Peroxoniobium(v)-catalyzed selective oxidation of sulfides with hydrogen peroxide in water: A sustainable approach

Article abstract of DOI:10.1039/c4cy00864b

An efficient and eco-compatible route for the selective oxidation of a variety of thioethers to the corresponding sulfoxide or sulfone with 30% aqueous H₂O₂ in water, using newly synthesized peroxoniobium (pNb) complexes as catalysts, is described. The catalysts with formulas Na₂[Nb(O₂)₃(arg)]·2H₂O (arg = arginate) (NbA) and Na₂[Nb(O₂)₃(nic)(H₂O)]·H₂O (nic = nicotinate) (NbN) have been synthesized from the reaction of sodium tetraperoxoniobate with 30% H₂O₂ and the respective organic ligand in an aqueous medium, and these have been comprehensively characterized by elemental analysis, spectral studies (FTIR, Raman, ¹H NMR, ¹³C NMR and UV-vis), EDX analysis and TGA-DTG analysis. The density functional theory (DFT) method has been used to investigate the structure of the synthesized pNb complexes. The catalysts are physiologically safe and can be reused for at least six reaction cycles without losing their activity or selectivity. The oxidation is chemoselective for sulfides or sulfoxides leaving the C=C or alcoholic moiety unaffected. The developed methodologies, apart from being high yielding and straightforward, are completely free from halogen, organic co-solvent, or co-catalysts.

Full text of DOI:10.1039/c4cy00864b

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PeroxoniobiumIJV)-catalyzed selective oxidation
of sulfides with hydrogen peroxide in water:
a sustainable approach†
Cite this: DOI: 10.1039/c4cy00864b
Sandhya Rani Gogoi, Jeena Jyoti Boruah, Gargi Sengupta, Gangutri Saikia,
*
Kabirun Ahmed, Kusum K. Bania and Nashreen S. Islam
An efficient and eco-compatible route for the selective oxidation of a variety of thioethers to the corre-
sponding sulfoxide or sulfone with 30% aqueous H2O2 in water, using newly synthesized peroxoniobium
(pNb) complexes as catalysts, is described. The catalysts with formulas Na2[Nb(O2)3(arg)]·2H2O (arg =
arginate) (NbA) and Na2[Nb(O2)3(nic)(H2O)]·H2O (nic = nicotinate) (NbN) have been synthesized from the
reaction of sodium tetraperoxoniobate with 30% H2O2 and the respective organic ligand in an aqueous
medium, and these have been comprehensively characterized by elemental analysis, spectral studies (FTIR,
1
Raman, H NMR, 13C NMR and UV-vis), EDX analysis and TGA-DTG analysis. The density functional theory
Received 4th July 2014,
Accepted 19th September 2014
(DFT) method has been used to investigate the structure of the synthesized pNb complexes. The catalysts
are physiologically safe and can be reused for at least six reaction cycles without losing their activity or
selectivity. The oxidation is chemoselective for sulfides or sulfoxides leaving the CC or alcoholic moiety
unaffected. The developed methodologies, apart from being high yielding and straightforward, are
completely free from halogen, organic co-solvent, or co-catalysts.
DOI: 10.1039/c4cy00864b
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otherwise efficient oxidation catalysts. Thus, notwithstanding
the enormous progress in the development of catalytic proto-
1 Introduction
Chemoselective oxidation of organic sulfides represents one
of the most fundamentally important reactions in the domain
of organic chemistry, which is fascinating from both chemical
and biological perspectives.1 The practical utility of sulfoxides
and sulfones as high-value commodity chemicals and their
versatility as precursors for gaining access to a variety of
chemically and biologically active molecules, including drugs
and chiral auxiliaries, have been adequately highlighted in
the literature.1a,d,e,2
A number of highly promising new catalytic strategies for
selective sulfoxidation using various oxidants have been
developed in the past few years based on transition metal
catalysts, particularly those from groups 4–7 in their highest
oxidation states such as titanium,3 vanadium,4 chromium,5
iron,6 molybdenum,7 tungsten7c,8 and rhenium.9 Many of the
available methods, however, rely upon the use of toxic and
volatile organic solvents and harmful oxidants or require harsh
reaction conditions, which lower the practical importance of
cols to achieve selective oxidation of sulfides, the important
criterion of ecological sustainability is still a challenging
issue to address. Sustainability of a chemical transformation
is mainly governed by the solvent, reagents and catalysts used
in addition to the work-up procedure employed.6b In view of
the current ecological concerns, the demand for catalytic oxi-
dation processes that use benign solvent, green oxidants and
reagents which co-produce only innocuous waste seems to
have intensified.7c,8,10 Out of the multitude of available
organic oxidants, 30% aqueous H2O2 has been recognized as
the best waste-preventing terminal oxidant because of the
high oxygen content, cost, safety and easy handling.10c–e,11
Water holds great promise as an alternative to traditional
organic solvents as it is inexpensive, safe and non-volatile,
with unique redox stability and high heat capacity.12 Although,
traditionally, water is referred to as “the universal solvent”, in
organic synthesis, water has been treated as a contaminant
mainly due to the concern regarding solubility.12a However,
since the ‘on water’ approach pioneered by Sharpless,12b which
demonstrated that solubility is not a requisite to reactivity
and that many organic transformations can be performed
efficiently in aqueous solvent, there has been phenomenal
progress in the field of water-based organic synthesis.12b,13
This also necessitated the development of water-tolerant cat-
alysts to support such transformations. Although very limited,
Department of Chemical Sciences, Tezpur University, Napaam, Tezpur - 784 028,
Assam, India. E-mail: nsi@tezu.ernet.in, nashreen.islam@rediffmail.com;
Tel: +91 3712 267007, +91 9435380222(Off.)
† Electronic supplementary information (ESI) available: FTIR, 13C NMR and
1H NMR spectra of complexes, hydrogen peroxide efficiency, characterization of
sulfoxides and sulfones, and bar diagram for recyclability of the catalyst. See
DOI: 10.1039/c4cy00864b
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there are reports on metal-catalyzed sulfoxidation reaction in
an aqueous medium.6a,7a,10h,14 Very recently, Chakravarthy
and co-workers reported a surfactant-based Mo catalyst for
selective sulfoxidation of a variety of sulfides in an aqueous
medium with 40% H2O2.7a
On the other hand, although a number of publications,
including those from our laboratory, have dealt with the
synthesis and characterization of peroxometal complexes
of V,16a–d,21 Mo22 and W16e,21f,23 with amino acid or nicotinic
acid as a co-ligand,24 as far as we are aware, arginine and
nicotinic acid have not been used so far to obtain a hetero-
leptic pNb complex in the solid state. Apart from being bio-
logically relevant, these ligands are reasonably inexpensive,
water soluble, and commercially available.
We report herein the catalytic activity of a pair of newly
synthesised pNb complexes for the controlled oxidation of
sulfides with H2O2 in an aqueous medium, in terms of selec-
tivity, yield, reusability and sustainability, to obtain sulfoxide
or sulfone.
In the recent past, we have successfully developed a set of
new recoverable heterogeneous catalysts based on polymer-
immobilized peroxotungsten (pW) and peroxomolybdenum
(pMo) complexes, which displayed excellent stability, selectiv-
ity and efficiency with respect to yield, TON and TOF for the
oxidation of a diverse range of thioethers and dibenzothio-
phene by H2O2 under very mild conditions.7b,8a Few of these
supported catalysts also effectively catalyzed the oxidative
bromination of a variety of activated aromatics at ambient
temperature and near-neutral pH.15 Furthermore, a number
of dimeric and macromolecular water-soluble peroxo com-
pounds of vanadium and tungsten that we prepared could
serve as stoichiometric oxidants of organic bromides and
sulfides in an aqueous organic medium.16
2 Results and discussion
2.1 Synthesis and characterization
A reasonably straightforward synthetic route has been
established to obtain a pair of stable and water-soluble
pNb complexes with arginine or niacin as a co-ligand.
