A facile one-pot synthesis of ruthenium hydroxide nanoparticles on magnetic silica: Aqueous hydration of nitriles to amides

Article abstract of DOI:10.1039/c2cc32566g

One-pot synthesis of ruthenium hydroxide nanoparticles on magnetic silica is described which involves the in situ generation of magnetic silica (Fe ₃O₄@SiO₂) and ruthenium hydroxide immobilization; the hydration of nitriles occurs in high yield and excellent selectivity using this catalyst which proceeds exclusively in aqueous medium under neutral conditions. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc32566g

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 6220–6222
www.rsc.org/chemcomm
COMMUNICATION
A facile one-pot synthesis of ruthenium hydroxide nanoparticles on
magnetic silica: aqueous hydration of nitriles to amidesw
R. B. Nasir Baig and Rajender S. Varma*
Received 10th April 2012, Accepted 1st May 2012
DOI: 10.1039/c2cc32566g
One-pot synthesis of ruthenium hydroxide nanoparticles on
magnetic silica is described which involves the in situ generation
of magnetic silica (Fe3O4@SiO2) and ruthenium hydroxide
immobilization; the hydration of nitriles occurs in high yield
and excellent selectivity using this catalyst which proceeds
exclusively in aqueous medium under neutral conditions.
that circumvents the extravagant use of stoichiometric reagents
and/or acidic and basic media is highly desirable. Magnetic
nanoparticles have emerged as a robust, high-surface-area
heterogeneous catalyst support.14,15 Magnetic recoverability, which
eliminates the necessity of catalyst filtration after completion of the
reaction, is an additional attribute of these materials16,17 compared
to most of the heterogeneous catalysts deployed. These catalysts
work well but suffer from the drawback of requirement of an
elaborate and tedious procedure for their synthesis which
involves three steps: (i) synthesis of nanoferrite, (ii) post-
synthetic modification via anchoring of ligands which may
be toxic and, (iii) immobilization of ruthenium metal. To
overcome this drawback and to avoid the use of toxic ligands
and reagents, a one-step procedure for the synthesis of magnetic
silica supported ruthenium as a magnetically retrievable catalyst
has been developed and its application in the hydration of
nitriles in benign aqueous media, which circumvents the use of
organic solvents, has been demonstrated.
The hydration of nitriles is an important reaction for the large-
scale synthesis of amides, which are a significant group of
compounds in the chemical and pharmaceutical industries.1
The hydration of nitriles has been conventionally achieved using
acids and bases, enzymes,2 and transition metals.3 However, under
these conditions, various by-products, notably carboxylic acids,
are formed by the hydrolysis of the starting nitriles and amides.
Several protocols, which predominantly use homogeneous metal
complexes, have been reported for the hydration of nitriles.3–6
These methods suffer from various drawbacks, especially the
difficulty in separation of the product and the catalyst from the
reaction mixture, as well as the use of an inert atmosphere
for handling air-sensitive metal catalysts. Many sensitive
functional groups do not withstand such harsh conditions,
which consequently decreases the selectivity of the reaction
protocol. Heterogeneous systems have been reported, such as
The first step in the accomplishment of this goal was the
easy synthesis of a magnetic silica supported ruthenium
catalyst by sequential addition of reagents in one-pot
(Scheme 1). The magnetic nanoferrite (Fe3O4) was generated
in situ via a hydrolysis method by stirring the solution of
FeSO4Á7H2O and Fe2(SO4)3 in water at pH 10 (adjusted using
(25%) ammonium hydroxide solution) followed by heating in a
water bath at 50 1C for 1 h. The reaction mixture was cooled to
room temperature, tetraethyl orthosilicate (TEOS) was added and
vigorous stirring continued for 18 h under ambient conditions. To
this solution, RuCl3 was added and the pH of the solution was
adjusted to B10 using ammonium hydroxide solution (25%) and
stirring continued for another 24 h (Scheme 1).
alumina,7 potassium fluoride doped Al2O3 and phosphates,9
8
silica supported manganese oxides,10 modified hydroxyapatite,11
and ruthenium hydroxide coated on alumina and ferrites.12
However, turnover numbers of these protocols are still very
low and the reusability of the catalyst is difficult. Pioneers in this
field, Cadierno et al., recently developed an excellent hydration
protocol in pure water under neutral conditions.13 Although this
work opened the way for the advancement of the hydration
reaction in aqueous medium, the protocol still needs traditional
workup, using organic solvents to isolate the product and exotic
ruthenium complexes as catalysts.
