. The broad charge transfer band appearing at 435 nm slowly decreases in intensity
(Fig. 12 (b)) along with further broadening and finally disappears upon carrying out a similar
titration with a concentrate solution (ca. 10-4 M) of complex 2. These changes indicate the
interaction of [MoVIO2{Hdfmp(smdt)2}(H2O)] (2) with H2O2 and the plausible formation of
[MoVIO(O2){Hdfmp(smdt)2}(H2O)] in methanol; Eq. (4). Similar changes have also been noted
for the other complexes; Fig. 12(c) and 12(d).
[MoVIO2{Hdfmp(smdt)2}(H2O)] + H2O2 → [MoVIO(O2){Hdfmp(smdt)2}(H2O)] + H2O (4)
To
understand
the
combined
effect
of
NaHCO3
and
H2O2
on
[MoVIO2{Hdfmp(smdt)2}(H2O)] (2) during catalytic action, a mixture of NaHCO3 (0.025g, 0.30
mmol) and 30% H2O2 (0.146 g, 1.3 mmol) dissolved in 5 mL of methanol was treated with 20
mL of 2 × 10-5 M solution of [MoVIO2{Hdfmp(smdt)2}(H2O)] (2). The spectral changes recorded
are presented in Fig. 13. Surprisingly, very similar spectral changes could be obtained as
15

exhibited by H2O2 alone after adding only a few drops of a mixture of NaHCO3 and H2O2, thus
hinting at the quick formation of the oxidoperoxido species. It has been reported that NaHCO3
–
reacts with H2O2 instantly and produces HCO4 (peroxymonocarbonate) [32-35]. This species
reacts with the dioxidomolybdenum(VI) complex to give the oxidoperoxidomolybdenum(VI)
intermediate more rapidly as compared to H2O2 alone, via the formation of another plausible
intermediate, as suggested by Bhattacharya and others [32,33], shown in Scheme 5.
<<Figures 12 and 13>> <<Scheme 5>>
4. Conclusions
The dioxidomolybdenum(VI)
complexes [MoVIO2{Hdfmp(sbdt)2}(H2O)] (1),
[MoVIO2{Hdfmp(smdt)2}H2O] (2) and [MoVIO2{Hdfmp(tsc)2}H2O] (3) have been prepared from
the potential tribasic pentadentate ligands H3dfmp(sbdt)2 (I), H3dfmp(smdt)2 (II) and
H3dfmp(tsc)2 (III), respectively; in the resulting complexes the ligands behave as dibasic
tridentate, with the other set of the coordinating atoms remaining free. In the solid state, the
asymmetric unit of [MoVIO2{Hdfmp(smdt)2}(dmso)]·dmso (2a·dmso) contains two complexes
which form an antiparallel dimer through π-π interactions between the C=S bonds and phenol
rings and between C=N bonds. Similar π-π interactions are also present in the crystal packing of
[MoVIO2{Hdfmp(tsc)2}(MeOH)]·MeOH (3a·MeOH), but here the interactions occur between
C=N bonds and phenol rings in an antiparallel disposition, while in 1a·2MeOH these interactions
appear only between the C=N bonds.
The complexes [MoVIO2{Hdfmp(sbdt)2}H2O](1), [MoVIO2{Hdfmp(smdt)2}H2O] (2) and
[MoVIO2{Hdfmp(tsc)2}H2O] (3) are good catalyst precursors for the oxidation of styrene and
cyclohexene. In the presence of sodium bicarbonate, oxidation of styrene using 30 % H2O2 as the
oxidant gave phenylacetaldehyde as the major product (ca. 93%) and styrene oxide as a minor
(ca. 7%) product under optimized reaction conditions. Oxidation of cyclohexene gave 2-
cyclohexene-1-ol selectively. No conversion of the substrates in the absence of NaHCO3 was
obtained, thus demonstrating the importance of NaHCO3 for such catalysts. These complexes
upon treatment with NaHCO3and H2O2 in methanol instantly generate oxidoperoxido species in
16