The procedure is based on the reaction of sodium tetra-
peroxoniobate with 30% H2O2 and the respective co-ligand
in an aqueous medium at near-neutral pH. Synthesis of
soluble niobium(V) compounds is considered to be rather
challenging because of very low solubility of the common
Nb-containing starting materials.18d In the present study,
water-soluble Na3[Nb(O2)4]·13H2O has been used as a precur-
sor complex, which was prepared by the reported method.18i
The maintenance of pH of ca. 6 was found to be essential
for the formation of the triperoxoniobium species and its
stabilisation by the chosen organic ligands, occurring in their
anionic form, leading to the isolation of the NbA and NbN
complexes. The procedure included other requirements such
as maintenance of temperature at ≤4 °C and limiting water
to that contributed by 30% H2O2. The compounds were
observed to be stable in the solid state for several weeks
when stored dry in a closed container at <30 °C.
The elemental analysis data (Table 1) for each of the
title compounds indicated the presence of three peroxide
groups and one ancillary ligand, either arginine or niacin per
NbIJV) centre with the formula Na2[Nb(O2)3(arg)]·2H2O or
Na2[Nb(O2)3(nic)(H2O)]·H2O. Energy-dispersive X-ray (EDX)
spectroscopic analysis clearly showed the presence of Nb, Na,
C, N and O in the complexes (Fig. 1). The obtained data on
the composition of the compounds from EDX analysis were
in good agreement with elemental analysis values (Table 1).
Despite many attempts, it was not possible to obtain crystals
of the compounds large enough for an X-ray crystal structure.
The compounds were diamagnetic in nature as was evident
from the magnetic susceptibility measurements, in confor-
mity with the presence of Nb in its +5 oxidation state.
In our continuing endeavours devoted to developing
newer catalysts and methodologies for oxidation under envi-
ronmentally acceptable reaction conditions, in the present
work, we have aspired to design new peroxometal-based cata-
lytic systems for selective sulfoxidation using H2O2 as an oxi-
dant in an aqueous medium. We selected the peroxoniobium
(pNb) system as an ideal candidate for our study for a variety
of reasons. Most importantly, Nb has been reported to be
non-toxic to animals, with LD50 values in several thousands
of milligrams per kilogram body weight in contrast to its
lighter group 5 element vanadium, which is known to be
moderately toxic.17 Catalysis by pNb compounds is a field of
growing interest.18 The pNb species generated in situ in
the presence of H2O2 have been shown to catalyze the oxida-
tion of sulfides18c–g and alcohol18h and the epoxidation of
alkene.18a,b,i For example, Egami et al. used Nb(salan) com-
plexes to catalyze the asymmetric epoxidation of allylic alcohol
with a urea–hydrogen peroxide adduct with good selectivity.18a
However, the catalytic potential of discreet synthetic hetero-
ligand pNb complexes as oxidation catalysts has rarely been
investigated.
A suitable choice of the co-ligand is an important pre-
requisite in order to obtain stable and well-defined peroxo-
metallates. Moreover, the coordinating ligand has been
shown to have a dramatic effect on the reactivity of peroxo-
metallates (pM) which enabled the activity of pM complexes
as stoichiometric or catalytic agents to be fine-tuned with
ligands.19 In the present investigation, since our focus was to
develop physiologically harmless catalysts, we have chosen
nicotinic acid (also known as niacin) and the amino acid
arginine as potential co-ligands. These ligands possess a
carboxylate functional group for easy attachment to the NbIJV)
centre in addition to the N-donor site. Carboxylate anions
have been known to be excellent co-ligands for stabilising
pNb species.20 Consequently, a host of pNb complexes have
been reported in recent years possessing ligands with a
carboxylate moiety.20
2.1.1 IR and Raman spectral studies. The NbA and NbN
complexes displayed a distinctive spectral pattern in the IR
region [Fig. S1 (ESI†)]. The Raman spectra of the compounds
complemented the IR spectra, confirming the presence of
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Table 1 Analytical data for the synthesized peroxo–niobium complexes
% O22− content
(theoretical %)
% Found from elemental analysis/EDX analysis (theoretical %)
Complexes
C
H
N
Nb
Na
11.14
10.47
(10.36)
11.63
Na2ijNbIJO2)3IJarg)]·2H2O
16.29
3.76
12.46
21.01a
20.84
(20.92)
23.52a
23.57
21.71
16.15
(16.21)
18.41
12.76
(12.61)
3.55
(3.83)
1.99
(21.61)
24.54
Na2ijNbIJO2)3IJnic)IJH2O)]·H2O
18.37
3.61
11.66
(18.32)
(2.03)
(3.56)
(23.64)
(11.70)
(24.43)
a
Determined by AAS.
as has been expected in the 870–810 and 500–600 cm−1
regions, respectively.18d,25
On the basis of the available reported data pertaining to
metal compounds with coordinated amino acid and niacin as
ligands, empirical assignments could be derived for the IR
and Raman bands observed for the catalysts NbA and NbN.26
The IR spectra of arginine and arginato–metal complexes
have been reported previously.27 In the spectrum of free arginine,
νasIJCOO−) and νs(COO−) modes are observed at 1606 and
1425 cm−1, respectively, with Δν = 181 [Δν = νas(COO−) − νs(COO−)],
whereas in the case of the NbA complex, the corresponding
absorptions appeared at 1636 cm−1 and 1406 cm−1, respectively.
The shift of νas(COO−) to a higher frequency and that of νs(COO−)
to a lower frequency compared to the free ligand values, with an
increase in the Δν (230 cm−1), are typical of the unidentate
coordination of the carboxylate group.26a In the Raman spec-
trum of the compound, weak intensity bands representing
νas(COO−) and νs(COO−) vibrations have been located at 1630
and 1410 cm−1, respectively. The ν(C–H) occurred as an intense
peak at 2934 cm−1 in the Raman spectrum in contrast to its
presence as a weak band in the IR. Two bands typical of
ν(NH2) are observed at 3352 and 3288 cm−1 in the free ligand
Fig. 1 EDX spectra of (a) NbA and (b) NbN.
coordinated peroxo groups and the respective co-ligand in
each of them. The significant general features of IR and
Raman spectra are summed up in Table 2. The Raman spec-
tra for the complexes are presented in Fig. 2.
A triperoxo niobium species with a triangularly bonded
peroxo group has been reported to exhibit a diagnostic IR pat-
tern with three νIJO–O) bands in the 800–880 cm−1 region.18d,25
The IR and Raman spectra of each of the NbA and NbN com-
pounds showed clear identification of three sharp absorptions
representing the characteristic ν(O–O) modes of the peroxo
group, in addition to the νasym(Nb–O2) and νsym(Nb–O2) vibrations,
Table 2 Experimental and theoretical infrared and Raman spectral data (cm−1) for the NbA and NbN compoundsa
Assignment
NbA
NbN
νIJO–O)
IR
R
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
846(m), 822(sh), 813(s)
847(m), 826(sh), 811(s)
837, 865, 889
823(sh), 847(s), 857(sh)
806, 853, 879
547(s)
528
538(s)
528
592(m)
604
567(sh)
575
1636(s)
1615
1630(vw)
1609
1406(m)
1375
1410(vw)
1403
807, 872
817(sh), 847(s), 867(sh)
813, 828, 876
547(s)
523
542(s)
554
593(sh)
605
571(sh)
598
1625(s)
1649
1627(sh)
1627
1388(s)
1378
1393(sh)
1335
νsIJNb–O2)
νasIJNb–O2)
νasIJCOO−)
νsIJCOO−)
IR
R
IR
R
IR
R
IR
R
a
s, strong; m, medium; vw, very weak; sh, shoulder.