Magnetic silica supported Ru-hydroxide nanoparticles were
separated using an external magnet, washed with water,
followed by acetone, and dried under vacuum at 50 1C for
8 hours. Catalyst characterization by X-ray diffraction (XRD)
(Fig. 1b and c) and transmission electron microscopy (TEM)
(Fig. 1a) confirms the formation of single-phase silica coated Fe3O4
nanoparticles, nano-Fe@SiO2Ru, with spherical morphology and
Consequently, the development of mild and efficient hydration
of nitriles under eco-friendly conditions for the synthesis of amides
Sustainable Technology Division, National Risk Management
Research Laboratory, U. S. Environmental Protection Agency,
MS 443, Cincinnati, Ohio 45268, USA.
E-mail: varma.rajender@.epa.gov; Fax: +1 513-569-7677;
Tel: +1 513-487-2701
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc32566g
Scheme 1 One pot synthesis of the nano-Fe@SiO2Ru catalyst.
c
6220 Chem. Commun., 2012, 48, 6220–6222
This journal is The Royal Society of Chemistry 2012

View Article Online
Using the above-optimized conditions, the scope of magnetic
silica supported ruthenium catalyst nano-Fe@SiO2Ru was then
explored for the hydration of a variety of nitriles (Table 2); the
catalyst displayed high activity for hydration of activated,
inactivated, aliphatic, and heterocyclic nitriles in pure water.
It also displayed high activity for a variety of benzonitrile
derivatives (Table 2, entries 1 and 11) as well as aliphatic
nitriles (Table 2, entry 11) that were smoothly hydrated to the
corresponding amides in excellent yield. The rates were barely
influenced by the electronic effects of the substituents on the
aromatic ring of the benzonitriles. However, hydration of
isonicotinonitrile N-oxide does not yield the corresponding
isonicotinoamide N-oxide, instead isonicotinoamide was
obtained (Table 2, entry 8). It was interesting to observe that
the hydration of 1,4-dicyanobenzene was time dependent
(Table 2 entries 9 and 10). 45 min exposure to MW at 100 1C
leads to the selective formation of the monohydrated derivative
(Table 2, entry 9) whereas exposure for longer time (2.5 h) resulted
in the dihydration of 1,4-dicyanobenzene to terephthalamide in
quantitative yield (Table 2, entry 10).
Fig. 1 (a) TEM of Fe3O4@SiO2Ru (Fe@SiO2Ru), (b) XRD of
Fe3O4, (c) XRD of Fe3O4@SiO2Ru (Fe@SiO2Ru).
a size range of 15–30 nm. The signals pertaining to Ru metal
and Si were not detected in XRD (Fig. 1c), indicating that the
Ru and Si species are highly dispersed on ferrites. The weight
percentage of Ru and Si was found to be 3.96% and 6.85%,
respectively, by inductively coupled plasma-atomic emission
spectroscopy (ICP-AES) analysis.
The utility of the magnetic silica supported ruthenium
nanocatalyst was then demonstrated in heterogeneously catalyzed
hydration of nitriles in aqueous medium as a benign solvent18
under microwave (MW) irradiation conditions (Scheme 2).
MW-assisted chemistry is used due to the efficiency of the
interaction of polar nanocatalysts as well as water molecules with
microwaves and the reaction mixture is rapidly heated to requisite
temperatures under MW irradiation with the precise control of the
reaction temperature.19,20 Initially, experiments were performed to
optimize the reaction conditions for the hydration of benzonitrile
as a substrate, in aqueous medium (Table 1).
The separation and recovery of the catalyst is an important step
in the synthesis of fine chemicals, which is generally performed
either by centrifugation or by filtration with reduced efficiency.
