Polymer Supported Nickel Nanoparticles as Recyclable Catalyst for the Reduction of Nitroarenes to Anilines in Aqueous Medium

Polymer supported nickel nanoparticles, generated by calcination under nitrogen of a Ni(II) containing polyacrylamide, catalyzed the hydrogenation of nitroarenes to anilines in aqueous medium at room temperature in the presence of NaBH4. The protocol generally favored the formation of the desired aniline as single product in high yields with short reaction times and proved to be highly selective in the hydrogenation of halonitrobenzene to haloaniline, avoiding the formation of hydro-dehalogenated side-products. The catalyst displayed excellent recyclability over at least five cycles (which is not trivial for nickel nanoparticle based catalysts used in water) and no leaching of metal into solution occurred, that made the overall system eco-friendly and economic.

STEM analyses revealed that the thermally formed Ni nanoparticles turned into cubic nanotwins under reaction conditions, which remained almost unchanged with the re-cycles.

Introduction
The last few years have witnessed an explosion in the use of new nanomaterials for catalytic application.
[1] In this field, noble metal nanoparticles (NPs) have been widely employed in catalysis [2] and several studies have been devoted to the recyclability of these expensive materials.
[3] Nevertheless, the high cost of noble metals has limited their practical use.The substitution of noble metals for abundantly available transition metals is an attractive strategy for lowering production costs and favoring noble-metal conservation in the planet.[4] In this context, recently, due to their low cost, Ni catalysts have been employed in several organic reactions (mainly hydrogenations).[5] The hydrogenation of nitroarenes to give the corresponding aromatic amines is a basic chemical reaction used both for removing toxic nitro aromatics from aqueous medium and for synthetizing anilines.In fact, aromatic amines are important bulk chemicals and intermediates to produce fine chemicals, pharmaceuticals, polymers, herbicides, and more.[6] The industrial hydrogenation of nitrobenzene for the synthesis of aniline uses copper, palladium or palladium-platinum supported on carbon or inorganic oxides, as catalysts, under high pressure (up to 3 MPa) and high temperature (up to 300 °C), and it is not able to avoid the formation of noxious azo-and azoxyderivatives, which lowers the yield into anilines.[6] Aiming at overcoming the above reported weaknesses, the scientific community tried to develop alternative protocols for the reduction of aromatic nitro compounds, [7] including the use of different reducing agents, such as hydrazine, [8] silane [9] and sodium borohydride [10] in the presence of Cu, [11] Pd, [12] Au, [13] Ru, [14] Ag, [15] and, more recently, Co [16] and Ni [17] catalysts.
Nevertheless, the major problems remain still unsolved and designing new systems with different chemical and physical compositions and morphologies for catalytic reduction of nitroarenes under sustainable conditions is an urgent topic of current research.
In the search for innovative catalytic processes that enable chemical transformations under mild, cheap and sustainable conditions with high efficiency, we prepared an acryl amidic polymer supported nickel catalyst (in the following Ni-pol) and we evaluated its catalytic activity in the hydrogenation of nitroarenes in aqueous medium.The real active species were Ni nanoparticles with a peculiar nanostructure stabilized by the insoluble support, which were active, selective and recyclable in the reduction of various nitro arenes into anilines in water in the presence of sodium borohydride, that is a cheap, easy-handling and ready available reducing agent with extensive applications in organic synthesis.[18] 2. Experimental Section

