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Cite this: Dalton Trans., 2014, 43,12365

Received 23rd April 2014,Accepted 28th May 2014

DOI: 10.1039/c4dt01189a

www.rsc.org/dalton

Magnetite nanoparticles coated with rutheniumvia SePh layer as a magnetically retrievable catalystfor the selective synthesis of primary amides in anaqueous medium†

Hemant Joshi,a Kamal Nayan Sharma,a Alpesh K. Sharma,a Om Prakash,a

Arvind Kumarb and Ajai Kumar Singh*a

The nanostructured magnetic oxide Fe3O4 has been coated with silica and then reacted with phenylsele-

nyl chloride under a N2 atmosphere and RuCl3·xH2O successively in an aqueous medium to prepare

Fe3O4@SiO2@SePh@Ru(OH)x nanoparticles (NPs) for the first time. These magnetically retrievable NPs

have been authenticated using TEM, SEM-EDX and powder-XRD and found to be an efficient catalyst for

one pot conversion (organic solvent not required) of aldehydes, nitriles and benzyl amine to primary

amides in water. For aldehydes and nitriles, the yields of primary amides are up to 93%. These NPs can be

recycled more than 7 times for the conversion of benzonitrile to the corresponding amide. Gram-scale

transformation carried out by using Fe3O4@SiO2@SePh@Ru(OH)x NPs as a catalyst gives ∼86% yield.

Introduction

The amide functional group is important in organic moleculesof biological interest, synthetic organic chemistry and pharma-ceuticals.1 For amide preparation, carboxylic acids or theirderivatives (halides, anhydrides or esters) can be reacteddirectly with amines.1,2 However, the direct methods use toxicand corrosive materials which are sometimes expensive. Also,the by products are sometimes toxic. The reactions are oftenhighly exothermic (difficult to control), complex, time consum-ing and intolerant to sensitive functional groups. Thereforethe synthesis of amides is a problematic, particularly inpharmaceutical industry and research in this area is widelyacknowledged to be a priority.3 Metal-catalyzed transform-ations have emerged in recent years as promising alternativeroutes for amide synthesis and many innovative protocols havebeen developed and covered in a couple of reviews.4 They aregenerally based on substrates other than carboxylic acids andtheir derivatives.5 The catalytic hydration of nitriles6,7 is onesuch promising protocol. The catalysts found to be efficientwere palladium(II) containing γ-Keggin silicodecatungstate,8

Pd NPs,9 supported silver NPs,10 Os–NHC complexes,11 gold,12

CeO2,13 MnO2,

14 hydrotalcite-clay supported nickel NPs,15

alumina,16 potassium fluoride doped Al2O3,17 silica-supported

manganese oxides,18 and ruthenium hydroxide coated onalumina and ferrites.19 The catalytic conversion of aldehydesto amides is another protocol in which Cu(II),20 indium, zinc,21

scandium(III) triflate22 and supported Rh NPs23 have beenreported as efficient catalysts. For these two protocols there areseveral disadvantages such as: the difficulty in separatingmany of these catalysts from the products (resulting in loss ofyield), low TON values, poor recyclability of the catalysts, therequirement of an inert atmosphere for handling air-sensitivemetal catalysts and harsh conditions8 (such as long reactiontimes e.g. 20–48 h, and reaction temperatures in the order of140 °C). Some functional groups do not withstand such harshconditions and the selectivity of the desired product decreases.The separation issues may be resolved and recyclabilityimproved with the use of nanoparticles (NPs) with para/ferro-magetic properties, as they can be separated from the reactionmedium using an external magnet. Fe3O4 magnetic NPs havealready emerged as robust, high surface area heterogeneouscatalyst supports.24,25 The Fe3O4 nanoparticles which arecoated with a thin layer of silica have further advantages suchas their stability and invariant catalytic activity.26 However,they are rarely used in amide synthesis.19,25d Furthermore, tothe best our knowledge, the use of a Se layer in designing suchsystems has not been reported before in the literature. There-fore it was thought worthwhile to layer Fe3O4 NPs with Se andRu (which has been found to be an efficient catalytic centre in

†Electronic supplementary information (ESI) available: NMR spectra, size distri-bution graphs of the NPs, TEM–EDX spectra, and PXRD patterns. See DOI:10.1039/c4dt01189a

aDepartment of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016,

India. E-mail: aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com;

