Selective oxidation of nonrefractory and refractory sulfides by cyclopentadienyl molybdenum acetylide complexes as efficient catalysts

Article abstract of DOI:10.1007/s10562-012-0898-x

The synthesis and catalytic properties of molybdenum acetylide complexes CpMo(CO)₃(-C≡CR), R = Ph(1), C₆H₄-p- CF₃ (2) and C₆H₄-p-CH₃ (3) has been studied. The molybdenum acetylide complexes were synthesized from CpMo(CO) ₃Cl and aryl acetylenes via Stephens- Castro coupling reaction. These complexes were characterized by single crystal X-ray diffraction analysis, FTIR and ¹H NMR spectroscopy. These complexes on treatment with hydrogen peroxide, formed corresponding molybdenum oxo-peroxo species. These in situ formed oxo-peroxo species were found very active (up to 100 % conversion) and selective (up to 100 %) oxidation catalysts for various refractory and nonrefractory sulfides. Interestingly, even though the molybdenum acetylide complexes are homogeneous, they could be recycled very efficiently by extracting the catalytically active molybdenum oxoperoxo species in aqueous phase. Springer Science+Business Media, LLC 2012.

Full text of DOI:10.1007/s10562-012-0898-x

Catal Lett (2012) 142:1352–1360
DOI 10.1007/s10562-012-0898-x
Selective Oxidation of Nonrefractory and Refractory Sulfides
by Cyclopentadienyl Molybdenum Acetylide Complexes
as Efficient Catalysts
•
Macchindra G. Chandgude Ankush V. Biradar
•
•
Trupti V. Kotbagi Vedavati G. Puranik
•
•
Mohan K. Dongare Shubhangi B. Umbarkar
Received: 31 May 2011 / Accepted: 23 August 2012 / Published online: 19 September 2012
Ó Springer Science+Business Media, LLC 2012
Abstract The synthesis and catalytic properties of
molybdenum acetylide complexes CpMo(CO)3(–C:CR),
R = Ph(1), C6H4–p-CF3 (2) and C6H4–p-CH3 (3) has been
studied. The molybdenum acetylide complexes were syn-
thesized from CpMo(CO)3Cl and aryl acetylenes via Ste-
phens–Castro coupling reaction. These complexes were
characterized by single crystal X-ray diffraction analysis,
FTIR and 1H NMR spectroscopy. These complexes on
treatment with hydrogen peroxide, formed corresponding
molybdenum oxo-peroxo species. These in situ formed
oxo-peroxo species were found very active (up to 100 %
conversion) and selective (up to 100 %) oxidation catalysts
for various refractory and nonrefractory sulfides. Interest-
ingly, even though the molybdenum acetylide complexes
are homogeneous, they could be recycled very efficiently
by extracting the catalytically active molybdenum oxo-
peroxo species in aqueous phase.
1 Introduction
Sulfur (S) is an essential element in the life processes of all
living things including micro-organisms, higher plants,
animals, and humans. Sulfur plays a major role in the
formation of the proteins that are essential to sustain life in
all biological organisms [1]. However, it is one of the
major problems in fuel that causes air pollution leading to
acid rains. Moreover, it poisons the catalyst in catalytic
convertor [2]. In order to control the pollution, Environ-
mental Protection Agency (EPA) has enforced stringent
regulation for substantial reduction in the total sulfur
content of gasoline and diesel fuel [3, 4]. Hence it is of
great importance to develop effective ways to remove
sulphur impurities from the fuel [5]. Commercially, these
impurities are removed by hydrodesulfurization (HDS),
which is a highly efficient process till date for removing
nonrefractory sulfides, although not very effective for
refractory sulfides [6]. An alternative route for removing
refractory sulfides is oxidative desulfurization (ODS) pro-
cess [7, 8]. Researchers worldwide have shown the use of
heterogeneous Co–Mo or Co–Ni catalysts for HDS [9]. The
refractory sulfides include dibenzothiophene (DBT) and
Keywords Homogeneous catalysis Á
Molybdenum acetylide Á Refractory sulfide oxidation Á
Oxo-peroxo species
especially
4,6-dimethyldibenzothiophene
(DMDBT),
which are difficult to remove by hydrodesulfurisation
(HDS) process, owing to the steric hindrance of methyl
groups of DMDBT that do not allow close contact with the
catalyst surface [10]. In ODS process the organosulfur
compounds are oxidized to their corresponding sulfones,
and these products are removed by extraction, adsorption,
distillation or decomposition [11, 12]. Though sulfur
compounds in fuel are poison for catalytic converter, many
sulphoxides and sulphones are used as intermediates in
chemically and biologically active compounds, including
therapeutic agents such as anti-ulcer (proton pump
Electronic supplementary material The online version of this
article (doi: 10.1007/s10562-012-0898-x) contains supplementary
material, which is available to authorized users.
