Ni-nanoparticles: An efficient green catalyst for chemo-selective oxidative coupling of thiols

Article abstract of DOI:10.1016/j.molcata.2006.12.042

Use of highly monodispersed, easily recyclable and cheap nickel nanoparticles for oxidative coupling of aliphatic, aromatic, cyclic and heteroaromatic thiols to their corresponding disulfides has been reported. Ni-nanoparticles act as a green catalyst that can selectively catalyse oxidative coupling of thiols to disulfides without producing any over-oxidized products. The catalytic reaction occurred at room temperature with excellent yield under air atmosphere and high TON and TOF value could be achieved. Effect of solvent polarity and size distribution of the Ni-nanoparticles on the catalytic efficiency have been investigated. Nickel nanoparticles could be easily recovered by mild centrifugation.

Full text of DOI:10.1016/j.molcata.2006.12.042

Journal of Molecular Catalysis A: Chemical 269 (2007) 35–40
Ni-nanoparticles: An efficient green catalyst for chemo-selective
oxidative coupling of thiols
Amit Saxena, Ajeet Kumar, Subho Mozumdar∗
Department of Chemistry, University of Delhi, Delhi 110007, India
Received 12 July 2006; received in revised form 23 December 2006; accepted 29 December 2006
Available online 9 January 2007
Abstract
Use of highly monodispersed, easily recyclable and cheap nickel nanoparticles for oxidative coupling of aliphatic, aromatic, cyclic and het-
eroaromatic thiols to their corresponding disulfides has been reported. Ni-nanoparticles act as a green catalyst that can selectively catalyse oxidative
coupling of thiols to disulfides without producing any over-oxidized products. The catalytic reaction occurred at room temperature with excellent
yield under air atmosphere and high TON and TOF value could be achieved. Effect of solvent polarity and size distribution of the Ni-nanoparticles
on the catalytic efficiency have been investigated. Nickel nanoparticles could be easily recovered by mild centrifugation.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Ni-nanoparticles; Green catalyst; Thiols; disulfides; Catalytic oxidation
1. Introduction
reagents. The oxidation of thiols to disulfides is a characteris-
tic reaction [30–36] and further oxidation to disulfide S-oxides
(thiosulfinates), 1,1-dioxides(thiosulfonates), andsulfonicacids
are possible. Weak S–S bonds in these compounds impart high
reactivity [37] and in natural products these moieties and related
cyclic analogues are associated with interesting biological activ-
ities including DNA-cleaving properties [38,39]. In recent years,
there has been growing interest in the catalytic properties of
transition metal nanoparticles [40]. The high surface area-to-
volume ratio of solid-supported metal nanoparticles [41] is
mainly responsible for their catalytic properties and this can be
exploited in many industrially important reactions [42]. Recent
literature shows that nanoparticles have hardly been used as cat-
alysts in organic synthesis. Ni-nanoparticles, in particular, being
cheap, require only mild reaction conditions for high yields
of products in short reaction times as compared to traditional
catalysts.
Thus, there is still a need to develop a mild and effi-
cient methodology of synthesizing aliphatic, aromatic, and
heteroaromatic disulfides. In the continuation of our work on
new applications of nanoparticles [43–45] in catalysis, we
wish to further disclose an efficient and rapid protocol using
monodispersed, cheap, easily recyclable Ni-nanoparticles as a
chemo-selective catalyst for the oxidative coupling of various
thiols to their corresponding disulfides under ambient condi-
tions.
Oxidative coupling of thiols to disulfides is of interest from
biological, synthetic and oil-sweetening point of view [1–6].
Since thiol can be over oxidized, extensive research has been
carried out in recent years to control their oxidation [7–12].
Most of the existing methods involve the use of reagents such
as molecular oxygen [13], metal ions [14], Bu3SnOMe/FeCl3
[15], nitric oxide [16], halogens [17–20], sodium perborate [21],
borohydride exchange resin (BER)-transition metal salt system
[22], a morpholine iodine complex [23], picolinium chlorochro-
mate (PCC) [24], ammonium persulfate [25], KMnO4/CuSO4
[26], H2O2 [27], solvent free permanganate [28], PVP–N2O4
[29] and cesium fluoride–celite, –O2 [30]. Some of these meth-
ods suffer from one or more of disadvantages such as long
reaction times, difficult workup, lack of general applicability
to thiol substrates bearing alkyl, aryl, cyclic and heteroaro-
matic moieties, formation of over-oxidation products leading
to lower yields, use of stoichiometric excess amounts of the
reagents for successful oxidation, requirements of strong oxi-
dizing agents, strong acidic or basic media, and use of expensive
∗
Corresponding author. Tel.: +91 11 4131 5109; fax: +91 11 766 6593.
E-mail addresses: Subhoscom@yahoo.co.in, subhom 2@yahoo.com
(S. Mozumdar).
1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.12.042