Fax: +91-01-26581102; Tel: +91-011-26591379bAnalytical Sciences Division, Indian Institute of Petroleum, Dehradun 248005, India

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amide synthesis). The utility and importance of such a cata-lytic system is evident from considerable current interest inmetal chalcogenide nanocrystals27 and their applications incatalysis. For example Pd17Se15 NPs grafted on graphene oxidehave been reported as an efficient catalyst for O-arylation reac-tions27a, along with PdP2 NPs grafted on graphene oxide forSuzuki–Miyaura coupling.28 Thus a novel catalytic system(Fe3O4@SiO2@SePh@Ru(OH)x) has been designed by graftingRu(OH)x on selenated silica coated on Fe3O4 and its catalyticapplication in the preparation of primary amides from alde-hydes, nitriles and benzylamine in an aqueous medium havebeen explored. The strong coordination of Se with Ru makesthe catalyst stable, nearly free from leaching (after two uses),and reusable for a greater number of cycles than the earlierreported catalysts.25d In the present paper we report the resultsof these investigations. The amide synthesis is complete in 7 hat a moderate temperature. For gram-scale preparation theyield is promising.

ExperimentalPhysical measurement1H and 13C{1H} NMR spectra were recorded using a BrukerSpectrospin DPX 300 NMR spectrometer at 300.13, and75.47 MHz respectively. The chemical shifts are reported inppm relative to the internal standard (tetramethylsilane) forthe 1H and 13C{1H} NMR spectra. All of the reactions werecarried out in glassware dried in an oven. Powder X-ray diffrac-tion (PXRD) studies were carried out using a Bruker D8Advance diffractometer using Ni-filtered CuKα radiation, ascan speed of 1 s and a scan step of 0.05°. Transmission elec-tron microscopy (TEM) studies were carried out using a JEOLJEM 200CX TEM instrument operated at 200 kV. The speci-mens for these studies were prepared by dispersing the pow-dered sample in ethanol by ultrasonic treatment. A few dropsof the resulting homogenized slurry were put on a porouscarbon film supported on a copper grid and dried in air. Theelemental composition of the NPs was studied with a CarlZEISS EVO5O scanning electron microscope (SEM) and theassociated EDX system Model QuanTax 200, which is based onthe SDD technology and provides an energy resolution of127 eV under Mn-Kα radiation. The sample was mounted on acircular metallic sample holder with a sticky carbon tape. Anestimation of ruthenium in the nanoparticles was carried outusing an ICP-AES instrument (DRE, PS-3000UV, Leeman Labs,Inc. USA).

Chemicals and reagents

FeCl3·6H2O, urea, FeSO4·7H2O, tetraethylorthosilicate (TEOS),RuCl3·xH2O, phenylselenyl chloride, obtained from Sigma-Aldrich (USA) were used as received. The AR grade solvents(toluene, acetone, ethyl acetate and ethanol) were dried anddistilled before use according to standard procedures.29

Synthesis of the magnetic NPs of Fe3O4.30 FeCl3·6H2O

(5.4 g) and urea (3.6 g) were stirred in water (200 mL) at 90 °C

for 2 h. The color of the solution turned brown. The reactionmixture was cooled to room temperature and FeSO4·7H2O(2.8 g) was added. It was followed by 0.1 M NaOH until a pH of10 was achieved. The resulting mixture was treated with ultra-sound in a sealed flask at 40 °C for 30 min. After ageing for5 h, a black powder of Fe3O4 was obtained, washed with water(2 × 50 mL) and dried in vacuo.

Synthesis of Fe3O4@SiO2 NPs.31 The mixture containing 1 g

of Fe3O4, 20 mL of water, 80 mL of ethanol, 3 mL of ammoniaand 3 mL of tetraethylorthosilicate (TEOS) was refluxed for10 h. The black precipitate of Fe3O4@SiO2 was collected usingan external magnet. It was washed with water until the filtratewas neutral and separated magnetically. The separated solidwas washed successively with ethanol and diethyl ether, anddried in vacuo to obtain Fe3O4@SiO2 NPs.

Synthesis of Fe3O4@SiO2@SePh NPs.31 Fe3O4@SiO2 NPs(1 g) were sonicated in 5 mL toluene and 2 mmol ofPhSeCl was added to it under a nitrogen atmosphere. Nitrogen(to remove any generated HCl) was passed through thereaction mixture for 12 h. The resulting brown coloredFe3O4@SiO2@SePh NPs were collected using an externalmagnet. They were washed with toluene and dried in vacuo.