M. G. Chandgude Á A. V. Biradar Á T. V. Kotbagi Á
M. K. Dongare Á S. B. Umbarkar (&)
Catalysis Division, CSIR-National Chemical Laboratory,
Dr. Homi Bhabha Road, Pune 411008, India
e-mail: sb.umbarkar@ncl.res.in
V. G. Puranik
Centre for Material Characterization, CSIR-National Chemical
Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
123

Molybdenum Acetylide Complexes for Selective Oxidation of Sulphides
1353
inhibitors) [13], antibacterial, antifungal, anti-atheroscle-
rotic [14, 15], and cardiotonic agents [16] as well as psy-
chotropic [17] and vasodilators [18]. Sulfoxides are
important in organic synthesis as an activating group and
have been utilized extensively in carbon bond formation
reactions [19]. Conventionally this is achieved by using
stoichiometric amount of t-butyl hypochlorite, N-halosuc-
cinimides, m-chloroperbenzoic acid, sodium metaperio-
date, nitrogen tetroxide, cerium ammonium nitrate (CAN),
tetrabutylammonium peroxydisulfate, 2,6-dicarboxy py-
ridinium chlorochromate as oxidizing reagents [20] as well
as large number of soluble metal complexes of Ti, V, Re,
Mn, Cr, and W have been used [11, 21–23]. Apart from
this, enzymatic methods have also been explored to remove
these sulfides [24]. However the major drawbacks of these
methods are their low operational stability and high cost of
operation.
2 Experimental
2.1 General
All preparations and manipulations were performed using
standard Schlenk techniques under argon atmosphere. All
reagents of commercial grade (Aldrich, SD fine) were used as
received unless stated otherwise. 30 % aqueous hydrogen
peroxide was used for oxidation reactions. Tetrahydrofuran
was dried over sodium wires and refluxed after addition of
benzophenone till appearance of blue color (ketyl formation)
under argon and freshly distilled prior to use. Cyclopentadiene
was obtained by freshly cracking dicyclopentadiene (Aldrich)
by distillation prior to use. The complex, CpMo(CO)3Cl was
prepared according to the reported procedure [34].
2.2 Preparation of CpMo(CO)3(–C:CPh) (1)
Sheldon and Van Doorn et al. [25] proposed that in
oxidation reactions, the main function of the metal
catalyst is to form co-ordinated peroxide complex that
acts as a Lewis acid and removes electron density from
the peroxidic oxygen. To prove these assumptions sev-
eral molybdenum compounds with different ligands
have been explored for oxidation of sulfides such as
molybdyldiacetylacetonate [MoO2(acac)2] [26], molyb-
denum hexacarbonyl [Mo(CO)6], and molybdenum
oxoperoxide [MoO(O2)2] [27, 28]. More recently Gam-
elas et al. [29] have reported the selective oxidation of
dialkyl, aryl–alkyl, benzylic, and benzothiophenic sul-
fides to either sulfoxides or sulfones, with stoichiome-
tric amounts of aqueous H2O2 or TBHP, in the presence
of complexes like CpMo(CO)3Cl, CpMoO2Cl. However
the catalytic species (oxo-peroxo or dioxo) which
perform actual catalysis is still unknown in these
complexes.
A mixture of CpMo(CO)3Cl (2.0 g, 0.0071 mol), H–
C:CPh (1.05 g, 0.010 mol) and catalytic amount of freshly
prepared CuI (5 mg) was stirred at room temperature in
diethyl amine (50 mL) for 15 min. The reaction was mon-
itored by TLC using hexane/dichloromethane (80/20 v/v) as
a mobile phase. After completion of the reaction, solvent
was removed in vacuum. The product was separated by
column chromatography using silica gel as stationary phase
and hexane/dichloromethane (80/20 v/v) as eluent.
The yield of complex 1 (CpMo(CO)3(–C:CPh)) = 1.5 g,
68.1 % based on CpMo(CO)3Cl. Elemental analysis: found
(calculated) for CpMo(CO)3(–C:CPh): C 55.53 (55.49), H
3.05 (2.89). Further, the product was analyzed by FT-IR,
1HNMR and 13CNMR. FT-IR (KBr) m(CO), 1941, 1960,
2035 cm-1, m(C:C), 2103.1 cm-1 NMR in CDCl3: d (1H);
5.5 (s, 5H, Cp), 7.15–7.38 (m, 5H, Ph) 13C in CDCl3: d (13C);
92.93 (Cp), 109.51 (C:CPh); 108.21 (C:CPh); 127.0,
129.39, 130.8, 130.87 (C:CC6H5) and 222.4, 238.78 (CO).
Shiu et al. [30] have reported the formation of oxo-
peroxo complexes (C5Me5)W(O)(O2)(C:CR) after
treatment of pentamethylcyclopentadienyl tungsten car-
bonyl acetylide complexes (C5Me5)W(CO)3(C:CR),
R=Ph, CH2OMe, Prn and C(Me)=CH2, with an acidic
solution of hydrogen peroxide at room temperature.
