PEGylated magnetic nanoparticles ([email protected]3O4) as cost effective alternative for oxidative cyanation of tertiary amines via CH activation

Applied Catalysis A: General 498 (2015) 25–31

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

PEGylated magnetic nanoparticles (PEG@Fe3 O4 ) as cost effective alternative for oxidative cyanation of tertiary amines via C H activation Vineeta Panwar, Pawan Kumar, Ankushi Bansal, Siddharth S. Ray, Suman L. Jain ∗ Chemical Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India

a r t i c l e

i n f o

Article history: Received 19 December 2014 Received in revised form 27 February 2015 Accepted 17 March 2015 Available online 24 March 2015 Keywords: Oxidative cyanation Polyethylene glycol Heterogeneous catalyst Magnetic nanoparticle C H activation

a b s t r a c t An efficient, cost effective and environmental friendly PEGylated magnetic nanoparticle catalyzed oxidative cyanation via C H activation of tertiary amines to corresponding ␣-aminonitriles using hydrogen peroxide as oxidant and sodium cyanide as cyanide source is described. The synthesized nanocatalyst was easily recovered with the help of external magnet and was successfully reused for several runs without any significant loss in catalytic activity. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Development of efficient and selective catalytic methods for C H bond functionalization is an area of current research interest [1–5]. The direct utilization of C H bonds offer several advantages such as it avoids the pre-functionalization of the substrates and reduces the synthetic procedures, which makes the synthesis more attractive from both environmental and economical viewpoints [6–11]. In this regard, transition-metal-catalyzed C H bond activation, particularly of unactivated sp3 C H bonds, has emerged to be a promising but highly challenging approach in organic synthesis [12–15]. Recently, the direct oxidative cyanation of C H bonds in tertiary amines to give corresponding ␣-aminonitriles has attracted much interest as these compounds are highly useful and versatile intermediates which find extensive applications in a wide range of natural products and nitrogen containing bioactive compounds such as alkaloids [16–19]. Further, these bi-functional organic compounds having adjacent functional groups show dual reactivity as the nucleophilic addition provides an easy access to various compounds such as ␣-amino aldehydes, ketones and ␤amino alcohols [20–25]. So far a number of metal-based catalysts, such as Fe [26–28], Ru [29,30], V [31], Mo [32], Au [33] and Re [34] in the presence of oxidants such as molecular oxygen, hydrogen

peroxide, tert-butyl hydroperoxide (TBHP) have been reported for the direct oxidative cyanation of tertiary amines. Photoredox catalysts mainly based on ruthenium have also been reported for the oxidative ␣-cyanation of sp3 C H bonds of tertiary amines [35–38]. However, the use of expensive metals such as ruthenium, rhenium, multi-step as well as high cost synthesis of catalysts make these methods less desirable from environmental and economic points of view. Furthermore, in some instances inefficient recovery of the catalyst still leave a scope for developing an efficient and improved method for the direct synthesis of ␣-aminonitriles from the oxidative cyanation of tertiary amines. In continuation to our efforts towards developing efficient and cost-effective methodologies for organic transformations [39–41], herein we report a facile, highly efficient and selective method for the oxidative cyanation of tertiary amines via C H bond activation using PEGylated magnetic nanoparticles as catalyst in the presence of hydrogen peroxide as oxidant and NaCN in acetic acid as a cyanide source (Scheme 1). Furthermore, facile recovery of the catalyst with the use of an external magnet makes the developed method attractive for wider applications. 2. Experimental 2.1. Materials

∗ Corresponding author. Tel.: +91 135 2525788; fax: +91 135 2660202. E-mail address: [email protected] (S.L. Jain). http://dx.doi.org/10.1016/j.apcata.2015.03.018 0926-860X/© 2015 Elsevier B.V. All rights reserved.

Ferric chloride hexa-hydrate (FeCl3 ·6H2 O), ferrous chloride tetra-hydrate (FeCl2 ·4H2 O) and ammonium hydroxide (NH4 OH,

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V. Panwar et al. / Applied Catalysis A: General 498 (2015) 25–31

2.4. Succinic acid functionalized Fe3 O4 nanoparticles [43]

Scheme 1. PEGylated magnetic nanoparticles catalyzed oxidative cyanation of tertiary amines.