Synthesis of Fe3O4@SiO2@SePh@Ru(OH)x NPs.25d TheFe3O4@SiO2@SePh NPs (2 g) were dispersed in water.RuCl3·xH2O (1 mmol) dissolved in water (60 mL) was added andthe mixture was stirred for 20 min. Aqueous sodium hydroxide(1 M) was added dropwise to the mixture until its pH reached∼13. The resulting slurry was stirred further for 36 h at roomtemperature. The product was separated magnetically, washedseveral times with water and methanol successively and driedin vacuo for 2 h. The weight percentage of Ru in the catalyst wasfound to be 5.48% which was obtained using ICP-AES analysis.

Hydration of nitriles

Nitrile (1.0 mmol) and Fe3O4@SiO2@SePh@Ru(OH)x (80 mg)were placed in an oven-dried flask with a magnetic stirrer.Water (4 mL) was added to the reaction mixture. The reactionmixture was heated using an oil bath (kept at 120 °C), underaerobic conditions (under reflux), and stirred until themaximum conversion of nitrile to amide occurred. After com-pletion of the reaction, the catalyst was removed from the reac-tion mixture using an external magnet. The clear solution wascooled slowly. Most of the time, analytically pure crystals ofthe corresponding amide were obtained, which were separatedby decantation. In the case of solid nitrile the resultingmixture was extracted with ethyl acetate (2 × 50 mL). Theorganic layer was washed with water (2 × 50 mL) and driedover anhydrous Na2SO4. The solvent of the extract was removedwith a rotary evaporator and the resulting residue (amide) waspurified using flash column chromatography on silica gelusing an ethyl acetate–hexane (5 : 95) mixture as the eluent.

Synthesis of amides from aldehydes. Aldehyde (1.0 mmol),NH2OH·HCl (1.2 eq.) and Fe3O4@SiO2@SePh@Ru(OH)x(80 mg) were placed in an oven-dried flask with a magneticstirrer and water (4 mL) was added to the mixture. The reactionmixture was heated using an oil bath (kept at ∼120 °C), under

Paper Dalton Transactions

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aerobic conditions (under reflux), and stirred until themaximum conversion of aldehyde to the corresponding amideoccurred. Using the work up procedure described for the con-version of nitriles, the amide was obtained.

Synthesis of amides from amines. Amine (1.0 mmol),Fe3O4@SiO2@SePh@Ru(OH)x (100 mg) and water (4 mL) wereplaced in an oven-dried flask with a magnetic stirrer. The reac-tion mixture was heated using an oil bath (temperature∼120 °C), under aerobic conditions, and stirred until themaximum conversion of the amine to the correspondingamide occurred. After completion of the reaction, the work upprocedure given above in the conversion of nitriles was fol-lowed to obtain the amide.

Recyclability

To check the recyclability of Fe3O4@SiO2@SePh@Ru(OH)xNPs, the catalyst was separated with an external magnet, aftercompletion of the reaction with benzonitrile. It was washedwith H2O and ethanol successively, dried in vacuo and reusedwith a new sample of benzonitrile.

Hot filtration experiment

In an oven dried flask water (4 mL), Fe3O4@SiO2@SePh@Ru-(OH)x NPs (80 mg) and benzonitrile (0.103 g, 1 mmol) wererefluxed, heated with an oil bath maintained at 120 °C, andstirred. The reaction was stopped after 1.5 h and the catalystwas separated using an external magnet. The reaction mixtureturned clear and was divided in two equal parts. One half ofthe reaction mixture was transferred into another reactionflask. In the remaining half the previously isolated NPs werereintroduced as a catalyst. Thereafter, the reactions in bothvessels were allowed to proceed for another 5 h and conver-sions were estimated with 1H NMR spectroscopy.