36
A. Saxena et al. / Journal of Molecular Catalysis A: Chemical 269 (2007) 35–40
Table 1
Oxidation of thiols to disulfides using 15 mol% nickel nanoparticles and acetonitrile as solvent at room temperaturea,b
Entry
Reactants
Product
Reaction time (min)
Yield
mp or bp (mmHg ◦C)
(found)
a
6
8
98
93
57–60
b
141–143
c
11
94
75–76
d
e
9
4
98
96
46–47
69–70
f
12
8
85
91
72–73
g
127–129
h
I
CH3(CH2)3–SH
CH3(CH2)7–SH
CH3(CH2)3–S–S–(CH2)3CH3
CH3(CH2)7–S–S–(CH2)7CH3
9
90
95
94–96
15
201–203
j
10
35
92
91
123–126
–
k
SH–CH2–CH2–SH
–(S–CH2–CH2–Sn)–
a
Confirmed by comparison with authentic samples (FT-IR, H NMR, C NMR, mass-analysis, TLC, mp, bp).
Yield of isolated pure product after chromatography/distillation and recrystallization.
b
Catalysis through nanoparticles is a well-established process.
2. Results and discussion
The ability of the nanoparticles to catalyze the chemical reac-
tion depends on many factors, such as—size, shape, electronic
structure, surface area, etc. Our process is highly economic
and eco-friendly as it requires neither elevated temperature,
nor harsh acids or bases, produces high yield with excellent
chemo-selectivity, reduces the reaction time and is applicable
to variety of aliphatic, aromatic, cyclic and heterocyclic thi-
ols substrates. Moreover, it is a one-pot synthesis that does
not require inert atmosphere, has an easy work up and prod-
uct isolation from the catalyst. In contrast, when Raney nickel
or other complexes are used as catalysts an excessive amount
of catalysts (usually in grams which may trigger toxicity) is
typically found in the form of waste after the reaction [46–49].
This has been significantly reduced to microgram (less chances
of exhibiting toxicity) using our methodology viz. using nickel
nanoparticles. As far as toxicity of nickel nanoparticles are con-
cerned there are many reports in which nickel nanoparticles have
been used for various applications including in vivo and in vitro
treatment of living cells/organisms and no evidence of toxicity
has been reported at a low level of dosage [50,51]. Therefore,
we claim that our method is much more ecofriendly and less
toxic.
To understand the possible mechanism of the Ni-
nanoparticles mediated oxidative coupling a control experiment
was conducted in the absence of a catalyst and it was observed
that no reaction occurred. However, the presence of Ni-
nanoparticles is not only the limiting factor, but the presence
of O2 is also absolutely necessary for the oxidation of thiols as
no reaction proceeded under N2 atmosphere. As evident from
the above-mentioned observation it can be proposed that Ni-
nanoparticlesareextremelyactiveandprovideenormoussurface
area that uses molecular oxygen as an oxidant for the oxidation
of thiols. It is apparent from the data (Table 1) that all aliphatic,
aromatic, cyclic and heteroaromatic thiols underwent the reac-
tion smoothly. The presence of electron withdrawing/donating
groups had negligible effect on the yield but had some influence
on the rate of the reaction.
2.1. Catalyst concentration
The effect of catalyst (Ni-nanoparticles) concentrations on
the reaction rate and yield of the product was also investigated
(Table 2). It was found that the optimum reaction rate and yield