25% of ammonia), succinic acid (>99%), polyethylene glycol (PEG-300) N-(3-dimethylaminopropyl)-N -ethylcarbodiimide hydrochloride (EDC) and ion exchange resin (Indion 130) was purchased from Aldrich were of analytical grade and used without further purification. Acetone, methanol was all analytical grade reagents and stored in cold and dark. Distilled water was used throughout. Hydrogen peroxide (35%) and ethanol was of analytical grade and procured from Alfa Aesar. All other chemicals were of A.R. grade and used without further purification. 2.2. Techniques used The rough surface morphology of the synthesized catalyst was determined with the help of scanning electron microscopy (SEM) by using Jeol Model JSM-6340F. Inner fine structure of samples was determined with high resolution transmission electron microscopy using FEI-TecnaiG2 TwinTEM operating at an acceleration voltage of 200 kV. The samples for TEM analysis were made by depositing very dilute aqueous suspension of samples on carbon coated TEM grid. Phase structure and crystalline state of material was determined on Bruker D8 Advance diffractometer at 40 kV and 40 mA with Cu K␣ radiation ( = 0.15418 nm). For XRD, samples were prepared on glass slide by adding well dispersed catalyst in slot and drying properly. Solid UV–visible spectra of samples were collected on Perkin Elmer lambda-19 UV–vis–NIR spectrophotometer using a 10 mm quartz cell, using BaSO4 as reference. The functional groups entities were confirmed by Fourier transform infrared spectra using Perkin–Elmer spectrum RX-1 IR spectrophotometer having Potassium bromide window. Nitrogen adsorption desorption isotherm was used for calculating surface properties like Brunauer–Emmet–Teller (SBET ) surface area, Barret–Joiner–Halenda (BJH) porosity (rp), pore volume (VP ) of samples at 77 K by using VP ; Micromeritics ASAP2010. The thermal degradation pattern of samples for calculating amount of PEG loading on Fe3 O4 nanoparticles was estimated by thermo gravimetric analyses (TGA) using a thermal analyzer TA-SDT Q-600. The TGA analysis was carried out in the temperature range of 40–800 ◦ C under nitrogen flow with heating rate 10 ◦ C/min. 1 H NMR and 13 C NMR spectra of the cyanation products were recorded at 500 MHz by using Bruker Avance-II 500 MHz instrument. 2.3. Synthesis of Fe3 O4 nanoparticles [42] The Fe3 O4 magnetic nanoparticles were synthesized by coprecipitation of Fe+2 and Fe+3 solutions under alkaline conditions. In brief 1.99 g (10 mmol) of FeCl2 ·4H2 O and 3.24 g (12 mmol) of FeCl3 ·6H2 O were dissolved in 50 mL of distilled water. A separate solution of NH4 OH was made by dissolving 30 mL NH4 OH (25% ammonia) in 50 mL of distilled water. Both the solutions in the beaker were allowed to stir for about half an hour, to achieve uniform mixing. After that NH4 OH solution was added drop wise into the first solution till a pH of 9 is obtained. The obtained solution was stirred continuously that generates magnetic nanoparticles. The obtained precipitates were separate with the help of external magnet and washed with distilled water and ethanol and dried at 120 ◦ C overnight then grinded. Finally the black coloured iron oxide nanoparticles were obtained.