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to the peroxo-to-metal charge transfer bands, as has been
observed in the case of other reported triperoxoniobiumIJV)
complexes.20f,i,29
1
2.1.3 H and 13C NMR studies. In Tables 3 and 4, relevant
1H and 13C NMR resonances for the catalysts are listed along
with those of the free ligand for comparison. Crucial structural
information regarding the complexes, including the coordination
mode of the ligand to the Nb atom, and their stability in
solution has been gathered from the complete analysis of
the NMR spectra. The major NMR resonances were interpreted
according to available literature data. The 1H NMR spectra
in D2O have shown the expected integration and peak
multiplicities [Fig. S2 (ESI†)].
The spectral pattern of the pNb complex with arginine as
a co-ligand resembled closely the spectrum of the free ligand
by exhibiting 4 major peaks at 3.52, 3.11, 1.74 and 1.59 ppm.30a
The spectrum of the complex, however, showed a distinct
upfield shift of all the resonances relative to the free ligand,
indicating coordination of the ligand to the metal centre.30
Similar observations were made previously in the case of metal
compounds containing complexed amino acid.30,31
Fig. 2 Raman spectra of (a) NbN and (b) NbA.
spectrum.27a A new peak appeared at 3193 cm−1 in the
spectrum of the NbA complex indicative of a coordinated
amino group.27a However, the other absorptions representing
ν(N–H) could not be assigned with certainty owing to their
overlapping with ν(OH) modes of lattice water, appearing as a
broad band in the 3500–3300 cm−1 region.
The 13C chemical shift induced by coordination has been
widely utilized as a convenient means of understanding
the bonding pattern of ancillary ligands in peroxo–metal
complexes.32 The 13C NMR spectrum of the free arginine in
D2O displays a typical resonance for a carboxylate carbon
atom at 183.17 ppm in addition to the five other well-resolved
peaks corresponding to carbon atoms C(2) to C(6).30a The
spectrum of NbA, on the other hand, displayed the carboxyl-
ate resonance at a lower field of 215.45 ppm, thus testifying
to the existence of a complexed carboxylate group.30a,33 The
substantial downfield shift relative to free carboxylate, with
Δδ (δcomplex − δfree carboxylate) ≈ 32 ppm, indicated strong
In the NbN catalyst, the presence of carboxylato-bonded
niacin has been clearly demonstrated in its IR and Raman
spectra (Table 2). A number of reports are available on IR
spectral characterization of metal complexes containing
coordinated niacin.26c,e,f Moreover, the IR and Raman spectra
of NIA have been thoroughly investigated by Kumar and
co-workers and others.26d Solid sodium nicotinate exhibits
major carboxylate IR bands at 1620 and 1416 cm−1 for
νasIJCOO−) and νs(COO−), respectively.26f The positions of
νas(COO−) and νs(COO−) stretching in both IR and Raman
spectra of NbN and the corresponding Δν value (237 cm−1
)
Table 3 1H chemical shifts for ligands and heteroligand peroxo–niobate
complexes
provide clear evidence of unidentate coordination of a non-
protonated carboxylato group to the metal atom. Further-
more, a significantly less intense aromatic ν(CC) band at
1568 cm−1 depicts monodentate coordination of a carboxylate
group to Nb.24 Since the ν(CC) mode, ν(CN) absorption and
pyridine ring vibrations at 1476, 1016 and 940 cm−1, respec-
tively, do not undergo any positive shift relative to the respec-
tive free ligand values, coordination via nitrogen in the
pyridine ring can be ruled out safely.26c The presence of water
molecules in the complex was apparent from the observed
broad and intense ν(OH) bands in the 3500 to 3400 cm−1
region. Further confirmation for the presence of coordinated
water in the compound was obtained from the consistent
appearance of a moderate intensity signal at 769 cm−1 attrib-
utable to the rocking mode of water.28 The intense band
appearing at 3077 cm−1 in the Raman spectrum has been
ascribed to ν(CH) vibration.
Chemical shift (ppm)a
Compound
H-2
H-3
H-4
H-5
H-6
Arginine
NbA
Niacin
NbN
3.76
3.52
8.97
8.82
1.89
1.74
—
1.66
1.59
8.68
8.13
3.23
3.11
7.93
7.40
—
—
8.76
8.49
—
a
See Fig. 4 for the atomic numbering.
Table 4 13C NMR chemical shift for ligands and triperoxoniobium
complexes, NbA and NbN
Chemical shift (ppm)a
Carboxylate
Compound carbon
C2
C3
C4
C5
C6
Arginine
NbA
Niacin
NbN
183.17
215.45
168.25
210.78
55.59
54.59
31.64
30.28
24.51
23.99
41.02 156.81
40.64 156.83
2.1.2 Electronic spectral studies. Close analogy was
observed between the UV-vis spectral patterns of compounds
NbA and NbN recorded in aqueous solution. The spectra
exhibited two bands at ca. 280 and ca. 325 nm attributable
143.04 135.49 142.49 126.94 145.89
149.06 137.69 142.46 128.76 150.51
a
See Fig. 4 for the atomic numbering.
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metal–ligand interaction as has been reported earlier in the
case of some other peroxo–metal carboxylate complexes.7b,33
The guanidyl C resonance along with the resonances of alkyl
groups (C-5 and C-4) showed very little shift, whereas the res-
onances of α-CH and β-CH2 groups shifted to a higher field
by ca. 1 ppm [Fig. S3(a) (ESI†)]. These results indicated the
coordination of carboxylate and amino groups of the ligand
to the niobium centre and are consistent with observations
made in the case of other reported arginine-containing metal
compounds.30a The 1H and 13C NMR spectra of nicotinic
acid in D2O have been studied and reported by Khan
and co-workers under varying pH conditions.34
A
1H NMR
pattern typical of a nicotinate anion was observed with four
well-resolved resonances in the spectrum of the NbN com-
pound.34 As expected for the nicotinate anion, the spectrum
showed a distinct upfield shift of each of the four resonances
corresponding to the aromatic protons H(2) to H(6) relative
to the zwitterionic free ligand values [Fig. S2(a) (ESI†)].30b
The 13C spectrum provided further persuasive evidence in
support of the presence of a nicotinate anion in the NbN
compound by displaying resonances for ring carbon atoms
in the region expected for the nicotinate anion.34 The peak
attributable to the metal-bound carboxylate carbon, as in the
case of NbA, appeared at a very low field of 210.78 ppm com-
pared to the carboxylate resonance of the free nicotinic acid
[Fig. S3(b) (ESI†)].7b,33 Occurrence of the carboxylate carbon
as a singlet in the spectrum of each of the catalysts, NbA and
NbN, indicated a single-carbon environment for complexed
carboxylate.33 Thus, the results of the NMR analysis showed
evidence of the presence of only one complex species in solu-
tion in each case. It is therefore apparent that catalysts did not
hydrolyze and retained their solid-state structure in solution.