Results and discussion

To design Fe3O4@SiO2@SePh@Ru(OH)x NPs, Fe3O4 (size:∼6–15 nm) prepared using a recently reported protocol30 wascoated with silica, and treated with phenylselenyl chlorideunder a N2 atmosphere and RuCl3·xH2O successively in anaqueous medium (Scheme 1). Magnetic Fe3O4@SiO2@-SePh@Ru(OH)x NPs were separated using an external magnet,washed with water, followed by Et2O and dried in vacuo at60 °C for 2 h. Neither Fe3O4@SiO2@SePh@Ru(OH)x nor itsprecursor was found to be soluble in water or the organicsolvent. However, the OH groups are present on the surfacethrough which these NPs show excellent catalytic activity (seethe mechanism in Fig. 3). The weight percentage of Ru inFe3O4@SiO2@SePh@Ru(OH)x NPs was found to be 5.48%,obtained using inductively coupled plasma-atomic emissionspectroscopic (ICP-AES) analysis, and the weight percentage ofSe in Fe3O4@SiO2@SePh@Ru(OH)x NPs was found to be2.55%, obtained using quantitative energy dispersive X-rayspectroscopy. The TEM images (Fig. 2), SEM-EDX spectra (seethe ESI Fig. S3 and S4†) and powder X-ray diffraction (PXRD)

patterns (Fig. 1, see the ESI Fig. S5a–c†), obtained confirmthese NPs as Fe3O4@SiO2@SePh@Ru(OH)x, with nearly spheri-cal morphology (size ∼6–15 nm; size distribution graphs ofFe3O4 and Fe3O4@SiO2@SePh@Ru(OH)x NPs are shown in theESI Fig. S1 and S2†). The nanoparticles have OH groups on thesurface and this has probably affected the quality of the TEMimages to some extent but they are as good as those reportedin an earlier work on a similar system.25d

Powder XRD of Fe3O4@SiO2@SePh@Ru(OH)x catalyst

The phase and purity of the different materials from Fe3O4 toFe3O4@SiO2@SePh@Ru(OH)x synthesized stepwise have beenstudied using powder XRD. The patterns are shown in Fig. 1and in the ESI Fig. S5.† The PXRD pattern of Fe3O4 exhibitsdiffraction lines (hkl) at 220, 311, 400, 333 and 440. Thesediffraction peaks in the pattern can be well indexed to thecubic phase of Fe3O4, as they are in good agreement with theliterature data (JCPDS 82-1533) (Fig. S5a in the ESI†). Thesilica coated nanoparticles (Fe3O4@SiO2) give a broader PXRDpattern due to their non-crystalline nature, at 2θ = 20–29°(JCPDS 83-2470). It has peaks (hkl) at 220, 311, 400, 333, and440, which correspond to the Fe3O4 core (Fig. S5b in the ESI†).On immobilization of selenium onto Fe3O4@SiO2, theadditional peaks due to Se (SePh) appear in PXRD pattern of

Scheme 1 Synthesis of Fe3O4@SiO2@SePh@Ru(OH)x NPs.

Fig. 1 PXRD pattern of Fe3O4@SiO2@SePh@Ru(OH)x NPs.

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Fe3O4@SiO2@SePh (Fig. S5c in the ESI†), with peaks ofFe3O4@SiO2. The selenium peaks are narrow as SePh is in agrafted form and is not nano-structured. Such an observationis reported in case of phosphorus.31 Most of these peaks e.g. at2θ = 22, 32 and 42 (ESI Fig. S5c†) are consistent with earlierreports on powdered/polycrystalline selenium.32 On layeringwith Ru the dispersity of Se increases as it becomes part of themetal’s coordination sphere. Thus peaks corresponding to Sehave not been detected in the PXRD pattern of Fe3O4@SiO2@-SePh@Ru(OH)x (Fig. 1). Ruthenium is present as a complexentity (coordinated with SePh and OH). There is a strongbinding of Ru with selenium donor sites. These features prob-ably result in the high dispersity of Ru also on Fe3O4@SiO2@-SePh. Therefore no Ru peaks were observed.25d No peakscorresponding to any impurities were detected, indicatinggood purity of Fe3O4@SiO2@SePh@Ru(OH)x NPs.

TEM and SEM-EDX of Fe3O4@SiO2@SePh@Ru(OH)x

Transmission electron microscopy (TEM) images of Fe3O4@-SiO2@SePh@Ru(OH)x NPs are shown in Fig. 2. The sizes ofthese NPs are approximately ∼6–15 nm (Fig. S2 in the ESI†).The SEM-EDX spectrum (Fig. S4 in the ESI†) supports the pres-ence of iron, oxygen, silicon, selenium, carbon and rutheniumin Fe3O4@SiO2@SePh@Ru(OH)x NPs. Similarly the SEM-EDXspectrum of Fe3O4@SiO2@SePh given in the ESI (Fig. S3†) alsosupports the presence of iron, oxygen, silicon, selenium, andcarbon in Fe3O4@SiO2@SePh NPs. This indicates that the lossof Se is insignificant during Ru layering.