Facile formation of oxo-peroxo species after reaction
with H2O2 makes this complex a very good catalyst for
oxidation reactions.
2.3 Preparation of CpMo(CO)3(–C:C–C6H4–p-CF3)
(2) and CpMo(CO)3(–C:C–C6H4–p-CH3) (3)
The same synthetic procedure as for complex 1 was fol-
lowed except addition of (H–C:C–C6H4–p-CF3)
(0.010 mol) for preparation of 2 and (H–C:C–C6H4–p-
CH3) (0.010 mol) for preparation of 3 instead of HC:CPh
for 1. Yield of CpMo(CO)3(–C:C–C6H4–p-CF3) =
2.15 g, 65.8 % based on CpMo(CO)3Cl. Elemental analy-
sis: found (calculated) for CpMo(CO)3(–C:CPh): C 50.03
(49.27), H 2.32 (2.17). The compound was confirmed using
In continuation to our efforts on development of
molybdenum acetylide complex CpMo(CO)3(C:CPh) as
a precatalyst for oxidation reactions using green oxidant
[31–33], herein we report the synthesis of molybdenum
acetylide complexes having electron withdrawing as well
as electron donating substituents on the phenyl ring of the
acetylide moiety and their effect on oxidation of various
sulfides including refractory sulfides.
FTIR spectroscopy. m(CO), 1938, 1966, 2037 cm-1; m(C:C)
,
2103.7 cm-1. NMR in CDCl3: d (1H); 5.58 (s, 5H, Cp),
7.35–7.49 (m, 4H, C6H4); d (13C); 93.3 (Cp), 113.6
(C:CPh), 108.18 (–C:CC6H4–p-CF3), 125.22, 128.12,
123

1354
M. G. Chandgude et al.
131.23, 132.4 (C:CC6H4); 124.2 (–C:C–C6H4–p-CF3),
222.4 and 238.78 (CO).
molecule is disordered. Anisotropic refinement was carried
out however the disordered atoms were refined
isotropically.
Yield of CpMo(CO)3(–C:C–C6H4–p-CH3) = 1.82 g,
71.2 % based on CpMo(CO)3Cl. Elemental analysis: found
(calculated) for CpMo(CO)3(–C:CPh): C 56.92 (56.67), H
3.46 (3.33). The compound was confirmed using FTIR
3 Typical Catalytic Reaction Procedure
spectroscopy. m(CO), 1936, 1963, 2036 cm-1; m(C:C)
,
2098.4 cm-1. NMR in CDCl3: d (1H); 2.23 (s, 3H, CH3), 5.56
(s, 5H, Cp), 7.0–7.23 (m, 4H, C6H4); d (13C); 21.2 (CH3);
93.3 (Cp); 108.0 (C:CC6H4); 109.32 (C:CC6H4); 129.1,
129.91, 131.23, 134.2 (C:CC6H4) and 222.0, 238.8 (CO).
In a 50 mL two necked round-bottom flask equipped with a
magnetic stirrer and oil bath with controlled heating, a
solution of desired sulfide, solvent, 30 % aqueous H2O2
and Mo-acetylide complex was stirred magnetically at
desired temperature. Reaction was monitored by with-
drawing the aliquots of samples at regular intervals of time
and analyzed by using gas chromatograph (Agilent 6890
Gas Chromatograph equipped with a HP-5 dimethyl
polysiloxane capillary column, 60 m length, 0.25 mm
internal diameter, 0.25 lm film thickness). Products were
also confirmed by GC–MS (GC Agilent 6890N with HP-5
MS 30 m capillary column, MS Agilent 5973 Network
2.4 Single Crystal X-Ray Diffraction Analysis
Single crystals of the complex 1 and 2 were grown by slow
evaporation of the solution of complex in acetonitrile. The
single crystal X-ray diffraction data for 1 and 2 were col-
lected on a SMART APEX CCD single crystal X-ray dif-
fractometer with x and U scan mode and different number
of scans and exposure times for both the crystals using k
1
MSD) and H and 13C NMR spectra on Bruker AC 200
˚
MoKa = 0.71073 A radiation, crystal to detector distance
(200 MHz) spectrometer. Chemical shifts were denoted
relative to the solvent residual peak. (1H: CDCl3-7.27 ppm,
TMS 0.0 ppm).
6.05 cm, 512 9 512 pixels/frame at room temperature. All
the data was corrected for Lorentzian, polarization and
absorption effects using SAINT and SADABS programs.
The crystal structures were solved by direct method using
SHELXS-97 and the refinement was performed by full
matrix least squares of F2 using SHELXL-97 [35].