2.1.4 TGA-DTG analysis. The TGA-DTG plots for the NbA
and NbN catalysts illustrated in Fig. 3 showed that the com-
pounds gradually undergo multistage decomposition on heating
to a temperature of 700 °C. Interestingly, the compounds do
not explode on heating, although a majority of the known
pNb compounds have been reported to decompose explo-
sively on heating.20b,f,35
Fig. 3 TGA-DTG plot of (a) NbA and (b) NbN.
value of 54.6% for the loss of the components, viz. water
molecules, coordinated peroxide, and the co-ligand, assum-
ing that four of the ligand oxygen atoms were being retained
to form the oxoniobate species as the final degradation
product.
It is notable that the thermogram for NbN [Fig. 3(b)]
displayed a two-step dehydration process in the temperature
range of 45–105 °C, providing conclusive evidence of the
presence of outer sphere coordinated water molecules
in the compound, consistent with the formula assigned. The
decomposition step occurring at relatively higher temperature
between 90 and 105 °C, after the initial liberation of the
outer sphere water molecule in the temperature range of
45–70 °C, is attributable to the loss of the coordinated water
molecule. The observed total weight loss of 9.8% correspond-
ing to the two steps combined is close to the calculated value
of 9.2% for the release of two molecules of water from the
complex. After the dehydration, the thermal behaviour of the
NbA and NbN catalysts is quite similar. The NbN compound
undergoes continuous degradation with loss of peroxide in
the temperature range of 158–189 °C analogous to the NbA
compound, followed by the loss of the niacin ligand up to
the final temperature of 700 °C. The residue remaining after
the complete degradation of each of the pNb compounds was
found to be oxoniobate species as indicated by the IR spectral
analysis which showed the typical νIJNbO) stretching and
was devoid of absorptions attributable to the peroxo group
and the respective co-ligands of the starting complex.
The first stage of decomposition for NbA [Fig. 3(a)] occurs
in the temperature range of 78–102 °C with the liberation of
the lattice water from the complex. The corresponding weight
loss of 8.9% is in good agreement with the value of 8.1%
calculated for two molecules of water of crystallization.
The next decomposition stage is in the temperature range of
157–189 °C attributable to loss of peroxo groups from the
complexes with a weight loss of 18.8%. As the observed
weight loss is slightly less than the expected value, it is likely
that part of the oxygen is retained with niobium to form
oxoniobium species, as has been observed previously in the
case of neat as well as heteroligand pNb complexes.20b,f,35
A
further increase in temperature leads to continuous degrada-
tion of the arginine ligand up to a final decomposition tem-
perature of 700 °C. The total weight loss which occurred
during the overall decomposition process was evaluated to be
54.3%, which agreed well with the theoretically calculated
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The above results are consistent with the proposed struc-
tures of the compounds NbA and NbN shown schematically
in Fig. 4. The structure of NbA shows the Nb atom displaying
a coordination number of 8, surrounded by the three peroxo
groups and the arginate ligand bonded via its unidentate
carboxylate group and the amino group. The structure of
NbN includes a nicotinate anion occurring as a unidentate
ligand bonded to the Nb centre through the carboxylate
group, the side-on bound peroxo groups and a coordinated
water molecule completing eightfold coordination around
the Nb.
2.1.5 Theoretical investigation. In order to verify the
feasibility of the proposed structures, density functional
theory (DFT)36 calculations were performed on the two
niobium complexes at the B3LYP/LANL2DZ level of theory.
The initial structures of the two complexes were modeled
based on the experimental data related to the compounds
(FTIR, Raman, 13C and 1H NMR spectroscopy, TGA and
elemental analysis). The optimized geometries of the two
niobium complexes are presented in Fig. 5. In both complexes
the coordination polyhedron around the niobium atom is a
dodecahedron as observed in a majority of the reported pNb
complexes.18d,20f
Fig. 5 Optimized geometry of (a) NbA and (b) NbN. The numerical
numbers represent the labeling of the atoms as in Table 5.
In the case of NbA, the central metal atom (Nb) is coordi-
nated to six oxygen atoms belonging to three η2-peroxo groups,
in addition to one oxygen atom from the carboxylato group
and a N (amino) atom from the deprotonated argininate
ligand. One of the bidentate peroxo groups is in trans position
to the co-ligand, whereas the other two are in cis configuration.
The coordination sphere of the NbN complex comprises a
niobium atom surrounded by seven oxygen atoms contrib-
uted by three η2-peroxo groups and one from the unidentate
carboxylato group of the nicotinate ligand. The eighth coordi-
nation site is satisfied by a water molecule which is weakly
bonded to the central atom, as has been observed previously
by Djordjevic et al. in the case of nicotinate containing a
peroxomolybdenum complex.24 Thus, the structure of NbN
can be described as a distorted octahedron with one of the
axial distances slightly longer. For both the complexes, the
coordination distances are within the range characteristic
of the heteroleptic peroxo complexes of niobiumIJV).20f The
geometrical parameters (bond angle and bond length)
obtained from DFT calculations are depicted in Table 5. The
Nb–O(peroxo) bond lengths are within the range of 1.977 to
2.086 Å, whereas the (O–O) distances range from 1.526 to
1.533 Å. The Nb–O (carboxylate) bond distances in NbA and
NbN are 2.162 Å and 2.138 Å, respectively. The Nb–N bond
Table 5 Selected bond lengths (Å) and bond angles (degree) for the two
complexes calculated at the B3LYP/LANL2DZ level of theory
Structural indexa
NbA
Structural indexa
NbN
Nb–O2
Nb–O3
Nb–O4
Nb–O5
Nb–O6
Nb–O7
Nb–O8
Nb–N10
O2–O3
2.072
2.008
1.999
2.066
2.162
2.086
2.011
2.370
1.526
1.533
1.530
1.537
1.307
43.9
Nb–O2
Nb–O3
Nb–O4
Nb–O5
Nb–O6
Nb–O7
Nb–O19
Nb–O20
O2–O3
O5–O6
O4–O7
C18–C11
2.047
1.999
1.977
2.033
1.985
2.040
2.138
2.831
1.527
1.530
1.532
1.521
O4–O8
O5–O7
C12–C15
C9–O6
∠O2–Nb–O3
∠O4–Nb–O8
∠O5–Nb–O7
∠O6–Nb–N10
∠O2–Nb–O3
∠O4–Nb–O7
∠O5–Nb–O6
∠O3–Nb–O19
44.3
44.8
44.7
82.9
44.9
44.2
72.1
a
Fig. 4 Proposed structure of (a) NbA and (b) NbN.
See Fig. 5 for the atomic numbering.
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distance in NbA is 2.370 Å. The geometrical parameters obtained
from our theoretical calculation correlated well with the reported
crystallographic parameters pertaining to other heteroleptic
triperoxo niobate complexes with coordination environment
comprising N,O- or O-donor co-ligands.18d–i,19a–d,20a–f The
small differences observed between the experimental and
calculated geometrical parameters are expected as the ground-
state geometries were obtained in the gas phase by full geometry
optimization.
We have further calculated the vibrational frequencies for
the optimized geometries of the compounds and compared
the data obtained with the experimentally determined frequen-
cies as illustrated in Table 2. The calculated IR and Raman
spectra for the two complexes are found to simulate well with
the experimental ones. The small deviations between the
calculated and experimental spectral data are anticipated as
the calculated spectral data are obtained for those of the gas-
phase-optimized geometries. These observed discrepancies
appear to be acceptable as the average error for frequencies
calculated with the B3LYP functional was reported to be of the
order 40–50 cm−1 for inorganic molecules.37 Moreover, such
deviations in the FTIR and Raman vibrational bands obtained
from DFT-based calculations on eight-coordinated peroxo
complexes of niobiumIJV) are not unprecedented.20f Thus, the
results obtained from our theoretical calculations truly sup-
port our experimental findings and completely validate the
predicted geometries for the synthesized complexes.