Catalytic applications

The magnetic Fe3O4@SiO2@SePh@Ru(OH)x NPs were exploredfor the catalytic synthesis of amides from aldehydes, nitriles,

and benzylamine in water, under aerobic reaction conditions.The experiments were performed to optimize the reaction con-ditions for the hydration of benzonitrile as a substrate (Scheme 2),in aqueous medium (see Table 1). The reactions were also con-ducted using nano-sized Fe3O4, Fe3O4@SiO2 and Fe3O4@-SiO2@SePh. The hydration reaction with them did not proceedat 120 °C even after 24 h (Table 1, entries 1–3). The reactionusing RuCl3·xH2O as the catalyst was carried out at 120 °C andonly gave a yield of ∼31% (Table 1, entry 4). The hydrationreaction in the presence of RuCl3·xH2O and diphenyl disele-nide (1 : 1 molar ratio) at 120 °C resulted in ∼27% yield ofamide (Table 1, entry 5). Most probably the insolubility ofdiphenyl diselenide in water may be one of the reasons attribu-ted to the poor yield of amide. On carrying out the reaction indichloromethane no conversion to amide was observed evenafter 7 h (Table 1, entry 8). The hydration reaction usingFe3O4@SiO2@SePh@Ru(OH)x NPs as the catalyst at a tempera-ture of ∼80 °C for 7 h converted only ∼57% of the nitriles toamides, while the reaction in the presence of these NPs at120 °C for 3 h converted ∼62% of the nitriles to amides. Thecontinuation of the reaction further for 7 h at 120 °C led to a92% conversion of benzonitrile to the corresponding amide(Table 1, entry 9). In Fe3O4@SiO2@SePh@Ru(OH)x NPshydroxyl groups are present on the surface which contribute totheir excellent catalytic activity as demonstrated in the mecha-nism shown in Fig. 3. The temperatures mentioned above areof the oil bath. The effect of the magnetic bar on catalytic reac-tion was found to be insignificant as a reaction of benzonitrilewith water under optimum conditions carried out with amechanical stirrer gave 90% conversion. It implies that withhigh speed stirring the magnetite NPs are suspended in solu-tion and the catalytic process occurs as expected (Scheme 2).

Fig. 2 TEM images (a) Fe3O4, (b) Fe3O4@SiO2, (c) Fe3O4@SiO2@SePh,(d) Fe3O4@SiO2@SePh@Ru(OH)x NPs.

Table 1 Optimization of the reaction conditions

Entry CatalystTime(h)

Temp.d

(°C)Yieldc

(%)

1 Fe3O4 24 120 02 Fe3O4@SiO2 24 120 03 Fe3O4@SiO2@SePh 24 120 04a RuCl3·xH2O 7 120 315a RuCl3·xH2O + (PhSe)2 (1 : 1) 7 120 276 Fe3O4@Ru(OH)x 7 120 367 Fe3O4@SiO2@Ru(OH)x 7 120 818b Fe3O4@SiO2@SePh@Ru(OH)x 7 120 09 Fe3O4@SiO2@SePh@Ru(OH)x 7 120 92

Reaction condition: catalyst (80 mg), substrate (1.0 mmol), H2O(4 mL).a 5 mol% Ru. b In dichloromethane. c Selectivity of the reactionis >99%. dOil bath temperature.

Scheme 2 Nano-Fe3O4@SiO2@SePh@Ru(OH)x catalyzed hydration.