3.1 Catalyst Recycle Study
In a typical catalyst recycle experiment, a two necked
50 mL round-bottom flask was charged with thioanisole
(1 mmol), 30 % hydrogen peroxide (2 mmol), 10 g solvent
and catalyst precursor 1, 2 or 3 (0.02 mmol). The reaction
mixture was stirred at desired temperature till the com-
pletion of the reaction. After completion of the reaction,
solvent was removed in vacuum. Organic products and
unreacted sulfides were extracted in ethyl acetate and fur-
ther organic phase was treated with sodium sulphate and
products were recovered by evaporation of solvent. Cata-
lytically active species (CpMo(O2)O(–C:CR) which are
soluble in aqueous phase were separated, concentrated and
used for further recycle studies.
2.4.1 Single Crystal Data
2.4.1.1 Compound 1 Pale yellow rectangular crystal of
approximate size 0.32 9 0.24 9 0.18 mm was used for data
collection. Total scans = 3, total frames = 1818, exposure/
frame = 10.0 s/frame, h range = 2.09–25.08, completeness
to h of 25.08 is 95.0 %, C16H10MoO3, M = 346.18. Crystals
belong to monoclinic, space group P21/c, a = 6.7308(4),
˚
b = 11.6071(6), c = 17.949(1) A, b = 93.046(1)8, V =
3
1400.3 (14) A , Z = 4, Dc = 1.642 g cc-1, l (Mo Ka) =
˚
0.939 mm-1, 9,068 reflections measured, 2324 unique
[I [ 2r(I)], R value 0.0342, wR2 = 0.1075.
CAUTION: Please note that ‘‘The molybdenum oxo-
peroxo species were found to be pyrophoric if completely
dried’’.
2.4.1.2 Compound 2 Brown plate of approximate size
0.44 9 0.36 9 0.12 mm was used for data collection.
Total scans = 3, total frames = 1271, exposure/frame =
5.0 s/frame, h range = 2.52–24.998, completeness to h of
24.998 is 99.6 %, C17H9F3MoO3, M = 414.18. Crystals
belong to Monoclinic, space group P21/c, a = 11.9333(7),
4 Result and Discussion
The complexes CpMo(CO)3(–C:CPh) (1), CpMo(CO)3(–
C:C–C6H4–p-CF3) (2) and CpMo(CO)3(–C:C–C6H4–p-
CH3) (3) were prepared in two steps as illustrated in
Scheme 1. First, CpMo(CO)3Cl was synthesized from
Mo(CO)6 by following the literature procedure [34]. Initially
the molybdenum hexacarbonyl [Mo(CO)6] was reacted with
the preformed cyclopentadienyl anion to produce the
˚
b = 12.8247(8), c = 21.023(1) A, b = 90.413(1)8, V =
3
3217.3(3) A , Z = 8, Dc = 1.710 g cc-1, l (Mo Ka) =
˚
0.859 mm-1, 15543 reflections measured, 5636 unique
[I [ 2r(I)], R value 0.0504, wR2 = 0.1398. There are two
molecules in the asymmetric unit with change of confor-
mation for the phenyl ring and –CF3 group in one of the
123

Molybdenum Acetylide Complexes for Selective Oxidation of Sulphides
1355
Scheme 1 General scheme for
preparation of molybdenum
acetylide complex
R
Cl
Mo(CO)6
R
Mo
CO
Mo
CO
OC
CuI,
Diethylamine
THF, RT
CO
CO
OC
where R= H (1), CF3 (2), CH3 (3)
intermediate, CpMo(CO)-3 Na?, which was acidified using
acetic acid and the hydride complex thus formed was further
treated with CCl4 (without isolation) to form molybdenum
chloride complex CpMo(CO)3Cl. In the second step, the
complexes CpMo(CO)3(–C:CR) (R=Ph 1, C6H4–p-CF3 2,
C6H4–p-CH3 3) were prepared by Stephens–Castro coupling
reaction with corresponding acetylenes in presence of CuI
[36]. In brief, the previously synthesized molybdenum chlo-
ride complex was treated with phenyl acetylenes in presence
of catalytic amount of freshly prepared copper iodide and
excess of diethylamine which acts as a solvent as well as base
to abstract proton from the acetylene at room temperature.
The yield of the complexes varied from 65 to 72 %. The
complexes were characterized by various spectroscopic
methods. The FTIR peaks showed carbonyl stretching at
1941, 1959, 2035 cm-1 for complex 1, 1938, 1966,
2037 cm-1 for complex 2 and at 1936, 1963, 2036 cm-1 for
complex 3 confirming the presence of metal carbonyls. The
FTIR peak at 2103.1, 2103.7 and 2098.4 cm-1 respectively
for complex 1, 2 and 3 confirmed the presence of C:C. The
1HNMR chemical shift at 5.5 ppm (s, 5H) for CH proton of
cyclopentadienyl group was also present in all the complexes.