2.1.General considerations
Tap water was de-ionized by ionic exchange resins (Millipore) before use.All other chemicals were purchased from commercial sources and used as received.Nickel content in Ni-pol was assessed after sample mineralization by atomic absorption spectrometry using a Perkin Elmer 3110 instrument.The experimental error on the nickel percentage was ± 0.3.Mineralization of Ni-pol prior to Ni analyses was carried by microwave irradiation with an ETHOS E-TOUCH Milestone applicator, after addition of HCl/HNO 3 (3:1 v/v) solution (12 mL) to each weighted sample.
Microwave irradiation up to 1000 W was used, the temperature being ramped from rt to 220 °C in 10 min and the sample being held at this temperature for 10 min.After cooling to room temperature, the digested Ni-pol was diluted to 1000 mL before submitting to Graphite Furnace Atomic Absorption Spectrometric nickel determination.
GC-MS data (EI, 70 eV) were acquired on a HP 6890 instrument using a HP-5MS cross-linked 5% PH ME siloxane (30.0 m × 0.25 mm × 0.25 m) capillary column coupled with a mass spectrometer HP 5973.The products were identified by comparison of their GC-MS features with those of authentic samples.Reactions were monitored by GLC or by GC-MS analyses.GLC analysis of the products was performed using a HP 6890 instrument equipped with a FID detector and a HP-1 (Crosslinked Methyl Siloxane) capillary column (60.0 m x 0.25 mm x 1.0 m).Conversions and yields were calculated by GLC analysis by using biphenyl as internal standard, or by column chromatography using silica gel and n-hexane/ethyl acetate as the eluent.
FT-IR spectra (in KBr pellets) were recorded on a Jasco FT/IR 4200 spectrophotometer.Elemental analyses were obtained on a EuroVector CHNS EA3000 elemental analyser using acetanilide as analytical standard material.The high-resolution mass spectrometry (HRMS) analysis was performed using a Bruker microTOF Q II mass spectrometer equipped with an electrospray ion source operated in positive ion mode.The sample solutions (CH 3 OH) were introduced by operated with endnebulizer pressure was 0.4 bar (N 2 ), and the drying gas (N 2 ) flow rate was 4.0 L min 1 .Capillary exit and skimmer voltages were 90 V and 30 V, respectively.The drying gas temperature was set at 180°C.The calibration was carried out with a sodium formate solution (10 mM NaOH in isopropanol/water 1:1 (+0.2% HCOOH)) and the software used for the simulations was Bruker Daltonics DataAnalysis (version 4.0).Thermogravimetric analyses (TGA) were performed in a nitrogen flow (40 mL min-1) with a Perkin-Elmer Pyris 6 TGA in the range from 30 to 800 °C with a heating rate of 10 °C min-1.Triplicate TGA runs have been performed to ensure reproducibility.
Surface morphology was investigated on a selected piece of Ni-supported catalyst considered to be representative of the material.Nova NanoSEM 450 manufactured by FEI Company, USA, was used to perform FESEM analysis on the selected samples.Tiny plate-like of the powdered catalyst were mounted on TEM copper grids, and gold-palladium sputtered (K550, Emitech Ltd, United Kingdom).Scanning Transmission Electron Microscopy (STEM) Detector allowed transmission images to be taken at 30 keV, lower energy level with respect to commonly used TEM, beam voltage 100-200keV.Resolution limits of this microscope are remarkable: 1.4 nm @ 1 kV in high vacuum mode.The particle sizes were analyzed by STEM image analysis using the ImageJ software (freeware software: http://rsb.info.nih.gov/ij/).

General experimental procedure for the reduction of nitroarenes catalyzed by
Nipol 0.5 mmol of nitroarene, 10.2 mg of Ni-pol (Ni% w =5.35, 9.3 10 -3 mmol of Ni) and 10.0 mmol of sodium borohydride were stirred under nitrogen at room temperature in 2.5 mL of double deionized water and 2.5 mL of diethyl ether for the appropriate amount of time, using a three-necked flask equipped by a gas bubbler to discharge the hydrogen excess produced during reaction.The progress of the reaction was monitored by GLC.After completion of the reaction, the reaction mixture was centrifuged to separate the catalyst.The solid residue was first washed with deionized water and then with acetone and diethyl ether to remove any traces of organic material.The filtrate containing the reaction mixture was extracted with ethyl acetate (3 × 5 mL) and then dried over anhydrous Na 2 SO 4 .The solvent was evaporated under reduced pressure to yield the crude product, which was then purified by column chromatography using silica gel and n-hexane/ethyl acetate as an eluent to afford the pure product.The products were characterized by GC-MS by comparison with authentic samples.For the assessment of the chromatographic yields, biphenyl (50.0 mg) was used as the internal standard.

2.4.Recycling experiments
At the end of reaction, the organic layer was removed with a syringe and the aqueous phase suspending the supported catalyst was washed with diethyl ether (3 × 5 mL), and, then, added of fresh reagents.Iteration of this procedure was repeated for five reuses of the catalytic system.