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Using the optimized conditions, the scope of usingFe3O4@SiO2@SePh@Ru(OH)x NPs as a catalyst was exploredfor the one pot synthesis of primary amides from a variety ofaldehydes (Table 2) and nitriles (Table 3). The catalyst dis-played high efficiency for the conversion of activated, unacti-vated, and heterocyclic aldehydes in pure water. Theconversion rates were not greatly influenced by the substituenton the aromatic ring of benzaldehyde. Interestingly, the reac-tions of the ortho and meta-derivatives (Table 2, entries 4 and 8)proceeded with similar rates, without a significant differencein yields. Heterocyclic aldehydes (Table 2, entries 9 and 10),which are extensively used as building-blocks in drug discov-ery, undergo the hydration reaction with high yields, provingthe suitability of this protocol for the assembly of bio-mole-cules. The one pot transformation of ferrocenecarboxaldehydeinto ferrocenecarboxamide (Table 2, entry 11) occurred with agood yield (88%). The method commonly used for the prepa-ration of ferrocenecarboxamide has a sequence of two-steps.First ferrocenecarboxylic acid is reacted with thionyl chloride(odour unpleasant) and then treated with aqueousammonia.33 Thus the present protocol has advantages overthis method. There is only one step, a cheap starting materialis used, the reaction proceeds in water which is better for theenvironment, and purification of the product is simple.

The efficiency of the Fe3O4@SiO2@SePh@Ru(OH)x NP cata-lyst (Table 3) for the hydration of activated, unactivated,heterocyclic and aliphatic nitriles in an aqueous medium isalso good. The rate of reaction does not alter much with thesubstituents on the aromatic ring of benzonitrile as the conver-sion rates for the ortho and meta-derivatives (Table 3, entries 4and 8) are almost similar. On a gram-scale, the catalytic con-version of benzonitrile (1 g) to the corresponding amide wascarried out (Scheme 3). The transformation proceeded effici-ently to give 1.04 g of benzamide (86% yield). There is only asmall decrease in the percentage yield with respect to small-scale conversion.

The lifetime and level of reusability are important factors inthe practical applications of a heterogeneous catalyst. In thisstudy, such properties of Fe3O4@SiO2@SePh@Ru(OH)x NPswere tested for the hydration of benzonitrile. After completionof the first reaction to afford the corresponding benzamide,the catalyst was recovered magnetically, washed with a metha-nol–ethanol mixture and dried at 50 °C. It was used again forfurther conversion of benzonitrile to the corresponding amide

Fig. 3 The proposed mechanism for amide synthesis catalyzed withFe3O4@SiO2@SePh@Ru(OH)x NPs.

Table 2 One pot synthesis of amides from aldehydesa

Entry Substrate Product Yield%

1 88 (>99)

2 91 (>99)

3 90 (>99)

4 82 (>99)

5 91 (>99)

6 92 (>99)

7 86 (>99)

8 85 (>99)

9 87 (>99)

10 86 (>99)

11 88 (>99)

a Reaction conditions: catalyst (80 mg), substrate (1.0 mmol),NH2OH·HCl (1.2 eq.), time (7 h), H2O (4 mL), and temp. (120 °C). Theselectivity of the reaction is given in the parentheses.

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under optimum conditions. There was no change in theefficiency as shown by the percentage yield values (Table 4).

The catalyst can be reused more than 7 times with a veryminor change in efficiency (Table 4). The scope of usingFe3O4@SiO2@SePh@Ru(OH)x NPs as a catalyst, was examinedfor the oxidation of amines to amides (Scheme 4). The reactionwas carried out using 1 mmol of benzylamine and 100 mg ofcatalyst at 120 °C under ambient conditions for 7 h and for-mation of the corresponding amide (I), by-product benzylidene-benzylamine (II) and benzaldehyde (III) were observed, withpoor conversion values. When the reaction time was increasedup to 15 h, (I) was obtained with 63% conversion.

The magnetic nature of the Fe3O4@SiO2@SePh@Ru(OH)xNPs means that they are able to be fully recovered with anexternal magnet, without a significant change in efficiency.After the separation of the catalyst, the clear liquid was cooledslowly and in most of the cases analytically pure amide crystalsappeared, which could be separated by simple decantation.Thus, the whole process was carried out in an aqueousmedium and no organic solvent used. The heterogeneity of thecatalyst Fe3O4@SiO2@SePh@Ru(OH)x NPs for the hydration ofbenzonitrile was examined using a hot filtration test. The reac-tion was stopped at 37% conversion and after 1 min the reac-tion mixture separated into a clear solution and a solid(catalyst) deposited on the magnetic bar. Half of the hot solu-tion was transferred into another reaction flask. The reactionin the two portions continued at 120 °C for a further 5 h. Theconversion in the portion containing the magnetic catalyst pro-gressed to 87%, while in the catalyst-free portion it reached42% from 37%, probably due to Ru leaching from the NPs.Thus the catalysis may be considered as largely heterogeneous.Metal leaching was studied using ICP-AES analysis of the cata-lyst before and after the two reaction cycles. The Ru in the cata-lyst was found to be 5.48% before the reaction and 5.37% aftertwo cycles of the reaction. Thereafter no change in Ru contentof the catalyst was observed even after the seventh catalyticcycle. These observations imply negligible Ru leaching aftertwo cycles of catalysis. This may be due to the strong bindingof Ru with Se.34 The outer rim of Ru on the surface of the NPs