In addition to this aromatic CH coupling peaks were observed
at 7.15–7.38 ppm for complex 1, at 5.58 (s, 5H, Cp),
7.35–7.49 (m, 4H, C6H4) for complex 2 and at d (1H); 2.23(s,
3H, CH3), 5.56 (s, 5H, Cp), 7.0–7.23 (m, 4H, C6H4) for
complex 3. The complexes were also characterized by 13C
NMR where characteristic peaks for Cp were observed in the
range of 92.9–93.3 ppm, acetylene carbons were observed in
the range of 108.2–113.6 ppm, phenylic carbons where
observed in the range of 124.2–134.2 ppm and carbonyl
carbons were observed in the range of 222–238.8 ppm.
The molecular structure of complexes 1 and 2 were
unambiguously determined by single crystal X-ray diffrac-
tion analysis. Crystals were grown by slow evaporation of
acetonitrile at 298 K. ORTEP diagram of complex 1 and 2 is
represented in Figs. 1 and 2 respectively. Crystals of 1
belong to monoclinic crystal system, space group P21/c.
Two inverted molecules of compound 1, come closer with
Mo(1)–C(9) 129.1(2), C(9)–C(10)–C(11) 176.9(5), C(3)–
Mo(1)–C(5) 96.4(2), C(9)–Mo(1)–C(5) 130.9(2), C(9)–
Mo(1)–C(4) 95.78(19).
Crystals of 2 crystallized in monoclinic crystal system
with two molecules in asymmetric unit and CF3 group of
one of the molecule is disordered and hence this group is
refined isotropically and rest of the molecule is refined
anisotropically. The cyclopentadienyl moiety of two sym-
metry related molecules face opposite to each other when
viewed along a-axis. The two symmetry related molecules
are packed alternatively in a zigzag manner when viewed
down b-axis (see Fig. S2). The overlap of Mo–C–C in two
symmetry related molecules of complex 2 shows that the
phenyl rings are oriented at 45o (Fig. 3).
˚
Bond lengths (A): Mo(1)–C(3) 2.007(6), Mo(1)–C(4)
2.344(7), O(2)–C(2) 1.129(7), C(4)–C(5) 1.376(11), C(4)–
C(8) 1.385(10), C(5)–C(6) 1.386(11); Bond angles [°]:
C(3)–Mo(1)–C(9) 72.2(2), C(3)–Mo(1)–C(5) 122.5(3),
C(9)–Mo(1)–C(5) 82.3(3), C(9)–Mo(1)–C(4) 104.6(3),
C(9)–C(10)–C(11) 177.7(6).
Attempts were also made to grow a single crystal of
complex 3 by varying the conditions. However the crystals
suitable for X-ray study could not be obtained.
To determine the structure of catalytically active spe-
cies, H2O2 was added to alcoholic solution of complexes 1,
2 and 3 respectively. Within few minutes after the initiation
of the reaction, effervescence was observed which could be
the liberation of carbonyl ligand as carbon monoxide gas
from catalysts [37]. When the effervescence ceased the
alcoholic solution was concentrated and the FTIR spectrum
of the yellow solid obtained was recorded immediately.
The formation of oxo-peroxo complex of molybdenum
acetytlide (1, 2 and 3) is represented in scheme 2. It was
observed that when the completely dried sample was
exposed to the atmosphere, it caught fire and due to the
pyrophoric nature of the dried sample, the extensive
characterization of these intermediates was not possible.
The FTIR spectrum of 1 after addition of H2O2 con-
firmed the formation of oxo-peroxo species. The IR band at
934 cm-1 confirmed the presence of Mo=O terminal bond.
The band at 854 cm-1 corresponds to the O–O stretching
vibration of peroxo species. The weak bands at 672 and
574 cm-1 can be assigned to the Mo–O2 (peroxo) asym-
metric and symmetric stretching vibrations, respectively.
The IR positions for molybdenum oxo and peroxo moieties
are in good agreement with the literature reports for various
˚
C–HÁÁÁC (distance being 2.849 A) bonding and intramolec-
ular C–HÁÁÁH bonding in the packing of the molecules when
viewed down a-axis (see Fig. S1). Two molecules packed in
a zigzag manner via a-axis.
˚
Bond lengths (A): Mo(1)–C(3) 1.980(6), Mo(1)–C(4)
2.292(5), O(2)–C(2) 1.127(6), C(4)–C(5) 1.387(7), C(4)–
C(8) 1.393(7), C(5)–C(6) 1.401(7); Bond angles [8]: C(3)–
123

1356
M. G. Chandgude et al.
H2O2, RT
-3CO
Mo
O
R
R
Mo
CO
O
O
OC
CO
R= H (1), CF3 (2), CH3 (3)
Scheme 2 In situ generation of oxo-peroxo Mo(VI) acetytlide
complex
molybdenum oxo-peroxo complexes [38, 39]. In all the
three cases, the acetylide moiety remained intact even after
addition of H2O2. However, a marginal shift in IR fre-
quency of the acetylide band was obsvered. The IR fre-
quency of acetylide group in 1 remained unchanged.