Synthesis of the catalyst and preliminary catalytic tests
The polymer supported catalyst used in this work was synthetized following an unconventional method, aimed at obtaining a material with a uniform distribution of the Ni(II) centers: i.e. by copolymerization of the metal-containing monomer [19] Ni(AAEMA) 2 [20] [AAEMA = deprotonated form of 2-(acetoacetoxy) ethyl methacrylate] with suitable co-monomer (N,Ndimethylacrylamide) and cross--methylenebisacrylamide) (Scheme 1).The copolymer supporting the metal was a non-hygroscopic green powder insoluble in all solvents.It swelled well in water, acetone, halogenated solvents and dioxane and shrink when treated with diethyl ether, ethyl acetate or petroleum ethers.Its IR spectra in KBr (Figure S3) showed the features of the coordinated -ketoesterate moiety (1527 and 1622 cm -1 ) at roughly the same wavenumbers of the corresponding Ni(AAEMA) 2 complex (1531 and 1623 cm -1 ), thus substantiating that the metalloorganic units in both soluble and supported materials are similar.
This polymer [Ni(AAEMA) 2 -pol] was alike the one already obtained in our labs by copolymerization of Pd(AAEMA) 2 with methacrylic monomers, [21] found active, selective and recyclable in several palladium catalyzed reactions.
[22] This Pd(II) based polymer turned into polymer supported Pd(0) nanoparticles (Pd-pol) under reaction conditions.In fact, as it has been reported for similar Pd nanoparticles supported on methacrylate based resins, [23] the reticular and macro porous polymeric support of Pd-pol could immobilize, stabilize and retain palladium nanoparticles, [24] that were the true catalytically active species.
Furthermore, the good swell ability in water of the methacrylic insoluble support rendered Pd-pol an ideal catalyst for reactions carried out in aqueous solvent, [25] since the migration of the reagents to the active sites would not be hampered by the solid matrix.Due to the analogy between Pd(AAEMA) 2 and Ni(AAEMA) 2 supported polymers, we decided to test the catalytic activity of Ni(AAEMA) 2 -pol in a benchmark reaction [26] in which Pd-pol was particularly active and recyclable: [27] the transfer hydrogenation of nitrobenzene in water using NaBH 4 as the reducing agent, hoping that Ni(II) centers could turn into Ni(0) nanoparticles under reaction conditions, as it happened for Pd-pol.Preliminary catalytic tests were carried out using nitrobenzene as the representative substrate.By employing the same optimized conditions used for the palladium catalyzed nitroarene reduction, [27] the yield into the desired aniline was poor (24 % after 1 h and 47 % after 4 h, entries 1 and 2 of Table 1, respectively), but it significantly increased by increasing NaBH 4 /substrate molar ratio up to 20 (entry 3, Table 1).However, attempting in recycling Ni(AAEMA) 2 -pol failed, since the yield into aniline dropped to 27 % already in the second run with the same catalyst (entry 4, Table 1).We observed also that the resin turned from green to black during reaction (presumably due to Ni reduction from +2 to 0 oxidation state) and, then, from black to white at the end of the reaction, probably due to metal leaching into solution (which in turn became green).
Calcination under hydrogen or nitrogen atmosphere is a well-known technique [16a,28] used for preparing metallic nanoparticles anchored to an insoluble support by thermal reduction of supported metal ions.Depending on the calcination temperature, the thermal treatment modifies also the insoluble matrix (which may lose water, crystallization solvent molecules and/or carbon-based moieties), thus enhancing or depressing the ability of the support in stabilizing and retaining the so formed metal nanoparticles.For example, Qiu and coworkers found that the calcination temperature is the key factor affecting the structure, morphology and the catalytic performance of their Ni/C catalyst in the hydrogenation of nitroarenes.[29] They identified in T=300°C the best calcination temperature for their catalyst in terms of activity.Thermal Gravimetrical Analyses (Figure S4) suggested us to set the calcination temperature for our system at 300°C as well, because over 300°C the loss in weight of the polymeric material exceeded 5% of the initial mass, due to important structural modification of the support [29].Therefore, by annealing Ni(AAEMA) 2 -pol under nitrogen flow for 30 min at 300°C, we obtained Ni-pol as a black powder (scheme 2).
Scheme 2. Preparation of Ni-pol by calcination under N 2 at 300°C.
To our delight, Ni-pol resulted active and recyclable in the reduction of nitrobenzene (entries 5-8, Table 1), although in neat water the reusability of the catalytic system dramatically decreased after the second run.Figure 1 summarizes the recyclability over five runs of Ni(AAEMA) 2 -pol and Ni-pol in neat water and in water/diethyl ether (v/v = 1/1), as the solvent.
It can be seen from Figure 1 that the best catalytic performance in terms of catalytic activity and recyclability was shown by Ni-pol in H 2 O/Et 2 O as the solvent.The biphasic reaction medium on one side facilitates the access of the water insoluble substrate to the catalytically active sites and on the other side renders more workable the acrylamide based catalyst, becoming the latter too jelly in neat water.In addition, the polymeric support seems to better retain the nickel centers by swelling (in water) and shrinking (in diethyl ether) at the same time.Furthermore, Zhao and coworkers demonstrated that water solvent rapidly deactivates Ni supported catalyst in the hydrogenation of nitroarenes by forming catalytically inactive Ni(OH) 2 species.[30] This problem has been resolved by coating the catalyst with a hydrophobic layer of carbon.[30] In our system, the hydrophobic property of diethyl ether might help to prevent catalyst deactivation.Additionally, it can be deduced that the catalytically active species are metallic Ni nanoparticles rather than Ni(II) sites.Therefore, high attention should be paid to the size and morphology of Ni sites, as well as to their distribution in the polymeric support and their modification after the use in the reaction media.
On the bases of the above results, we concluded that the optimum reaction conditions to be used for converting 0.5 mmol of nitrobenzene into aniline in the presence of 10.2 mg of Ni-pol (containing 5.35 % w Ni) were the ones reported in entries 7 and 8 of Table 1.