solution, which is considered to be an intermediate species responsible for the transfer of oxygen
to the substrates during the catalytic oxidation.
Appendix A. Supplementary data
CCDC 991414 for 1a, CCDC 990602 for 2a and 990603 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.
Acknowledgments
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 the gas chromatograph. S.D.
acknowledges the University Grant Commission, New Delhi for a Junior Research fellowship.
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Schemes
N
S
N
S
OH
N
S
N
S
OH
NH
S
HN
S
NH
HN
S
S
Me
Me
H3dfmp-(smdt)2(II)
H3dfmp-(sbdt)2(I)
N
N
S
N
S
N
OH
N
N
OH
OH
N
N
N
HN
H2N
NH
NH2
HN
S
HS
SH
HS
NH2
NH2
H2N
H2N
enol-form
keto-form
H3dfmp-(tsc)2(III)
Scheme 1. Structure of the ligands designated by I, II and III used in this work. Keto-enol
tautomerism is shown in only one of the ligands.
20

N
N
N
O
N
O
N
N
O
N
NH
S
N
NH
O
O
Mo
O
Mo
N
NH
O
Mo
O
S
S
OH2
S
S
S
OH2
NH2
H2N
S
O
S
OH2
S
S
Me
Me
[MoO2{Hdfmp-(tsc)2}H2O](3)
[MoO2{Hdfmp-(smdt)2}H2O](2)
[MoO2{Hdfmp-(sbdt)2}H2O](1)
Scheme 2. Proposed structures of the complexes.
O
O
Catalyst
H2O2, NaHCO3
Scheme 3. NaHCO3 activated oxidation products of styrene using H2O2 as the oxidant.
OH
Catalyst
H2O2, NaHCO3
Scheme 4. NaHCO3 activated oxidation product of cyclohexene using H2O2 as the oxidant.
21

-
-
H2O2
H2O
HCO4
O
HCO3
N
O
O
Mo
C
O
S
OH
O
O
-
-
HCO4
HCO3
N
O
N
O
Mo
O
Mo
O
S
S
O
O
O
Scheme 5. Proposed catalytic mechanism for the oxidation of alkenes.
Figures
Fig. 1. ORTEP plot of the complex [MoVIO2{Hdfmp(sbdt)2}(MeOH)]·2MeOH (1a·2MeOH). All
the non-hydrogen atoms are presented by their 30% probability ellipsoids. Hydrogen atoms are
omitted for clarity.
22

Fig. 2. ORTEP plot of the complex [MoVIO2{Hdfmp(smdt)2}(dmso)]·dmso (2a·dmso). All the
non-hydrogen atoms are presented by their 30% probability ellipsoids. Hydrogen atoms and
solvent dmso are omitted for clarity.
Fig. 3. ORTEP plot of the complex [MoVIO2{Hdfmp(tsc)2}(MeOH)]·MeOH (3a·MeOH). All the
non-hydrogen atoms are presented by their 30% probability ellipsoids.
23

Fig. 4. Intermolecular π-π interactions in 1a·2MeOH. Dashed lines link the centres of the π
clouds involved in each interaction.
Fig. 5. Intermolecular π-π interactions in 2a·dmso. Dashed lines link the centres of the π clouds
involved in each interaction.
24

Fig. 6. Intermolecular π-π interactions in 3a·MeOH. Dashed lines link the centres of the π clouds
involved in each interaction.
[MoVIO2{Hdfmp(smdt)2}(H2O)] (2)
–CH3
*
–CH=N–
–NH
Ar–H
*
H3dfmp(smdt)2 (II)
Ph–OH
25

Fig. 7. 1H NMR spectra of H3dfmp(smdt)2 (II) and the complex [MoVIO2{Hdfmp(smdt)2}(H2O)]
(2)
[MoVIO2{Hdfmp(smdt)2}(H2O)] (2)
C1, S-CH3
C8, C7
C9, C3,C4,C2,C5,C6
C10
C11
C8/C9
H3dfmp(smdt)2 (II)
C10/C11
C3/C4, C2 C5/C6
C7
Fig 8. 13C NMR spectra of H3dfmp(smdt)2 (II) and the complex [MoVIO2{Hdfmp(smdt)2}(H2O)]
(2)
26