However, the IR frequency of acetylide band in 2 and 3
Fig. 1 ORTEP diagram of the complex 1 (Ellipsoids are drawn at
40 % probability)
shifted to 2112 and 2100 cm-1 from 2103 and 2096 cm-1
,
respectively. The bands due to C–H stretching vibrations of
phenyl ring are observed in the range 2854–2955 cm-1 and
the C=C stretching vibrations of the ring are observed at
1464 and 1377 cm-1. The bands due to carbonyl stretching
vibrations in the range of 1935–2035 cm-1 disappeared
after addition of H2O2. This clearly indicates elimination of
all the CO ligands after addition of hydrogen peroxide
forming higher oxidation state Mo(VI) complex with
retention of acetylide moiety attached to Mo centre.
Whereas the FTIR band at 930 cm-1 confirmes the pres-
ence of Mo=O terminal bond in the complex 2 and 3 both.
The band at 843 and 849 cm-1 corresponds to the O–O
stretching vibration of peroxo species and weak bands at
652, 578 and 655, 568 cm-1 respectively can be assigned
to the Mo–O2 (peroxo) asymmetric and symmetric
stretching vibrations, for 2 and 3 respectively. Thus the
structure of the active species formed after addition of
H2O2 to the acetylide complexes 1–3 was confirmed by
FTIR which proved the presence of acetylide moiety, Cp
ligand as well as formation of oxo-peroxo species.
4.1 Catalytic Activity
Fig. 2 ORTEP diagram of the compound 2 (Ellipsoids are drawn at
40 % probability)
Initially, the catalytic activity of the three complexes 1–3
was compared (Scheme 3) for sulfide oxidation particularly
choosing the easily available thioanisole as a model sub-
strate and hydrogen peroxide as an oxidant. When complex
1 was used as catalyst, 56 % thioanisole conversion was
obtained with 92 % selectivity for sulfoxide within 2 h of
reaction time at room temperature (Fig. 4a). Furthermore,
thioanisole was completely (*100 %) consumed in 10 h
giving 72 % selectivity for sulfoxide and 28 % for sulfone
(Fig. 4a). These results suggest this reaction to be two step;
initial oxidation to sulfoxide and in second step further
oxidation of sulfoxide to sulfone. Hydrogen peroxide being
an electrophilic oxidant, the initial oxidation of the highly
nucleophilic sulfide to sulfoxide is an easier process than
Fig. 3 Overlap of two molecules of complex 2
123

Molybdenum Acetylide Complexes for Selective Oxidation of Sulphides
1357
O
S
O
O
difference in the rate of reaction and selectivity pattern was
observed with varying amount of H2O2. When amount of
H2O2 was increased, the rate of the reaction was found to
increase with decrease in the selectivity for sulfoxide.
When one equivalent H2O2 was used the reaction was
complete in 6 h with 84 % selectivity for sulfoxide
(Table 2 entry 2). However when 3 equivalent H2O2 was
used the reaction was complete in 1 h with only 15 %
selectivity for sulfoxide (Table 2 entry 4). Thus the results
clearly prove the fact that higher concentration of H2O2
enhances the rate of the reaction leading to over oxidation
to sulfone as the major product.
S
S
30 % H2O2
+
Cat, 1-3 ,
Acetonitrile,
R.T.
Cat= CpMo(CO)3(C=CR), R=Ph(1), C6H4-p-CF3(2) and C6H4-p-CH3(3).
Scheme 3 Molybdenum acetylide complexes catalyzed oxidation of
thioanisole
the second oxidation of the resulting much less nucleo-
philic sulfoxide to sulfone which is the rate determining
step [40, 41].
When catalyst precursor 2 was used for oxidation of
thioanisole with H2O2 as oxidant, 64 % conversion was
obtained after 2 h with 71 % selectivity for sulfone and the
reaction was complete (*100 % conversion) in 7 h with
85 % selectivity for sulfone (Fig. 4b). However, when
complex 3 was used as catalyst precursor under identical
conditions (Fig. 4c) 100 % conversion of thioanisole was
obtained only in 1.5 h; with very high selectivity for sulfone
(90 %). When catalyst precursor 1 was used, the rate of
reaction was found to be the slowest compared to 2 and 3.
The reactivity order of the catalysts was found to be
3 [ 2 [ 1 but the selectivity order for the sulfoxide for-
mation was found to be 1 [ 2 [ 3 at the complete con-
version, which can be related to the substituent on the aryl
acetylide. The substituent on phenyl ring of acetylide
moiety can lead to the activation of phenyl ring which in
turn may affect the catalytic activity of the complexes 2
and 3. Presently there is no evidence to support the reason
for enhancement in the catalytic activity of complex 2 and
3 with electron withdrawing and donating groups respec-
tively compared to 1.