Selectivity of the catalytic system in the hydrogenation of halonitrobenzene to haloaniline
Using the optimized reaction conditions, the selectivity of the catalyst was tested in the reduction of different halonitrobenzenes.In fact, the selective hydrogenation of halonitrobenzenes catalyzed by transition metals is regarded as a high atom efficiency and environmentally friendly process for the synthesis of haloanilines, which are important fine chemicals, widely used in the production of pharmaceuticals, dyes, herbicides, pesticides.[31] However, up to now, the major challenge is avoiding the undesired hydrodehalogenation reaction, which is a side-reaction favored by the electron withdrawing effect of the nitro group in para and/or ortho positions (with respect to the halogen), that enhances the rate of the Ar-X (Cl, Br, I) oxidative addition to the metal center.
Selective halonitroarene reductions have been reached by poisoning Pd [32] or Pt [33] nanocatalysts.Our Ni based catalytic system resulted active and selective in the reduction of halonitrobenzenes (Table 2), even in the case of challenging bromo-and iodo-nitrobenzene (entries 3 and 4, Table 2).This chemoselectivity, much higher than the one observed with the analogues Pd catalyst, [27] should be ascribable to reluctance of aryl halides to give oxidative addition to nickel active site, which is the well-known first step of the Ni and Pd catalyzed hydrodehalgenation reaction.[34] Table 2 Reduction of various halonitroarenes catalyzed by Ni-pol a Ni-pol system showed the same activity and selectivity over various cycles.In fact, after completion of the reaction, the organic phase was pipetted out and the water solution (containing the catalyst on its upper layer) was washed twice with diethyl ether to remove all organic reactants and products.

Entry
Fresh reagents and diethyl ether were then added to the water mixture and the catalytic system was successfully reused for five subsequent runs, where it showed the unaltered catalytic activity and selectivity, giving 4-bromoaniline as the product in excellent yield (89-95%) for every cycle (Figure 2).(v/v = 1/1, 5 mL) at room temperature, t = 12 h.

Activity of the catalytic system
The catalytic system was then tested for the reduction of various nitroarenes under the optimized reaction conditions (Table 3).
Ni-pol could catalyze the reduction of substrates bearing either electron-withdrawing and electrondonating substituents in the aromatic ring.2-, 3-and 4-Nitrotoluene, as well as 3-and 4methoxynitrobenzenes, gave very high yields into 2-, 3-and 4-toluidine, and 3-and 4-anisidine, respectively, in 2-5 hours (entries 1-5).Regarding 1,2-dinitrobenzene (entry 6), both nitro groups were converted in 12 hours, and 2-nitro-aniline was detected only in slight amount during the  a Reaction Conditions: 0.5 mmol of nitroarene, 10.0 mmol of NaBH 4 and Ni-pol (9.3•10 -3 mmol of Ni) were stirred in 2.5 mL of double deionized water and 2.5 mL of diethyl ether at room temperature for the appropriate amount of time.b Yield determined by GLC with the internal standard (biphenyl) method.Isolated yield after column chromatography in parenthesis.

3.4.Heterogeneity tests
To verify whether the observed catalysis was truly heterogeneous or not, the reduction of 4bromonitrobenzene was taken as the model reaction.The reaction system was removed of the supported catalyst by filtration after 3 h reaction (30% conversion of the substrate) and added of 10.0 mmol NaBH 4 .Additional stirring of the filtrate under the optimized reaction conditions did not increase the conversion of 4-bromonitrobenzene.On the contrary, the amount of 4-bromoaniline in the reaction mixture decreased after further 9 h stirring (vide infra).
This mixture was then mineralized and subjected to AAS analyses, showing negligible nickel amount.In addition, Ni-pol recovered from the filtration showed the same nickel content (within the experimental error) found in the catalyst before its use (AAS analyses) as well as in Ni-pol recovered at the end of the fifth cycle.All these data suggested that the catalysis was truly heterogeneous, since the presence of catalytically active leached nickel species was not observed .