A. Saxena et al. / Journal of Molecular Catalysis A: Chemical 269 (2007) 35–40
37
Table 2
Effect of catalyst concentration on yield of the product and reaction time
Entry
Catalysts (x, mol%)
Yield (%)
Reaction time (min)
1
2
3
4
5
6
7
8
2
5
8
10
15
18
20
25
18
50
65
84
94
94
93
94
17
15
13
10
6
6
6
6
Fig. 1. Effect of recycling of Ni-nanoparticles on catalytic oxidative coupling
of Thiophenol and 2-aminothiophenol using 15 mol% Ni-nanoparticles at room
temperature stirred under air atmosphere. Acetonitrile was used as solvent.
could be achieved at the catalyst concentration of 15 mol% with
the thiol concentration kept constant.
n-hexane with all the other parameters kept constant and the
progress of the reaction was checked through TLC. The yields
were found to be 96, 75, 40, 32 and 8%, respectively. Hence,
CH3CN was used for all the reactions through the work. The
nanoparticles thus prepared could be stored in absolute ethanol
or as dry powder under N2 atmosphere for a long period. The
advantage of the nanoparticles is that unlike other catalyst,
their use is not restricted by their solubility, as the nanoparti-
cles can be dispersed in desired solvent by agitation or slight
sonication.
2.2. Effect of particles size on the catalytic efficiency
Ni-nanoparticles of sizes ranging from 10 nm to 85 nm in
diameters were prepared in the aqueous cores of the reverse
micellar droplets. The catalytic efficiency of the nanoparticles
was found to be dependent on the size of the nanoparticles
(Table 3). The maximum conversion was obtained for parti-
cles in average diameter of about 20 nm. Particles below this
diameter exhibited a trend of decreasing reaction rate with the
decrease in particle size, while those above this diameter showed
a steady decline of reaction rate with increasing size. It has been
postulated that in the case of particles smaller than 20 nm in
average size, a downward shift of Fermi level with a consequent
increase in band gap energy takes place. As a result, the par-
ticles require more energy to pump electrons to the adsorbed
ions for the electron transfer reaction. This leads to a reduced
reaction rate catalyzed by smaller particles. On the other hand,
for nanoparticles larger than 20 nm in diameter, the change of
Fermi level is not appreciable. However, these particles with
larger size exhibit less surface area for adsorption. As a result,
the catalytic efficiency of the particles is also decreased with
increasing particle size.
2.4. Recyclability
After the completion of the reaction the Ni-nanoparticles
were recycled by separating them from the reaction mixture by
mild centrifugation at 2000–3000 rpm and used as a catalyst for
the same reaction again. It was observed that with increasing
numbers of cycles the catalytic activity of the Ni-nanoparticles
decreased and that it was nearly lost after six cycles, as shown
in (Fig. 1). This may be explained on the basis of slow but con-
tinuous oxidation of nickel nanoparticles during the course of
reaction.
3. Experimental
3.1. General information
2.3. Solvent effect
The materials were purchased from Sigma–Aldrich and
Merck and were used without any additional purification. All
reactions were monitored by thin layer chromatography (TLC)
on gel F254 plates. The silica gel (250–400 meshes) for column
chromatography was purchased from Spectrochem Pvt. Ltd.
The oxidative coupling of thiols was also investigated in
various solvents such as CH3CN, CHCl3, CH2Cl2, THF and
Table 3
Effect of size of the Ni-nanoparticles on catalytic oxidative coupling of Thiophe-
noland2-aminothiophenolusing15 mol%Ni-nanoparticlesatroomtemperature
under air atmospherea
1
India. H NMR (300 MHz) and 13C NMR (300 MHz) spectra
were recorded on a Bruker Spectrospin 300 MHz spectrome-
ter in CDCl3 (with TMS for 1 H and CDCl3 for 13C NMR as
internal references). MS were recorded on a TOF-Mass spec-
trometer Model No. KC455. Melting points were recorded on
a Buchi melting point 540 instrument. The size and morphol-
ogy of Ni-nanoparticles were characterized with the help of
a transmission electron microscope (TEM, FEI Philips Mor-
gagni 268Dmodel) Acc. voltage: 100 kV with magnification:
Particle size ( 2 nm)
Thiophenol
Yield (%)
2-Aminothiophenol
Time (min)
Yield (%)
Time (min)
10
20
30
85
89
95
93
71
10
6
9
87
93
89
62
12
8
15
25
22
a
Acetonitrile was used as solvent in all the reactions.