Table 2 Hydration of nitriles catalyzed by the nano-
Fe@SiO2Ru catalyst
Temperature/
Entry Substrate Time 1C
Producta
Conversionb,c
First, the reaction was conducted using nanoferrites nano-Fe3O4
and magnetic silica nano-Fe3O4@SiO2. The hydration reaction did
not proceed at 100 1C or at 150 1C even after 30 min of MW
exposure (Table 1, entries 1–4). The magnetic silica catalyst
Fe3O4@SiO2 was modified using RuCl3 (particle size 15–30 nm),
termed magnetic silica supported ruthenium catalyst nano-
Fe@SiO2Ru (Scheme 1), and tested for the hydration of
benzonitrile with nano-Fe@SiO2Ru at 90 1C for 30 min under
MW irradiation. The low conversion of nitrile to amide (up to
50%, monitored by GC-MS) was observed (Table 1, entry 5).
However, MW exposure for 1 h at 100 1C leads to the
quantitative conversion of benzonitrile to the corresponding
amide (Table 1, entry 6).
1
2
60 min 100
60 min 100
499%(88%)
499%(85%)
3
4
5
6
7
8
9
1.2 h 100
45 min 100
45 min 100
45 min 100
60 min 100
45 min 100
45 min 100
499%(83%)
499%(87%)
499%(88%)
499%(88%)
499%(84%)
499%(78%)
484.3%(72%)
Scheme 2 Nano-Fe@SiO2Ru-catalyzed hydration of nitriles.
Table 1 Optimization of reaction conditionsa
Entry
Catalyst
Time
Temperature/1C
Yieldb
1
2
3
4
5
6
Fe3O4
Fe3O4
Fe3O4@SiO2
Fe3O4@SiO2
nano-Fe@SiO2Ru
nano-Fe@SiO2Ru
30 min
30 min
30 min
30 min
30 min
60 min
100
150
100
150
90
—
—
—
—
450%
499%
10
2.5 h 100
1.2 h 100
499%(86%)
499%(83%)
11
100
a
a
Reactions were carried out with 1 mmol of nitrile, 100 mg nano-
b
catalyst, in water under MW irradiation. Reactions were monitored
Reactions were carried out with 1 mmol of benzonitrile, 100 mg
b
nanocatalyst, in water under MW irradiation. Reaction was
monitored by GCMS.
c
by GCMS. Isolated yield.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6220–6222 6221

View Article Online
In this catalytic system, because of the super-paramagnetic
nature of nano-Fe@SiO2Ru, it can be recovered simply by
using an external magnet. Upon completion of the reaction,
the reaction mixture turned clear and the catalyst deposited on
the magnetic bar, which can be easily separated from reaction
mixture using an external magnet. After the removal of the
catalyst, pure crystals of amides usually appeared in the
aqueous layer, which is isolated from the water medium by
simple decantation. Thus, synthesis and purification can be
carried out in a benign aqueous medium, without the use of
organic solvents.
4 K. Ingvosersen and J. Kamphuis, in Enzyme Catalysis in Organic
Synthesis, ed. K. Drauz and H. Waldmann, VCH, Weinheim,
1995, vol. 1, p. 365.
5 (a) J. H. Kim, J. Britten and J. Chin, J. Am. Chem. Soc., 1993,
115, 3618; (b) R. Breslow, R. Fairweather and J. Keana, J. Am.
Chem. Soc., 1967, 89, 2135; (c) K. L. Breno, M. D. Pluth and
D. R. Tyler, Organometallics, 2003, 22, 1203; (d) S. I. Murahashi,
S. Sasao, E. Saito and T. Naota, J. Org. Chem., 1992, 57, 2521;
(e) M. C. K. B. Dojoman and A. N. Ajjou, Tetrahedron Lett., 2000,
41, 4845; (f) G. Villain, A. Gasket and P. H. Kalck, J. Mol. Catal.,
1981, 12, 103; (g) N. V. Kaminskaia and N. M. Kostic, J. Chem.