To getting succinic acid functionalized Fe3 O4 , succinic acid (2.4 g) was added to an aqueous solution of Fe3 O4 nanoparticles (150 mL, 0.3 mg) and this reaction mixture was allowed to stir for 24 h under vigorous stirring. The synthesized succinic acid functionalized Fe3 O4 nanoparticles were recovered by external magnet and washed with water continuously to wash away unreacted succinic acid. 2.5. Synthesis of PEGylated magnetic nanoparticles (PEG@Fe3 O4 ) EDC (120 mg, 0.6 mmol) and ion exchanger (180 mg, 1.5 mmol) were added to an aqueous solution of Fe3 O4 –succinic acid nanoparticles (25 mL, 60 mg) and the mixture was stirred for 30 min at room temperature. Afterwards, PEG-300 (3.33 mL) was added to the reaction mixture and stirred for 24 h at room temperature. Finally, Fe3 O4 –succinic acid–PEG nanoparticles were recovered and purified as above by means of three steps of magnetic separation, removal of the supernatant and washing with water to obtain 1.2 g of Fe3 O4 –succinic acid–PEG nanoparticles as a black powder. 2.6. General experimental procedure For the cyanation reaction, tertiary amine (1 mmol), NaCN (1.2 mmol), MeOH (4 mL), catalyst (0.1 g), and AcOH (1 mL) was charged in a 25 mL round bottomed flask equipped with a magnetic stirrer. Aqueous hydrogen peroxide (2.5 mmol, 35 wt%) was added drop wise over a period of 30 min to the resulting stirred reaction mixture, and the stirring was continued at room temperature. The progress of reaction was monitored by TLC. After completion of the reaction, the catalyst was recovered by an external magnet. Dichloromethane was added in the obtained solution. The organic layer was washed with water, dried over anhydrous Na2 SO4 and concentrated under vacuum to give crude product, which was purified by flash chromatography to afford pure ␣-aminonitrile. The product of cyanation of tertiary amines to corresponding ␣aminonitriles was identified with the help of GC–MS (EI quadrupol mass analyzer, EM detector) by comparing their spectral data with authentic samples. The yield and selectivity of product was also determined with GC–MS. 3. Results and discussion 3.1. Synthesis and characterization of catalyst The magnetic Fe3 O4 nanoparticles were synthesized by using co-precipitation method by mixing of acidic solution of Fe+2 , Fe+3 and dropping it in a weak alkaline solution as following the literature procedure [42]. The obtained Fe3 O4 nanoparticles were subsequently functionalized with succinic acid as a linker to provide active COOH groups for ester bond formation with PEG [43]. Thus obtained succinic acid modified magnetic nanoparticles were treated with polyethylene glycol (PEG300 ) in the presence of EDC (N-(3-dimethylaminopropyl)-N -ethylcarbodiimide hydrochloride) and ion exchanger (Indion 130) as shown in Scheme 2. The rough surface morphology of magnetic nanoparticles (Fe3 O4 ) and PEG@Fe3 O4 catalyst was determined by scanning electron microscopy (SEM) as shown in Fig. 1. The SEM image of Fe3 O4 confirmed that small sized particles in the range of 50–100 nm were obtained (Fig. 1a). However, in case of PEGylated nanoparticles some lumps type morphology was observed, which is probably due to the coating of magnetic nanoparticles with PEG molecules (Fig. 1b). The EDX pattern of Fe3 O4 nanoparticles (Fig. 1c) gave sharp

V. Panwar et al. / Applied Catalysis A: General 498 (2015) 25–31

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Scheme 2. Synthesis of PEGylated magnetic nanoparticles (PEG@Fe3 O4 ).

peaks of iron, which was found to be decreased significantly after coating with PEG molecules (Fig. 1d). The fine structure of Fe3 O4 nanoparticles and PEG@Fe3 O4 was executed with TEM. TEM image of Fe3 O4 showed that most of the nanoparticles were of spherical in shape and are in 10–25 nm in size (Fig. 2a). After the coating of PEG the spherical geometry remained intact, however the surface has become rough which is assumed due to the coating of magnetic nanoparticles with PEG (Fig. 2b). The bright spots in SAED pattern clearly indicated that crystalline nature of Fe3 O4 remained intact during the coating step (Fig. 2c). FT-IR spectra of Fe3 O4 (Fig. 3a) showed characteristic peaks at 574 cm−1 and 1384 cm−1 due to Fe O bending and Fe O stretch vibrations. The peak at 1627 cm−1 was attributed to the bending vibration of water adsorbed on the surface of Fe3 O4 . The peak at 3415 cm−1 was appeared due to the OH present on the surface of Fe3 O4 nanoparticles [44,45]. The FT-IR spectra of PEG300 showed its characteristic vibrations at 3405 cm−1 and 1403 cm−1 due to the OH stretching and bending vibration respectively along with at 2918 cm−1 (C H stretch), 1595 cm−1 ( COO stretch), 1460 cm−1 (C H bending and scissoring), 1342 cm−1 ( OH bending), 1209 cm−1 (C O stretch), and 1164 cm−1 ( C O C stretch) and other peaks in fingerprint region (Fig. 3b) [46,47]. The presence of characteristic peaks of PEG in the FTIR spectra of PEG@Fe3 O4 further confirmed the successful formation of PEGylated magnetic Fe3 O4 nanoparticles (Fig. 3c). XRD diffractogram of Fe3 O4 showed the characteristic peaks at 2 value 30.28◦ (2 2 0), 35.48◦ (3 1 1), 43.36◦ (4 0 0), 54.04◦ (4 2 2), 57.28◦ (5 1 1) and 62.96◦ (4 4 0) that were matched well with JCPDS card No. 65–3107 (Fig. 4a) [48–50]. The high intensity of peaks confirmed the crystalline nature of the magnetic Fe3 O4 nanoparticles. While in PEGylated magnetic (PEG@Fe3 O4 ) nanoparticles the peaks of Fe3 O4 at 35.48◦ (3 1 1), 43.36◦ (4 0 0), 57.28◦ (5 1 1) and 62.96◦ (4 4 0) were observed but the intensity of peaks was significantly