38
A. Saxena et al. / Journal of Molecular Catalysis A: Chemical 269 (2007) 35–40
Scheme 1. Preparation of nickel nanoparticles.
up to 280,000) and a Quasi Elastic Light Scattering instrument
(QELS, photocor-FC, model-1135 P) and the metallic nature
of the particles was confirmed with a UV-spectrophotometer
(Shimadzu).
lar NaBH4 solution (5%, w/v in 2% NaOH (w/v) aqueous
solution) (RM-2) was added dropwise with constant stir-
ring maintained under the nitrogen atmosphere. The resulting
solution was further stirred for 3 h to allow complete Ost-
wald ripening (particle growth). The nickel nanoparticles were
extracted by adding absolute ethanol to the reverse micellar
solution containing Ni-nanoparticles followed by centrifuga-
tion at 3000–4000 rpm for 10 min. By varying the water content
parameter W0 (defined as the molar ratio of water to surfactant
concentration, W0 = [H2O]/[surfactant] the size of the nanopar-
ticles could be controlled.
The sizes of the nickel-nanoparticles prepared at W0 = 5 (the
water content parameter W0 can be defined as the ratio of molar
concentration of water to surfactant, W0 = [H2O]/[surfactant])
were found to be as 15–18 nm through quasi elastic light
scattering (QELS) data (Fig. 2a) and transmission electron
microscopy (TEM) (Fig. 2b). The Ni-nanoparticles pre-
pared were round in shape and black in color (colloidal
state).
3.2. Preparation of nanoparticles
A chemical method involving reduction of Ni2+ ions to
Ni(0) in a reverse micellar system was employed to prepare
the nickel nanoparticle (Scheme 1). Poly (oxy ethylene)(tetra
methyl butyl)-phenyl ether, commercially known as TritonX-
100 (TX-100) was used as the surfactant, cyclohexane as the
solvent (continuous phase), hexanol as co-surfactant and aque-
ous solution of salts as dispersed phase (water core in which
particles formation occurs).
The reverse micelles were prepared by dissolving TX-100
in cyclohexane (usually 0.08–0.15 M of TX-100 solution). A
typical preparative method is as follows:
To 100 ml of (0.1 M TX-100 solution in cyclohexane) 900 ␮l
of Ni(NO3)2 aqueous solution (2%, w/v) and hexanol (q.s.) was
added to afford an optically clear reverse micellar solution (RM-
1). To another 100 ml of (0.1 M TX-100 solution in cyclohexane)
NaBH4 solution (5%, w/v in 2% NaOH (w/v) aqueous solution)
and hexanol (q.s.) was added to afford RM-2.
3.3. Typical procedure for the oxidative coupling of thiols
In a 50 ml round-bottomed flask a solution of thiol
(2 mmol) in a solvent (20 ml) was prepared. nickel nanopar-
ticles (15 mol%) were added to the solution and the reaction
mixture was stirred at room temperature in air atmosphere.
To the prepared reverse micellar solution of Ni(NO3)2
aqueous solution (2%, w/v) (RM-1), another reverse micel-
Fig. 2. (a) QELS data of Ni-nanoparticles: plot of population distribution in percentile vs. size distribution in nanometers and (b) TEM image of Ni-nanoparticles.
The bar corresponds to 50 nm in the TEM.

A. Saxena et al. / Journal of Molecular Catalysis A: Chemical 269 (2007) 35–40
39
The progress of the reaction was monitored by TLC (eluent: n-
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10 ml of the same solvent used during the reaction. Excess
solvent was evaporated using a rotatory evaporator and concen-
trating to dryness gave the desired product, which-followed by
recrystallization afforded the pure disulfides in 85–98% yields
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