Soc., Dalton Trans., 1996, 3677; (h) T. Ghaffar and A. W. Parkins,
Tetrahedron Lett., 1995, 36, 8657; (i) T. Ghaffar and
A. W. Parkins, J. Mol. Catal. A: Chem., 2000, 160, 249.
6 (a) A. Goto, K. Endo and S. Saito, Angew. Chem., 2008, 120, 3663;
(b) T. Oshiki, H. Yamashita, K. Sawada, M. Utsunomiya,
K. Takahashi and K. Takai, Organometallics, 2005, 24, 6287;
(c) V. Y. Kukushkin and A. J. L. Pombeiro, Inorg. Chim. Acta,
2005, 358, 1; (d) N. A. Bokach and V. Y. Kukushkin, Russ. Chem.
Rev., 2005, 74, 153; (e) V. Y. Kukushkin and A. J. L. Pombeiro,
Chem. Rev., 2002, 102, 1771.
7 C. P. Wilgus, S. Downing, E. Molitor, S. Bains, R. M. Pagni and
G. W. Kabalka, Tetrahedron Lett., 1995, 36, 3469.
8 C. G. Rao, Synth. Commun., 1982, 12, 177.
9 S. Sebti, A. Rhiil, A. Saber and N. Hanafi, Tetrahedron Lett., 1996,
37, 6555.
10 K. T. Li, M. H. Shih, H. W. Huang and C. J. Hu, Synthesis, 1988,
715.
11 (a) F. Bazi, H. El Badaoui, S. Tamani, S. Sokori, A. Solhy,
D. J. Macquarrie and S. Sebti, Appl. Catal., A, 2006, 311, 211;
(b) K. Mori, K. Yamaguchi, T. Mizugaki, K. Ebitani and
K. Kaneda, Chem. Commun., 2001, 461.
The lifetime of the catalyst and its level of reusability are
important considerations for practical applications. To clarify
this issue, a set of experiments for the hydration of 4-cyano
pyridine using the nano-Fe@SiO2Ru catalyst were established.
After the completion of the first reaction to afford the corres-
ponding amide, the catalyst was recovered magnetically,
washed with acetone, and dried at 50 1C. A new reaction was
then performed with fresh 4-cyano pyridine under the same
conditions. The nanomagnetic silica supported ruthenium
catalyst nano-Fe@SiO2Ru could be used at least three times
without any change in the activity (ESIw and Table 1).
Metal leaching was studied by ICP-AES analysis of the
catalyst before and after the three reactions. The Ru concen-
tration was found to be 3.96% before the reaction and 3.87%
after the reaction. The TEM image of the catalyst taken after
the third cycle of the reaction did not show significant change
in the morphology or in the size of the catalyst nanoparticles
(15–30 nm) (ESIw and Fig. 1), which indicates the retention of
the catalytic activity after recycling. No Ru metal was detected
in the reaction solvent (water) after completion of the reaction.
This confirms the fact the nanomagnetic silica held the ruthenium
metal very tightly thus minimizing the deterioration of the
catalyst, minimizing metal leaching, and facilitating efficient
catalyst recycling.
12 (a) K. Yamaguchi, M. Matsushita and N. Mizuno, Angew. Chem.,
2004, 116, 1602; (b) M. Kotani, T. Koike, K. Yamaguchi and
N. Mizuno, Green Chem., 2006, 8, 735.
13 V. Cadierno, J. Francos and J. Gimeno, Chem.–Eur. J., 2008,
14, 6601.