reduced due to coating of amorphous PEG on the surface of Fe3 O4 (Fig. 4b). Nitrogen adsorption desorption isotherm was used for determining the surface properties. For Fe3 O4 the loop of isotherm was of type (IV) confirms the mesoporous nature of material (Fig. 5a) [51] BET surface area (SBET ), total pore volume (VP ) and mean pore diameter (rp ) for Fe3 O4 was found to be 53.24 m2 g−1 , 0.12 cm3 g−1 and 2.58 nm respectively. For PEG@Fe3 O4 the isotherm was type (IV) and BET surface area (SBET ), total pore volume (VP ) and mean pore diameter (rp ) for Fe3 O4 was found to be 83.93 m2 g−1 , 0.3759 cm3 g−1 and 7.98 nm respectively (Fig. 5b). This change in surface properties was assumed due to coating of PEG on the surface of Fe3 O4 nanoparticles the rough surface provides more pore for the adsorption of nitrogen. Solid UV–visible absorption spectra of magnetic Fe3 O4 nanoparticles gave a broad absorption pattern from 200 to 600 nm, which is attributed to the d-orbital transitions of Fe3 O4 (Fig. 6a) [52]. In case of PEGyted Fe3 O4 nanoparticles, the intensity of absorption pattern was increased, but there was no specific absorption band was appeared due to absence of conjugation in PEG (Fig. 6b). The increased intensity of absorption in PEG@Fe3 O4 was assumed due to mixed transition of composite. Thermal degradation behaviour of the synthesized Fe3 O4 nanoparticles and PEGylated Fe3 O4 nanoparticles was elucidated by TGA (Fig. 7). Because the TG was measured at N2 atmosphere, the oxidation of Fe3 O4 NPs was greatly reduced. The weight loss below 200 ◦ C could be attributed to the adsorbed water in the samples (Fig. 7a) [53]. In PEGylated Fe3 O4 nanoparticles a sharp decrease in weight loss between 200 and 800 ◦ C was observed as compared to neat Fe3 O4 NPs. The main weigh loss at 200–350 ◦ C and 450–800 ◦ C could be attributed to first decomposition and second decomposition of organic components, which were presented on the surface of Fe3 O4 NPs (Fig. 7b) [54].

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V. Panwar et al. / Applied Catalysis A: General 498 (2015) 25–31

Fig. 1. SEM images of (a) Fe3 O4 magnetic nanoparticles and (b) PEG@Fe3 O4 and EDX pattern of (c) Fe3 O4 and (d) PEG@Fe3 O4 .

Fig. 2. TEM images of (a) Fe3 O4 , (b) PEG@Fe3 O4 and (c) SEAD pattern of PEG@Fe3 O4 .