(a)
(b)
100
80
60
40
20
0
100
80
60
40
20
0
0.0005g
0.0010g
0.0015g
10 mmol
15 mmol
20 mmol
0
20
40
60
80
100 120
0
20
40
60
80
100 120
Time/min
Time/min
100
80
60
40
20
0
100
80
60
40
20
0
(d)
(c)
1 mmol
2 mmol
3 mmol
5ml
7ml
10ml
0
20
40
60
80
100 120
0
20
40
60
80
100 120
Time/min
Time/min
100
80
60
40
20
0
100
80
60
40
20
0
(e)
(f)
600C
500C
400C
[MoO {Hdfmp(tsc) }H O]
2
2
2
[MoO {Hdfmp(sbdt) }H O]
2
2
2
[MoO {Hdfmp(smdt) }H O]
2
2
2
20
40
60
80
100 120
0
20
40
60
80
100 120
Time/min
Time/min
Fig. 9. (a) Effect of catalyst amount on the oxidation of styrene. Reaction conditions: styrene
(0.520 g, 5 mmol), 30 % H2O2 (2.27 g, 20 mmol), NaHCO3 (0.168 g, 2 mmol), temperature (60
oC and acetonitrile (5 ml). (b) Effect of oxidant amount on the oxidation of styrene. Reaction
conditions: styrene (0.520 g, 5 mmol), catalyst (0.0005 g), NaHCO3 (0.168 g, 2 mmol),
o
temperature (60 C) and acetonitrile (5 ml). (c) Effect of NaHCO3 amount on the oxidation of
27

styrene. Reaction conditions: styrene (0.520 g, 5 mmol), catalyst (0.0005 g), 30 % H2O2 (2.27 g,
o
20 mmol), temperature (60 C) and acetonitrile (5 ml). (d) Effect of solvent amount on the
oxidation of styrene. Reaction conditions: styrene (0.520 g, 5 mmol), 30 % H2O2 (2.27 g, 20
o
mmol), NaHCO3 (0.168 g, 2 mmol), catalyst (0.0005 g) and temperature (60 C). (e) Effect of
temperature on the oxidation of styrene. Reaction conditions: styrene (0.520 g, 5 mmol), 30 %
H2O2 (2.27 g, 20 mmol), NaHCO3 (0.168 g, 2 mmol), catalyst (0.0005 g) and acetonitrile (5 ml).
(f) Plot showing conversion for the oxidation of styrene using different metal complexes.
100
75
100
75
% Conversion
Styrene Oxide
50
50
Phenyl Acetaldehyde
25
25
25
50
75
Time/min
100
125
Fig. 10. Profiles showing the conversion and selectivity of products as a function of time.
28

(a)
100
80
60
40
20
0
(b)
100
80
60
40
20
0
10 mmol
20 mmol
30 mmol
0.0005g
0.0010g
0.0015g
0
40
80
120
160
0
40
80
Time/min
120
160
Time/min
100
80
60
40
20
0
1 mmol
2 mmol
3 mmol
(c)
(d)
5 ml
7 ml
10 ml
100
80
60
40
20
0
0
40
80
120
160
0
40
80
120
160
Time/min
Time/min
100
80
60
40
20
0
100
80
60
40
20
0
600C
500C
400C
(e)
(f)
[MoO {Hdfmp(tsc)2}H O]
2
2
[MoO {Hdfmp(sbdt)2}H O]
2
2
[MoO {Hdfmp(smdt)2}H O]
2
2
0
40
80
120
160
0
40
80
120
160
Time/min
Time/min
Fig. 11. (a) Effect of catalyst amount on the oxidation of cyclohexene. Reaction conditions:
cyclohexene (0.410 g, 5 mmol), 30 % H2O2 (2.27 g, 20 mmol), NaHCO3 (0.168 g, 2 mmol),
29