(1 equiv.) in the presence of the water-soluble catalyst NbA,
maintaining a catalyst : substrate molar ratio of 1 : 1000, was
conducted in water at ambient temperature under magnetic
stirring. The reaction proceeded rapidly within a reasonably
short period in the presence of each of the catalysts. The
reaction under these conditions was, however, observed to be
mildly exothermic, leading to the formation of a mixture of
1a and 1b with a molar ratio of 85 : 15, as presented in Table 6
(entry 1). Subsequently, the reaction conditions for selective
sulfoxidation including substrate : oxidant stoichiometry, cata-
lyst concentration, solvent type and reaction temperature
were optimized using MPS as the model substrate and the
NbA complex as the catalyst. We have attempted to control
the degree of oxidation by lowering the reaction temperature
to 0 °C by conducting the reaction in an ice bath.
As has been anticipated, the reaction indeed proceeded
smoothly to yield sulfoxide with 100% selectivity and nearly
70% conversion under these conditions. Complete oxidation
of MPS to pure sulfoxide could be attained with excellent
TOF (without affecting the selectivity) by increasing the
oxidant : substrate molar ratio to 2 : 1 without altering the
other reaction conditions.
Next, we have examined the effect of the catalyst amount
on the rate and selectivity of the reaction under otherwise
identical reaction conditions. As illustrated in Table 6,
although the rate was faster at higher catalyst concentration
and a reasonably good TOF could be attained even at a
catalyst : substrate ratio of 1 : 3000, the optimal catalyst : substrate
molar ratio was found to be 1 : 2500 for achieving the highest
TOF along with complete selectivity. The pNb species has an
important contribution in facilitating the reactions, which
was confirmed by conducting a control experiment without
the catalyst. The reaction was extremely slow and non-
selective in the absence of the catalyst, affording a mixture of
sulfoxide and sulfone in <9% yield under the optimized
2.2 Catalytic activity of the NbA and NbN complexes
2.2.1 Oxidation of sulfides to sulfoxides. Catalytic
performances of the title pNb complexes in oxidation of
various organic sulfides using 30% aqueous H2O2 as a
terminal oxidant in neat water have been investigated. In a
preliminary experiment, the reaction of MPS with H2O2
Table 6 Optimization of reaction conditions for selective oxidation of methyl phenyl sulfide (MPS) by 30% H2O2 catalyzed by pNb complexesa
Entry
Molar ratio (catalyst : MPS)
30% H2O2 (equiv.)
Solvent
Time (min)
Isolated yield (%)
1a : 1b
TON
TOF (h−1
)
1
2
3
4
5
6
7
8
1 : 1000b
1 : 1000c
1 : 500c
1 : 1000c
1 : 2000c
1 : 2500c
1 : 3000c
1 : 2500b
1 : 2500b
1 : 2500c
1 : 2500c
—
1
1
2
2
2
2
2
2
2
2
2
2
H2O
H2O
H2O
H2O
H2O
H2O
H2O
CH3OH
CH3CN
H2O
H2O
H2O
75
75
38
40
43
50
75
115
120
95
76
70
97
95
96
97
95
75
70
97
96
9
85 : 15
100 : 0
85 : 15
100 : 0
100 : 0
100 : 0
100 : 0
100 : 0
100 : 0
55 : 45
100 : 0
90 : 10
760
700
485
608
560
766
1425
2679
2910
2275
978
950
1920
2425
2844
1875
1750
2425
2400
—
9
875
10d
11e
12f
1531
7200
—
20
50
a
b
Reactions were carried out with 5 mmol of the substrate in 5 mL of solvent. Catalyst amount = 2.22 mg for 0.005 mmol of NbA. Reaction
c
d
e
f
at room temperature. Reaction at 0 °C in ice bath. Na3ijNbIJO2)4]·13H2O as the catalyst. Using H2SO4 (1 mmol). Blank experiment (without
a catalyst).
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reaction conditions (Table 6, entry 12). We have also
compared the catalytic efficiency of the newly developed
catalysts with the neat tetraperoxoniobate (TPNB) complex
Na3ijNbIJO2)4]·13H2O under analogous reaction conditions. As
shown in Table 6 (entry 10), the reaction proceeded smoothly
in the presence of TPNB as well, although product selectivity
could not be obtained under the maintained reaction con-
ditions. From these observations, it is apparent that the
co-ligand environment influences the catalytic activity of the
pNb compounds.
The aforementioned findings were further exploited to
obtain pure sulfoxide from a series of aryl alkyl, aryl vinyl,
aryl alcohol and dialkyl sulfides listed in Table 7. Evidently,
both the catalysts were effective in leading to the facile and
selective transformation of each of the substrates to the cor-
responding sulfoxide with impressive yield, although the
NbN catalyst displayed relatively superior activity. The trans-
formations worked well for both aliphatic and aromatic sub-
strates irrespective of having electron-donating or electron-
withdrawing moieties. The nature of the substrate and the
attached substituent, however, appeared to influence the
rates of oxidation.8g,11b The observed trend in variations in
the rate of oxidation of the chosen substrates is consistent
with the previous findings that with increasing nucleophilicity
of the sulfide, the rate of oxidation with H2O2 increases.8g,11b
It is therefore not unexpected that dialkyl sulfides were
oxidized by H2O2 at a faster rate leading to the highest TOF,
relative to conjugated systems such as allylic and vinylic sul-
fides or aromatic sulfides. It is notable that even in the case
of a less nucleophilic diaromatic sulfide the corresponding
sulfoxide was effectively obtained.
A salient feature of the methodology, which enhances the
synthetic utility of the oxidation, is the excellent chemo-
selectivity displayed by the catalysts for the sulfur group of
substituted sulfides such as allylic, vinylic and alcoholic
sulfides, with co-existing sensitive functional groups (Table 7,
entries 3–5). Importantly, allylic and vinylic sulfoxides were
obtained without an epoxidation product. Moreover, the –OH
group of alcoholic sulfides and the benzylic C–H bond
remained unaffected when the benzylic and alcoholic sulfides
could be oxidized to the corresponding sulfoxide under
the maintained reaction conditions. The potential of the
developed protocol for scaled-up synthetic application has
been demonstrated by conducting the oxidation with 6.24 g
of thioanisole (tenfold scale) under optimized conditions
(Table 7, entry 1e). The H2O2 efficiency in the oxidation in
the presence of both the catalysts NbA and NbN was found to
be higher than 90%. The H2O2 efficiency, which is a measure
of the effective use of H2O2, has been defined as 100 × moles
of H2O2 consumed in the formation of oxyfunctionalized
products per mole of H2O2 converted11a [text S4, (ESI†)].
2.2.2 Oxidation of sulfides to sulfones. Clean conversion
of MPS to sulfone could be achieved with high yield in an
aqueous medium, in the presence of each of the catalysts,
simply by extending the reaction time after initial formation
of sulfoxides and conducting the reaction at room temperature.
The optimization of reaction conditions, accomplished by
using MPS as a substrate and NbA as a catalyst, revealed that
the best rate and TOF could be obtained by maintaining the
catalyst : substrate ratio of 1 : 1000 using 2 equiv. of H2O2
(Table 8, entry 1).