Table 3 Hydration of nitrilesa

Entry Substrate Product Yield%

1 92 (>99)

2 93 (>99)

3 91 (>99)

4 84 (>99)

5 93 (>99)

6 90 (>99)

7 87 (>99)

8 86 (>99)

9 89 (>99)

10 87 (>99)

11 81 (>99)

a Reaction conditions: catalyst (80 mg), substrate (1.0 mmol), time(7 h), H2O (4 mL), and temp. (120 °C). The selectivity of the reaction isgiven in the parentheses.

Scheme 3 Gram-scale transformation.

Table 4 Recycling experimentsa

Run 1 2 3 4 5 6 7

Yield% 92 92 92 91 91 90 90

a Reaction conditions: catalyst (100 mg), substrate (1.0 mmol), time(7 h), H2O (4 mL), and temp. (120 °C). The selectivity of the reaction is>99%.

Scheme 4 Catalytic transformation of benzyl/2-thiophene amines.

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is easily accessible for catalysis. It is desirable in a metal pro-moted reaction for there to be no remnant of the metal in theend product, which happens in the present case after thesecond cycle. Thus Fe3O4@SiO2@SePh@Ru(OH)x NPs may besuitable for pharmaceutical preparations also after two uses.

The leached Ru (also from RuCl3·xH2O) appears to havepoor catalytic activity in comparison to the one anchored onmagnetite NPs. Thus strong binding of Ru via Se (strong sigmadonor) results in a significant improvement in the catalyticactivity. Due to this reason, the catalytic performance (underidentical conditions) of Fe3O4@SiO2@SePh@Ru(OH)x (Table 1entry 9) is better than that of the closely related NPsof Fe3O4@SiO2@Ru(OH)x (Table 1 entry 7). In a procedurereported25d previously for a similar amide synthesis withFe3O4@SiO2@Ru(OH)x NPs, there is a microwave effect. Thustheir results are not directly comparable with ours. Further-more, their recyclability is only 3 times. However, our catalysthas a very promising efficiency without a microwave effect.The possible mechanism for amide synthesis catalyzed withFe3O4@SiO2@SePh@Ru(OH)x NPs is shown in Fig. 3. Thiscatalytic amide synthesis probably occurs in four steps. Thefirst step is based on an aldoxime. In the second step a nitrileis formed and coordinated to Ru. The intramolecular nucleo-philic attack of the hydroxide species on the nitrile carbonatom takes place to afford the ruthenium iminolate species inthe third step. The transformation into the original rutheniumhydroxide species with the formation of the amide producttakes place in the last step.

Conclusions

Fe3O4@SiO2@SePh@Ru(OH)x NPs, which have been designedfor the first time, were used as a catalyst in the preparation ofprimary amides from aldehydes (including ferrocenecarbox-aldehyde), nitriles, and benzylamine in an aqueous mediumwith high yield and selectivity. The catalyst is magneticallyseparable which makes the work up of the reaction very easy.The catalyst is reusable for more than 7 reaction cycles for theconversion of benzonitrile to the corresponding amide. It isalso efficient for gram-scale preparation of amides from benzo-nitrile. There is virtually no leaching of Ru after the secondcycle which was confirmed by using ICP-AES analysis and car-rying out a hot filtration test.

Acknowledgements

The authors thank the Council of Scientific and IndustrialResearch (CSIR), New Delhi, India for the project no. 01(2421)10/EMR-II and the JRF/SRF to KNS and AS. The Department ofScience and Technology (India) is acknowledged for theresearch project (SR/S1/IC-40/2010), and the financial supportfor the use of the HR-TEM (NSIT) facility at IIT Delhi. HJ, OPand AK thank the University Grants Commission (India) forthe JRF/SRF.

Notes and references

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Paper Dalton Transactions

12372 | Dalton Trans., 2014, 43, 12365–12372 This journal is © The Royal Society of Chemistry 2014

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