14 (a) A. Hu, G. T. Yee and W. Lin, J. Am. Chem. Soc., 2005,
127, 12486; (b) N. T. S. Phan, C. S. Gill, J. V. Nguyen, Z. J. Zhang
and C. W. Jones, Angew. Chem., 2006, 118, 2267; (c) R. Abu-
Reziq, H. Alper, D. Wang and M. L. Post, J. Am. Chem. Soc.,
2006, 128, 5279; (d) C. O. Dalaigh, S. A. Corr, Y. Gunko and
S. J. Connon, Angew. Chem., 2007, 119, 4407; (e) D. Guin,
B. Baruwati and S. V. Manorama, Org. Lett., 2007, 9, 1419;
(f) T. Hara, T. Kaneta, K. Mori, T. Mitsudome, T. Mizugaki,
K. Ebitanic and K. Kaneda, Green Chem., 2007, 9, 1246;
(g) B. Baruwati, D. Guin and S. V. Manorama, Org. Lett., 2007,
9, 5377; (h) A. H. Lu, E. L. Salabas and F. Schuth, Angew. Chem.,
2007, 119, 1242; (i) A. H. Latham and M. E. Williams, Acc. Chem.
Res., 2008, 41, 411; (j) C. Gonzalez-Arellano, J. M. Campelo,
D. J. Macquarrie, J. M. Marinas, A. A. Romero and R. Luque,
ChemSusChem, 2008, 1, 746.
15 (a) V. Polshettiwar, M. N. Nadagouda and R. S. Varma, Chem.
Commun., 2008, 6318; (b) M. N. Nadagouda, V. Polshettiwar and
R. S. Varma, Aust. J. Chem., 2009, 62, 260; (c) V. Polshettiwar,
B. Baruwati and R. S. Varma, Green Chem., 2009, 11, 127;
(d) V. Polshettiwar, B. Baruwati and R. S. Varma, Chem. Commun.,
2009, 1837; (e) R. Luque, B. Baruwati and R. S. Varma, Green
Chem., 2010, 12, 1540; (f) A. M. Balu, B. Baruwati, E. Serrano,
J. Cot, J. Garcia-Martinez, R. S. Varma and R. Luque, Green
Chem., 2011, 13, 2750; (g) R. B. Nasir Baig and R. S. Varma, Green
Chem., 2012, 14, 625; (h) R. B. Nasir Baig and R. S. Varma, Chem.
Commun., 2012, 48, 2582.
We have developed a novel magnetic silica supported
ruthenium nanocatalyst, which can be readily prepared in gram
quantity in one-step under ambient conditions in aqueous media.
This nanomaterial then catalyzed the hydration of nitriles with
high yield and excellent selectivity, which proceeds exclusively in
aqueous medium under neutral conditions. Magnetic separability
eliminated the requirement of catalyst filtration after completion
of the reaction, which is an additional attribute of the catalyst.
Nasir Baig R. B. was supported by the Postgraduate
Research Program at the National Risk Management Research
Laboratory administered by the Oak Ridge Institute for Science
and Education through an interagency agreement between the
U.S. Department of Energy and the U.S. Environmental
Protection Agency.
16 V. Polshettiwar and R. S. Varma, Chem.–Eur. J., 2009, 15, 1582.
17 S. E. G. Garrido, J. Francos, V. Cadierno, J. M. Basset and
V. Polshettiwar, ChemSusChem, 2011, 4, 104.
18 J. H. Clark and S. J. Tavener, Org. Process Res. Dev., 2007,
11, 149.
19 (a) D. Dallinger and C. O. Kappe, Chem. Rev., 2007, 107, 2563;
(b) R. B. Nasir Baig and R. S. Varma, Chem. Soc. Rev., 2012,
41, 1559; (c) R. B. Nasir Baig and R. S. Varma, Chem. Commun.,
2012, 5853.
Notes and references
1 (a) C. E. Mabermann, in Encyclopedia of Chemical Technology,
ed. J. I. Kroschwitz, Wiley, New York, 1991, vol. 1, p. 251;
(b) The Chemistry of Amides, ed. J. Zabicky, Wiley-Interscience,
New York, 1970.
2 R. Opsahl, in Encyclopedia of Chemical Technology, ed. J. I. Kroschwitz,
Wiley, New York, 1991, vol. 2, p. 346.
3 P. K. Mascharak, Coord. Chem. Rev., 2002, 225, 201, and references
therein.
20 C. J. Li and L. Chen, Chem. Soc. Rev., 2006, 35, 68.
c
6222 Chem. Commun., 2012, 48, 6220–6222
This journal is The Royal Society of Chemistry 2012