In addition to water, we have screened the sulfoxidation
reaction in relatively safer organic solvents such as CH3OH
and CH3CN. The solvent effect has been evaluated in the
oxidation of MPS. Interestingly, the catalytic protocol for
sulfoxidation was found to be compatible with these organic
solvents as well; however, the efficiency of the catalysts was
observed to vary with the nature of the solvent. Although the
catalysts are insoluble in neat organic solvents, including
methanol or acetonitrile, in the presence of aqueous H2O2
used as an oxidant, each of the catalysts dissolves completely
in these water-miscible solvents, leading to the homogeneity
of the catalytic process. It is pertinent to note that we have
strategically avoided the use of hazardous chlorinated
solvents in the present work. The data presented in Table 6
(entries 8 and 9) demonstrate that although the NbA is highly
potent in methanol as well as in acetonitrile, MeOH proved
to be a relatively better solvent, affording both product selec-
tivity and high yield at ambient temperature (Table 6, entry 8).
Significantly, the selective sulfoxidation in the chosen organic
solvents could be achieved at room temperature. It is thus
remarkable that by using the same set of catalyst, it is pos-
sible to achieve selective oxidation of sulfide in water and
organic solvents under mild reaction conditions. Water, how-
ever, proved to be the best solvent with respect to catalyst
efficiency as demonstrated by higher TOF and product selec-
tivity, notwithstanding the insolubility of most of the chosen
organic substrates in water. This is not surprising, considering
the observations made by Sharpless et al.12b that several reac-
tions involving water-insoluble organic reactants could proceed
optimally in pure water. Our results are also in agreement with
earlier findings that chemoselective sulfoxidation is favoured
in polar protic solvent with strong hydrogen bonding ability.38
The sulfoxidation reaction has been carried out at the
natural pH attained by the reaction mixture (ca. 5). However,
a substantial increase in TOF was noted on addition of acid
to the reaction medium (Table 6, entry 11). The finding is in
accord with the reports related to other peroxo metal systems
(Ti, Mo and W),39 where it has been demonstrated that the
use of acidic additives led to an improvement in catalytic
activity of the peroxometallates. The role of protons in the
activation of titanium peroxo complexes has been extensively
investigated by Kholdeeva and co-workers.39b In the present
work, however, since our goal has been to maintain a mild
reaction condition, addition of acid or other additives was
avoided as far as possible. Therefore no attempt has been
made to adjust the pH of the reaction.
Apart from MPS, the protocol could be conveniently
applied to obtain pure sulfone from a variety of aromatic and
aliphatic sulfides as shown in Table 9. The transformations
were chemoselective (Table 9, entries 3–5) and amenable for
scale-up (Table 9, entry 1e) as has been observed in the case
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Table 7 Selective oxidation of sulfides to sulfoxides catalysed by NbA and NbNa
NbA
NbN
Isolated yield
Isolated yield
Time (min) of sulfoxide (%) TONb
Entry Substrate
1.
Time (min) of sulfoxide (%) TONb
TOFc (h−1
)
TOFc (h−1
)
50
50
50
97
2425
2325
2400
2910
2790
2880
45
45
45
96
2400
2350
2375
3200
3133
3166
93d
96e
94d
95e
2.
3.
45
97
2425
3233
40
95
2375
3562
85
93
2325
1641
75
94
2350
1880
4.
240
95
2375
593
225
93
2325
620
5.
6.
115
55
97
96
2425
2400
1265
2618
105
50
95
95
2375
2375
1357
2850
7.
8.
210
30
93
97
2325
2425
664
195
25
94
96
2350
2400
723
4850
5760
9.
40
40
96
97
2400
2425
3600
3637
35
35
93
95
2325
2375
3985
4071
10.
a
b
Optimized conditions: 5 mmol of substrate, 10 mmol of 30% H2O2 and 0.002 mmol of catalyst in H2O at 0 °C. TON (turnover number) =
c
millimoles of the product per millimole of the catalyst. TOF (turnover frequency) = millimoles of the product per millimole of the catalyst per
hour. d Yield of the 6th reaction cycle. Scale-up data (6.24 g of MPS).
e
Table 8 Optimization of reaction conditions for NbA-catalyzed selective oxidation of methyl phenyl sulfide (MPS) to sulfonea
Entry
Molar ratio (catalyst : MPS)
H2O2 (equiv.)
Solvent
Time (min)
Isolated yield (%)
1a : 1b
TON
TOFIJh−1
)
1.
2.
3.
4.
5.
1 : 1000
1 : 2000
1 : 2500
1 : 1000
1 : 1000
2
2
2
2
2
H2O
H2O
H2O
CH3OH
CH3CN
80
175
240
240
240
96
95
96
96
93
0 : 100
0 : 100
25 : 75
70 : 30
75 : 25
960
1900
2400
960
720
651
600
240
232
930
a
Reactions are carried out with 5 mmol of the substrate in 5 mL of solvent at room temperature.
of sulfoxidation reaction. These findings underscore the
synthetic value of the methodology.
2.2.3 Recyclability of the catalysts. The reusability of the
catalysts for subsequent cycles of oxidation was assessed by
employing MPS as the substrate. The catalysts afforded
regeneration in situ after separation of the organic product
from the reaction mixture on completion of the reaction and
could be reused without further conditioning. Regeneration
was accomplished by simply charging the aqueous part of
the spent reaction mixture with 30% H2O2 and a fresh batch
of substrates on completion of each reaction cycle. In an
alternative approach, the regenerated catalysts could also be
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Table 9 Selective oxidation of sulfides to sulfones catalyzed by NbA and NbNa
NbA
NbN
Isolated yield
of sulfone (%)
Isolated yield
of sulfone (%)
Entry
1.
Substrate
Time (min)
TONb
TOFc (h−1
)
Time (min)
TONb
TOFc (h−1
)
80
80
80
96
960
930
950
720
697
712
70
70
70
97
970
930
940
831
797
805
93d
95e
93
94e
2.
3.
4.
5.
75
95
93
94
97
950
930
940
970
760
384
221
306
65
97
94
93
95
970
940
930
950
895
431
235
316
145
255
190
135
250
180
6.
7.
90
95
93
950
930
633
177
80
94
93
940
930
705
180
315
310
8.
65
70
70
94
93
95
940
930
950
867
797
814
55
60
60
96
93
94
960
930
940
1047
930
9.
10.
940
a
b
Optimized conditions: 5 mmol of the substrate, 10 mmol of 30% H2O2 and 0.005 mmol of the catalyst in H2O at RT. TON (turnover
c
number) = millimoles of the product per millimole of the catalyst. TOF (turnover frequency) = millimoles of the product per millimole of the
catalyst per hour. d Yield of the 6th reaction cycle. Scale-up data (6.24 g of MPS).
e
isolated into the solid state by solvent-induced precipitation
with acetone after treating the aqueous extract of the spent
reaction mixture with 30% H2O2. Nevertheless, the in situ
regeneration of the catalyst offers obvious advantages as the
procedure does not require tedious separation and subse-
quent purification steps, which are usually associated with
soluble catalysts.12a The activity and selectivity of the cata-
lysts remained unaltered for at least up to 6 reaction cycles
as illustrated in Table 7 (entry 1d), Table 9 (entry 1d) and
Fig. S6 (ESI†). In order to further confirm that the catalysts
remain intact after their use in catalytic cycles, the recovered
catalysts were characterized by elemental analysis and spec-
tral studies. The IR and Raman spectra of each of the regene-
rated catalysts resembled closely the corresponding spectrum
of the starting catalyst, displaying the typical absorptions for
the triperoxoniobium moiety and the respective metal bound
co-ligand, indicating that the coordination environment of
the complexes was not altered during the catalytic process.