3.2. Catalytic activity The catalytic activity of the synthesized PEGylated magnetic NPs was tested for the oxidative cyanation of tertiary amines to ␣-aminonitriles using hydrogen peroxide as oxidant and NaCN in acetic acid as a cyanide donor at room temperature (Scheme 1). N,N-Dimethylaniline was chosen as the model substrate to optimize the reaction conditions with various oxidants and solvents at room temperature using NaCN in acetic acid as the cyanide source

(Table 1). No product was obtained when the reaction was carried out without catalyst or using PEG without containing iron NPs as catalyst under otherwise similar reaction conditions (Table 1, entries 1 and 2). For the comparison purpose we also performed the reaction using neat magnetic NPs as catalyst under identical experimental conditions. The neat magnetic particles were found to be ineffective and gave only trace yield of the desired reaction product (Table 1, entry 3). The poor activity of neat MNPs could be ascribed to the aggregation of MNPs and oxidation of nanoparticles

V. Panwar et al. / Applied Catalysis A: General 498 (2015) 25–31

Fig. 6. UV–vis absorption spectra of (a) Fe3 O4 and (b) PEG@Fe3 . Fig. 3. FTIR spectra of (a) Fe3 O4 , (b) PEG and (c) PEG@Fe3 O4 .

Fig. 4. XRD diffractogram of (a) Fe3 O4 and (b) PEG@Fe3 O4 .

Fig. 7. TGA thermogram of (a) Fe3 O4 and (b) PEG@Fe3 O4 .

Fig. 5. BET Ads Des isotherm and pore size distribution of (a) Fe3 O4 and (b) PEG@Fe3 O4 .

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Table 1 Results of the optimization experimentsa .

Table 2 PEG@Fe3 O4 catalyzed oxidative cyanation of tertiary aminesa .

Entry

Catalyst

Solvent

Oxidant

Yield (%)b

TOF (h−1 )

1 2 3 4 5 6 7 8 9 10 11 12

– PEG Fe3 O4 Succinic acid@Fe3 O4 PEG@Fe3 O4 PEG@Fe3 O4 PEG@Fe3 O4 PEG@Fe3 O4 PEG@Fe3 O4 PEG@Fe3 O4 PEG@Fe3 O4 PEG@Fe3 O4

MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH CH2 Cl2 Water CH3 CN EtOH

H2 O2 H2 O2 H2 O2 H2 O2 H2 O2 – O2 TBHP H2 O2 H2 O2 H2 O2 H2 O2

– – Trace 62.4 75c , 94a , 94.5d – 22 48 5 22 34 86

– – – 12.5 18.8 – 4.4 9.6 1.0 4.4 6.8 17.2

a Reaction conditions: substarte (1 mmol), catalyst (0.1 g), NaCN (1.2 mmol), AcOH (1 mL), solvent (4 mL) in the presence of H2 O2 ; reaction time 5 h. b Isolated yield. c Using 0.05 g of catalyst. d Using 0.2 g of catalyst.