4.4 Effect of Type of Solvents on Conversion
and Selectivity
The effect of different solvents on thioanisole conversion
and product selectivity was studied and the results are
given in Table 3.
The use of aprotic solvents like acetonitrile or toluene
(Table 3, entries 1 and 4) lead to higher selectivity for
sulfone at 100 % conversion. On the other hand, the use of
protic solvents like methanol or tert-butanol (Table 3,
entries 3 and 5) gave sulfoxide as major product. More-
over, dichloromethane (Table 3, entry 2), led to the for-
mation of sulfoxide with high selectivity of 97.5 % at 60 %
conversion of sulfide. The choice of solvent was indeed
very important as it allowed the tuning of selectivity
towards the desired products.
4.5 Scope of Substrate
Wider applicability of the acetylide complex 1 was studied
for the oxidation of a range of sulfides containing various
functionalities (Table 4). For simple sulfides maximum
conversion was achieved in short period of time at room
temperature (Table 4, entries 1–4). However for bulky
sulfides the maximum conversion was obtained at slightly
higher temperature (60 °C or more) (Table 4 entries 5–10).
In most of the cases very high selectivity for sulfoxide was
obtained under mild reaction conditions. In case of thio-
anisole electron donating or withdrawing substituents on
phenyl ring did not have any adverse effect on conversion
as well as sulfoxide selectivity (Table 4, entries 5 and 6
respectively). Tetrahydrothiophene could be oxidized with
100 % conversion at 60 °C in only 2 h (entry 8).
4.2 Effect of Loading of Catalyst Precursor 1
Oxidation of thioanisole using H2O2 without catalyst gave
9 % conversion with 100 % selectivity for sulfoxide. When
the catalyst precursor loading was varied gradually from
0.1 to 1 mol % with respect to substrate, the conversion
increased gradually from 21 to 100 % in 2 h (Table 1). At
lower catalyst precursor loading the selectivity for sulfox-
ide was higher whereas at high catalyst precursor loading
the selectivity for sulfone was maximum (100 % at 0.01
and 0.004 mmol catalyst precursor loading).
Interestingly, refractory sulfides such as DBT and
DMDBT (Table 4 entries 9 and 10 respectively) which are
very difficult to remove by HDS, could also be oxidized
efficiently using complex 1 and hydrogen peroxide as
oxidant. At 100 °C, 100 % DBT conversion was achieved
with 80 % selectivity for sulfoxide whereas 75 % conver-
sion with 100 % selectivity for sulfone was achieved for
4.3 Effect of Hydrogen Peroxide Molar Concentration
The effect of amount of H2O2 used on the conversion and
the product selectivity was studied at various substrate/
H2O2 molar ratios in the range of 1:0.5–1:3 (Table 2) for
oxidation of thioanisole using complex 1. Significant
123

1358
M. G. Chandgude et al.
100
80
60
40
20
0
100
80
60
40
20
0
Sulfide conv.
Sulfoxide
Sulfone
Sulfide conv.
Sulfoxide
Sulfone
A
B
1
2
3
4
5
6
7
0
2
4
6
8
10
Time, h.
Time, h.
100
80
60
40
20
0
Sulfide conv.
Sulfoxide
Sulfone
C
1
2
3
4
5
6
7
Time, h.
Fig. 4 Thioanisole oxidation with H2O2 using catalyst precursors a 1, b 2, c 3 (reaction conditions: thioanisole: 1 mmol, H2O2 (30 %): 2 mmol,
catalyst: 0.002 mmol, acetonitrile: 10 g, temperature: RT)
Table 1 Effect of catalyst loading on thioanisole oxidation using
precursor 1
Table 2 Effect of varying H2O2 amount on oxidation of thioanisole
using 1
Entry
Catalyst loading,
mmol (mol%)
Conversion
(%)
Selectivity (%)
Entry
H2O2 used
(mmol)
Time
(h)
Conversion
(%)
Selectivity (%)
Sulfoxide
Sulfone
Sulfoxide
Sulfone
1
2
3
4
5
Blank
9
21
100
100
92
47
0
00
00
1
2
3
4
0.5
1
15
6
48
100
100
100
93
84
47
15
07
16
53
85
0.001 (0.1)
0.002 (0.2)
0.004 (0.4)
0.01 (1)
56
08
2
2
100
100
53
3
1
100
Reaction conditions Thioanisole: 1 mmol, Catalyst precursor:
0.004 mmol, Acetonitrile: 10 g, Temperature: RT
Reaction conditions Thioanisole: 1 mmol, H2O2: 2 mmol, Acetoni-
trile:10 g, Time: 2 h, Temperature: RT
Table 3 Effect of solvents on oxidation of thioanisole using 1
DMDBT. In case of 4,6-DMDBT, formation of sulfone is
desired as the solubility of this sulfone is very low in
petroleum feedstock and hence removal from petroleum
feedstock becomes easier.