The IR spectrum of the regenerated catalyst NbA is presented
in Fig. S1(c) (ESI†). No significant change was observed in
the niobium and peroxide contents of the recovered catalysts
in comparison to the respective original complex. It is thus
evident that the metal complexes retain their structural integ-
rity even after several catalytic cycles.
Interestingly, the procedure with NbA used as a catalyst
provided an overall TOF of ca. 17 130 h−1 (ca. 18 999 h−1
for NbN) after 6 cycles of oxidation of MPS to sulfoxide
and ca. 4259 h−1 (ca. 4884 h−1 with NbN) for conversion
to sulfone. The results further demonstrate the superior
activity of the catalysts under mild conditions over other
reported methods for sulfide oxidation involving NbIJV)
catalysts,20a–f as well as many other protocols based on
7b–e,8b–e,g,10c,g,h,38a,40
Mo(VI) or W(VI)/H2O2
systems including
the polymer-supported heterogeneous Mo(VI) catalysts that we
reported recently.7b
2.2.4 The proposed mechanism. Based on our results and
taking into account our earlier findings on the catalytic
activity of some other peroxometal systems,7b,8a a credible
mechanism for the pNb-catalyzed selective oxidation of sul-
fides to sulfoxides or sulfone by H2O2 has been proposed
(Fig. 6), which satisfactorily describes the principal features
of our findings from the present study.
As shown in Fig. 6, with the NbA catalyst as a representa-
tive, it is possible that the reaction proceeds through the
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Fig. 6 The proposed mechanism.
formation of the diperoxoniobate intermediate II, subsequent
to the facile transfer of electrophilic oxygen from the triper-
oxoniobium complex I to the substrate V to yield sulfoxide
(reaction a). The intermediate II combines with the peroxide
of H2O2 to regenerate the starting triperoxoniobate complex
(reaction b) leading to a catalytic cycle. The resulting sulfoxide
formed may undergo further oxidation in a separate cycle by
reacting with a triperoxo Nb species to yield sulfone (reaction c).
characteristic of a DPNb moiety [Fig. S1(d) (ESI†)] in contrast
to the three peroxo absorptions of the original triperoxo-
niobium catalyst. Similar reaction of MPS conducted with the
isolated DPNb complex was noted to be extremely slow, as
has been anticipated, remaining incomplete even after 14 h of
reaction time. The aforementioned findings lent further
credence to the proposed mechanism.
The sulfone formation thus seems to be a two-step process. 3. Experimental section
The greater ease of oxidation of sulfide to sulfoxide compared
to the second oxidation of sulfoxide to sulfone is likely to be
3.1 Materials
Acetone, hydrogen peroxide, acetonitrile, methanol, ethyl-
a consequence of higher nucleophilicity of sulfide relative to
sulfoxide. The proposed mechanism is in line with the reac-
tion pathway proposed previously for peroxoniobate-catalysed
oxidation of sulfide.18a,d,20g
acetate, petroleum ether, diethyl ether, silica gel (60–120 mesh)
(RANKEM), L-arginine (CDH, New Delhi, India), nicotinic acid
(HIMEDIA), sodium hydroxide, and sodium sulfate (E. Merck,
India) were used in this study. Niobium pentoxide, methyl
phenyl sulfide (MPS), methyl p-tolyl sulfide (MpTS), ethyl
phenyl sulfide (EPS), dimethyl sulfide (DMS), dibutyl sulfide
(DBS), phenylvinyl sulfide (PVS), 2-IJphenylthio)ethanol (PTE),
dihexyl sulfide (DHS), diphenyl sulfide (DPS) and allyl phenyl
sulfide (APS) were obtained from Sigma-Aldrich Chemical
Company, Milwaukee, USA. The water used for solution prep-
aration was deionized and distilled.
It is worth noting that although the mechanism of action
of peroxo complexes of other d0 metals such as VIJV), Mo(VI)
and W(VI) has been extensively investigated in organic oxida-
tion, the chemistry of Nb(V) peroxide still remains relatively
unexplored.7b,d,8a,16e,21d,24,40a,d,41 The previous studies from
several laboratories including ours7b,8a,16e, f have shown that
during substrate oxidation performed by active diperoxo com-
plexes of Mo(VI) or W(VI), a more stable monoperoxo species
is formed which is practically inactive in oxidation.7d,24,41k–n
Taking into account this finding, it is reasonable to expect
the formation of a less reactive diperoxoniobate (DPNb) inter-
mediate from an active triperoxoniobate (TPNb) species dur-
ing sulfide oxidation, as shown in the proposed mechanism
(reaction intermediate II). In order to establish the involve-
ment of such an intermediate in the reaction pathway, a sepa-
rate experiment was conducted using NbA as a stoichiometric
oxidant of MPS, maintaining an NbA : substrate molar ratio
of 1 : 1 in the absence of H2O2 at 0 °C. The substrate was
completely and selectively transformed into sulfoxide within a
reaction time of ca. 40 minutes. The product isolated from
the aqueous part of the spent reaction mixture was subse-
quently subjected to spectral and elemental analyses. The
data obtained indicated a Nb : peroxo ratio of 1 : 2, clearly
suggesting the formation of a DPNb species. This was further
confirmed from the IR spectrum which showed two distinct
bands, in addition to the coordinated amino acid ligands,
3.2 Synthesis of the complexes
3.2.1 Preparation of sodium tetraperoxoniobate18i. Sodium
tetraperoxoniobate, Na3ijNbIJO2)4]·13H2O, was prepared by the
reported method.18i In a nickel crucible, 1 g of Nb2O5 and
1.85 g of NaOH were fused together at 700 °C. The solid
obtained was cooled and dissolved in 100 ml of 1 M aqueous
H2O2. Unreacted Nb2O5 was filtered off and the filtrate was
allowed to settle at 5 °C for 24 h to afford the white crystalline
Na3[Nb(O2)4]·13H2O.
3.2.2 Synthesis of the peroxoniobate complex
Na2ijNbIJO2)3IJarg)]·2H2O (NbA). Solid Na3[Nb(O2)4]·13H2O
(0.6550 g, 1.25 mmol) was dissolved in 30% H2O2 (4 ml,
25 mmol) in a 250 ml beaker in an ice bath. L-Arginine was
added to this solution gradually under constant stirring while
maintaining the temperature below 4 °C. At this stage, the
pH of the solution was recorded to be ca. 8. Dilute HNO3
solution (4 M) was added dropwise to the solution under
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continuous stirring until the pH was lowered to 6. The
resulting solution was allowed to stand for 3 h in an ice bath.
A white pasty mass separated out on adding pre-cooled ace-
tone to this mixture under vigorous stirring. The supernatant
liquid was decanted off and the residue was treated repeat-
edly with acetone under scratching. The microcrystalline
product obtained was separated by centrifugation and dried
in vacuo over concentrated sulfuric acid.
3.2.3 Synthesis of the peroxoniobate complex
Na2ijNbIJO2)3(nic) (H2O)]·H2O (NbN). The procedure consisted
of gradual addition of nicotinic acid (1.25 mmol) under
constant stirring to a solution of Na3[Nb(O2)4]·13H2O (0.6550 g,
1.25 mmol) in 30% H2O2 (4 ml, 25 mmol) in an ice bath. The
pH 3 of the reaction solution at this stage was raised to ca. 6
by dropwise addition of NaOH solution (8 M) under stirring.