Entry Reactant

N

CH3

Br

N Br

N

N

CH3

N

9

N

N

11

(n-Bu)3 N

18.8

4.5

78

17.3

4.5

89

19.7

4.5

80

17.7

5.0

72

14.4

5.0

81

16.2

4.0

86

21.5

5.5

84

15.2

5.5

86

15.6

5.0

91

18.2

CH3 CH2CN

CH3 CH2CN

Br

N

CH3

CH3 CH2CN

CN

CH3 N

C2H5

C2H5

NC

N NC

10

94

CH3 CH2CN

N

8

13

N

CH3

5

CH3 CH2CN

Me

CH3

7

12

CH3 CH3

CH3

N

Br

CH2CN

N

CH3

N

N

6

CH3

Me

4

CH3

N

CH3

N

2

5

under the reaction conditions. Similarly, succinic acid functionalized magnetic nanoparticles exhibited poor catalytic activity as compared to the PEGylated magnetic NPs for the oxidative cyanation of N,N-dimethylaniline to corresponding ␣-aminonitrile under otherwise identical reaction conditions (Table 1, entries 4 and 5). The superior catalytic activity of the PEGylated magnetic nanoparticles was assumed due to their higher chemical stability, non-aggregation and preventive oxidation due to the presence of PEG coating. Next, we studied the effect of various oxidants such as molecular oxygen, hydrogen peroxide and TBHP under described reaction conditions (Table 1, entries 5 and 7–8). Among the various oxidants, hydrogen peroxide was found to be best oxidant for this transformation (Table 1, entry 5). However there was no reaction occurred in the absence of any oxidant (Table 1, entry 6). The presence of acetic acid was found to be essential and in its absence no reaction was occurred even after prolonged reaction time (10 h). Among the various solvents such as acetonitrile, methanol, dichloromethane and water studied (Table 1, entries 5 and 9–12), methanol was found to be best solvent for the present transformation. Further, we evaluated the effect of catalyst amount on the conversion of N,N-dimethylaniline in methanol at room temperature under otherwise identical experimental conditions. Initially the reaction was found to be increased with increase in catalyst amount from 0.05 to 0.1 g with respect to 1 mmol of the substrate, however further increase in catalyst amount from 0.1 to 0.2 g did not affect the reaction to any significant extend (Table 1, entry 4). With the optimal conditions for the highly efficient and selective oxidative cyanation of tertiary amines in hand, the scope of the reaction was explored for the different substrates under described reaction conditions. The results of these experiments are summarized in Table 2. As shown in Table 2, substituted N,N-dimethylanilines with electron-donating and electron-withdrawing groups were selectively and efficiently converted into the corresponding ␣-aminonitriles in good to excellent yields (Table 2, entries 2–6). N,N-Dimethyl-o-toluidine offered a slightly lower yield (78% yield) than N,N-dimethyl-p-toluidine (89% yield) owing to steric hindrance. In case of N-methyl-Nethylaniline, the N-methyl group was oxidized chemoselectively to give the corresponding N-ethyl-N-phenylaminoacetonitrile in 80% yield (Table 2, entry 7). The developed catalytic system could also be applied efficiently for oxidative cyanation of cyclic amines such as piperidine, and tetrahydroisoquinoline to give the corresponding ␣-aminonitriles in moderate to high yields (Table 2, entries 8–10). Aliphatic tertiary amines like tert-butyl amine did not produce any product (Table 2, entry 11). But the tertiary amines having benzyl

CH3

N

1

3

Time (h) Yield (%)b TOF (h−1 )

Product

N

Ph

CN

Ph

– CH3 N CH3

CH3 N CH3

CH2CN N CH3

CH2CN N CH3

32

–

–

24

34

1.4

24

47

1.9

a Reaction conditions: Substrate (1 mmol), PEG@Fe3 O4 catalyst (0.1 g), NaCN (1.2 mmol), AcOH (1 mL), MeOH (4 mL) in the presence of H2 O2 at room temperature. b Isolated yield.

groups were sluggish in reaction with poor yield (Table 2, entries 12–13). Furthermore, we checked the recycling of the PEGylated MNPs by using N,N-dimethylaniline as a model substrate. After completion of the reaction, the catalyst was easily recovered from reaction mixture by using an external magnet, washed with methanol and dried. The recovered catalyst was used for six runs using fresh substrates and oxidant. The results of recycling experiments are summarized in Fig. 8. As shown in Fig. 8, the yield of the desired product in all cases was found to be almost similar which confirmed that the developed catalyst can be reused efficiently without any significant loss in activity for several runs. Although, the mechanism of reaction is not clear at this stage, the probable mechanistic pathway is shown in the Scheme 3. In analogy to the mechanism proposed by Murahashi et al. [29] we

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References

Fig. 8. Results of recycling experiments.

Scheme 3. Possible mechanism of the reaction.

can assume that the reactive oxo-iron (IV) species derived from iron and H2 O2 abstracts hydrogen from the ␣-carbon of the tertiary amines to give cationic intermediate. The nucleophilic attack of the HCN, generated in situ by the reaction of NaCN and AcOH yielded corresponding ␣-aminonitrile as shown in Scheme 3. 4. Conclusion In summary, we have described a novel, highly efficient and cost effective PEGylated magnetic nanoparticles as catalyst for the oxidative cyanation via C H activation of tertiary amines with hydrogen peroxide in the presence of sodium cyanide in acetic acid as cyanide source at room temperature to give ␣-aminonitriles in high to excellent yields. The developed catalyst was easily recovered with the effect of an external magnet and efficiently recycled for several runs without significant loss of activity. Acknowledgement We are thankful to the Director, CSIR-IIP for his kind permission to publish these results. VP and PK acknowledge the CSIR, New Delhi, for their Research Fellowships. Analytical department of institute is kindly acknowledged for the analysis of samples.

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