Entry Solvent
Conversion (%) Selectivity (%)
Sulfoxide Sulfone
1
2
3
4
5
Acetonitrile
100
60
47
97.5
53
2.5
One important aspect of catalyst precursors 1–3 is that
even though the complexes are homogeneous in nature, the
catalytically active species could be recycled and reused.
The recyclability of the catalyst precursors 1–3 was tested
for 5 cycles after extracting the catalytically active species
in aqueous phase (Table 5) which proves the stability of
these catalysts. As the quantity of catalyst precursors used
Dichloromethane
tert-Butanol
Toluene
84
70.5
0
29.5
100
14
100
83
Methanol
86
Reaction conditions Thioanisole: 1 mmol, H2O2: 2 mmol, Catalyst
precursor: 0.004 mmol, Temperature: RT, Time: 2 h
123

Molybdenum Acetylide Complexes for Selective Oxidation of Sulphides
1359
Table 4 Oxidation of various sulfides with H2O2 using complex 1
Entry
Substrate
Time (h)
Conversion (%)
Selectivity (%)
Sulfoxide
Sulfone
1
2
Me2S
Et2S
Et2S
Ph2S
2
2
100
76
92
68
70
87
87
79
74
100
81
79
75
7
8
32
30
13
13
21
26
0
6
89
3
4
5
6
2
64
4
86
Methyl p-tolyl sulfidea
p-Chloro thioanisolea
Benzyl phenyl sulfidea
2
62
6
100
55
2
8
100
65
19
21
25
93
20
100
2
8
100
94
7
Tetrahydrothiophene
DBTb,c
4,6-DMDBTb,c
2
9
10
8
100
75
80
0
10
Reaction conditions Substrate: 1 mmol; H2O2: 2 mmol; Catalyst precursor: 0.002 mmol; Acetonitrile: 10 g; Temperature: RT, a60 °C, b100 °C;
DBTc: Dibenzothiophene, DMDBTc: 4, 6-dimethyldibenzothiophene
Table 5 Catalysts recycle studies for catalyst precursors 1–3
Cycle
Catalyst 1a
Catalyst 2b
Catalyst 3c
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
SO
SO2
SO
SO2
SO
SO2
0
1
2
3
4
5
100
100
99
0
0
0
0
0
0
100
100
100
100
100
100
100
99
99
99
98
98
0
0
0
0
0
0
100
100
100
100
100
100
100
99
99
98
98
98
0
0
0
0
0
0
100
100
100
100
100
100
99
99
98
Reaction conditions Thioanisole: 1 mmol, H2O2: 2 mmol, Catalyst: 0.01 mmol, Acetonitrile: 10 g, Temperature: RT, Reaction time for each
a
cycle 2 h; 1 h; 0.5 h, SO: sulfoxide, SO2: sulfone
b
c
in all previous experiments was very less (0.002 mmol),
handling of such small quantities of catalyst is difficult
during extraction and concentration, hence slightly higher
catalyst precursor concentration (0.01 mmol) was used for
recycle studies in case of all three acetylide complexes.
Due to the higher catalyst concentration used 100 %
selectivity for sulfone was obtained with all the three cat-
alysts with difference in the time for completion of the
reaction. In case of catalyst precursor 1, time required for
complete conversion was 2 h, in case of 2 it was 1 h
whereas with 3 it was only 0.5 h showing the highest
catalytic activity of 3. The difference in product selectivity
was prominent only at lower catalyst concentration. In case
of all the catalysts precursors the conversion and the
product selectivity did not decrease significantly even after
five recycles. Hence recycling of these highly efficient
homogeneous catalysts is the major achievement of this
work.
5 Conclusion
Molybdenum acetylide complexes have been proved to be
very efficient catalyst precursors for selective oxidation of
various sulfides to sulfoxide or sulfone using H2O2 as an
oxidant under very mild reaction conditions. The selec-
tivity could be tuned to either sulfoxide or sulfone effec-
tively. Sulfides with various functional groups could be
oxidized with high efficiency. The refractory sulfides like
DBT and DMDBT were also oxidized to corresponding
123

1360
M. G. Chandgude et al.
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they were successfully recycled for five cycles without
appreciable loss in the conversion and selectivity.
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6 Supporting Information Available
Crystallographic data for structural analysis has been
deposited with the Cambridge Crystallographic Data Cen-
tre, CCDC Nos. 719072 and 719073 for complex 1 and 2.
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.
¨
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Acknowledgments SBU acknowledges the financial support by
Department of Science and Technology (project code: SR/S1/IC-08/
2004). AVB and TVK acknowledges Council of Scientific and
Industrial Research (CSIR New Delhi Govt. of INDIA) for Senior
Research Fellowships.
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