After allowing the solution to stand for 3 h, pre-cooled ace-
tone was added to it under continuous stirring. The white
pasty mass obtained from the solution after decanting the
supernatant liquid was treated repeatedly with acetone to iso-
late the white microcrystalline solid product. The product
obtained was separated by centrifugation and dried in vacuo
over concentrated sulfuric acid.
acid and NbN were recorded using a JEOL JNM-ECS400 spec-
trometer with the following parameters: a carbon frequency
of 100.5 MHz, 4096 X-resolution points, the number of scans
equal to 1000–20 000, 1 s of acquisition time, and 30° pulse
length (D2O as solvent). 1H NMR study for the synthesised
complexes along with free ligands has been carried out with
a JEOL JNM-ECS400 spectrometer with 16 scans, 2 s of acqui-
sition time and 45° pulse length (D2O as solvent). The 1H and
13C NMR spectra of organic sulfoxides and sulfones were
recorded using a JEOL JNM-ECS400 spectrometer (CDCl3 as
solvent and TMS as an internal standard). Melting points were
determined using open capillary tubes on a Büchi melting
point B-540 apparatus and were uncorrected. GC analysis was
carried out using a CIC model 2010 gas chromatograph and
an SE-52 packed column (length 2 m, 1/8 inch OD) with a
flame ionization detector (FID) and nitrogen as the carrier gas
(30 mL min−1).
3.5 Catalytic activity
3.5.1 General procedure for oxidation of sulfides to
sulfoxides (catalyst: NbA or NbN) at 0 °C. In a representative
procedure, an organic substrate (5 mmol) was added to a
solution of a catalyst (0.002 mmol) in water (5 mL) and
then a 30% H2O2 oxidant (1.13 mL, 10 mmol) was added,
maintaining a catalyst : substrate molar ratio of 1 : 2500 and a
substrate : H2O2 ratio of 1 : 2 in a 50 mL round-bottomed
flask. The reaction was conducted at 0 °C in an ice bath
under continuous stirring. The progress of the reaction was
monitored by thin-layer chromatography (TLC). After comple-
tion of the reaction, the products were extracted with diethyl
ether and dried over anhydrous Na2SO4 and distilled under
reduced pressure to remove excess diethyl ether. The corre-
sponding sulfoxide obtained was purified by column chroma-
tography on silica gel using ethyl acetate and n-hexane (1 : 9).
3.3 Elemental analysis
The Perkin-Elmer 2400 series II elemental analyzer was used
for C, H and N elemental analysis of the synthesized com-
pounds. The niobium content was estimated gravimetrically
as NbOIJC13H10NO2)3 (ref. 42) and by atomic absorption spectros-
copy (AAS). Niobium, sodium, C and N composition contents
were also obtained by EDX analysis. In addition, the sodium
content was quantified with an ionometer (ORION VERSASTER).
The peroxide content of the compounds was determined by
adding a weighed amount of the compound to a cold solu-
tion of 1.5% boric acid (w/v) in 0.7 M sulfuric acid (100 mL)
and titrating with standard sodium thiosulfate solution.43
1
The products obtained were characterized by IR, H NMR,
and 13C NMR spectroscopy, and in the case of solid sulfoxide
products, in addition to the above spectral analysis, we have
also carried out melting point determination (see the ESI†).
3.5.2 General procedure for oxidation of sulfides to
sulfones (catalyst: NbA or NbN) at room temperature. To a
stirred solution of the catalyst (0.005 mmol) and 30% H2O2
(10 mmol), the organic substrate (5 mmol) was added in
water (5 mL) maintaining a catalyst : substrate molar ratio of
1 : 1000 and a substrate : H2O2 ratio of 1 : 2 at room temperature
(RT) under continuous stirring. The reaction was monitored
by thin-layer chromatography (TLC). After completion of the
reaction, the products were extracted with diethyl ether and
dried over anhydrous Na2SO4 and distilled under reduced
pressure to remove excess diethyl ether. The corresponding
sulfone obtained was purified by column chromatography on
silica gel using ethyl acetate and n-hexane (1 : 9).
3.4 Physical and spectroscopic measurements
The IR spectra were recorded at ambient temperature by
making pressed pellets of the compounds with KBr pellets
using a Perkin-Elmer spectrum 100 FTIR spectrophotometer.
Raman spectra of the compounds were recorded using a
Renishaw InVia Raman microscope equipped with an argon
ion laser with an excitation wavelength of 514 nm and a laser
maximum output power of 20 mW. The measurement param-
eters were 10 s of exposure time, 1 accumulation, laser power
10% of the output power and 50× objective, and the spectral
resolution was set to 0.3 cm−1. Thermogravimetric analysis
was performed in a SHIMADZU TGA-50 system at a heating
rate of 10 °C min−1 under an atmosphere of nitrogen using
an aluminium pan. Energy-dispersive X-ray analysis was done
by using a JEOL JSM-6390LV scanning electron micrograph
attached to an energy-dispersive X-ray detector. Atomic
absorption spectrometry was done using a Thermo iCE 3000
series atomic absorption spectrophotometer model analyst
200. The 13C NMR spectra for arginine, NbA, nicotinic
The IR, 1H NMR, and 13C NMR spectroscopy tools were
used to characterize the products. In addition to the above
spectral analysis, we have also carried out melting point
determination for the products (see the ESI†).
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3.6 Regeneration of the catalyst
a non-toxic pNb catalyst which can be reused in subsequent
cycles without losing its activity. In addition, the oxidation is
absolutely free from organic co-solvent, co-catalyst or any
other auxiliaries, involves easy work-up procedure and is
amenable for ready scalability. All of these features make the
presented catalytic strategies attractive and interesting from
both economic and environmental perspectives.
Any of the following procedures could be adopted to regener-
ate the catalysts for reuse. The regeneration of the catalyst
was carried out for the reaction using methyl phenyl sulfide
(MPS). After completion of the oxidation reaction and subse-
quent extraction of the organic reaction product, the aqueous
part of the reaction mixture was transferred to a 250 mL
beaker. Keeping the solution in an ice bath, 30% H2O2 was
added to it maintaining a Nb : peroxide ratio of 1 : 2, followed
by addition of pre-cooled acetone under constant stirring
until a white pasty mass separated out. From this precipitate,
the catalyst was finally obtained as a microcrystalline solid by
following the work-up procedure mentioned under Section
3.2.2. The regenerated pNb catalyst was then placed into a
fresh reaction mixture consisting of MPS and hydrogen
peroxide in water, and the reaction was allowed to proceed
under optimised conditions as mentioned under Section
3.5.1 (for sulfoxide) or Section 3.5.2 (for sulfone). The prog-
ress of the reaction was monitored by thin-layer chromatogra-
Acknowledgements
The authors gratefully acknowledge the Department of
Science and Technology, New Delhi, India, for providing
financial support. We are also grateful to the University
Grants Commission, Basic Science Research Fellowship,
New Delhi, India, for providing a junior research fellowship
to S.R.G. We thank Dr. B. Choudhury, Department of Physics,
Tezpur University, Tezpur, Assam, India, for the Raman spectra.
phy (TLC). After completion of the reaction, the process was Notes and references
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Conclusions
In summary, a pair of new heteroligand triperoxoniobiumIJV)
complexes has been synthesized, characterized and success-
fully applied in the selective oxidation of variously substituted
sulfides to selectively obtain sulfoxide or sulfone with 30%
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