3.5.STEM analyses
To gain insight into the structure of the catalyst, Scanning Transmission Electron Microscopy (STEM) was carried out.Ni(AAEMA) 2 -pol and Ni-pol were subjected to STEM analyses as well as the catalyst recovered after the first and the fifth run, respectively, with the aim of ascertaining whether the reaction cycles affected the morphology and the dispersion of the nickel active species on the polymeric support.
STEM images of Ni(AAEMA) 2 -pol and of Ni-pol before and after duty are reported in Figure 3.

Ni(AAEMA) 2 -pol was constituted by Ni(II) species only (not visible in STEM micrographs)
supported onto a macro-porous polymer (figure 3a).STEM image of Ni-pol before use in catalysis shows metallic Ni nanoparticles ranging from 11 to 37 nm in size and a few amount of Ni(0) nanocubes with a cube side of 85-200 nm (Figure 3b), formed during the thermal treatment procedure.In the STEM picture of Ni-pol recovered after the first run, the previously observed metal nickel nanoparticles were not visible anymore, due to their transformation under reductive reaction conditions into cubic nanotwins ranging from 28 to 70 nm in side size (figure 3c).The peculiar morphology of the latters might arise from the matrix effect of the polymer support which controlled both nanoparticle shape (cubic) and growth.In fact, it seemed that for each nanoparticle a second nano-cube grew up along the diagonal (preferential direction) of the primary nano-cube, giving rise to two compenetrated cubes as final overall nanoparticle.On passing from the first to the fifth cycle (Figure 3d), the Ni(0) nanotwin side sizes remained almost the same (27-65 nm) as well as nanoparticle number density.The quantity of the largest nanoparticles (45-70 nm) seemed even to decrease with the re-uses (figure 3c and 3d), probably for a dissolution/re-deposition mechanism [36] due to the presence of water, which is known to oxidize polymer embedded Ni(0) to Ni(II) [30], that in turn would be again reduced under reaction conditions to form Ni(0) nanoparticles.The polymer matrix might act as a stabilizer of the metal nanoparticles; thus, their size and morphology did not dramatically change with the re-uses.
Ni nanoparticles formed under reaction condition, because NaBH 4 acts as a reducing agent of the metal in a dual way, by providing hydrides and by generating hydrogen gas.In fact, the hydrides from sodium borohydride can displace negatively charged ligands bound to Ni(II), generating Nihydrides directly, which upon -hydride elimination of H 2 , give the reduced metal.In addition, the in situ formed H 2 in water (scheme 3) in the closeness of Ni(II) can reduce it to Ni(0).

3.6.Mechanistic consideration
By using NaBH 4 in water in the presence of Ni-pol, abundant formation of H 2 gas was observed in the reaction flask, according to the reaction reported in scheme 3. [37] Scheme 3: nickel catalyzed formation of H 2 by NaBH 4 It is widely recognized [38] that the hydrogenation of nitroarenes (I) could follow two pathways: the direct route and the condensation route (scheme 4).The first one gives the aniline product (IV) through the formation of hydroxylamine (III), while the second one affords diazoarene (VI) as intermedium.
Pd and Au based catalytic systems generally prefer the direct route, [39] even if Pd/C catalyst might follow other mechanistic ways.[40] For a Ni catalyst, density functional study has demonstrated that the direct reduction pathway is more favorable than the indirect one.[41] During the reduction with NaBH 4 /Ni-pol system, diazo-intermediates (V and VI) were always detected (GC-MS analysis), as well as the nitroso compound (II).The presence of arylhydroxylamine (III) was never observed.When the substrate (I) disappeared, the concentration of II, V and VI became lower, because they reacted to give the product (IV).Probably, III was never detected because its reduction was faster compared to the one of II and/or because III quickly reacted with II to give V.These results suggest that Ni-pol reducing system might go through both direct and condensation routes (scheme 4).
An increasing of diazobenzene (VI) amount and a decreasing of aniline (IV) quantity were observed (GLC Analyses) in the mother liquor removed of Ni-pol during reaction (30 % conversion of nitrobenzene) and stirred for additional nine hours after addition of fresh NaBH 4 .In the absence of the supported catalyst, nitroso benzene (II) and aniline (IV) condensed into diazobenzene (VI) (scheme 4), because the hydrogenation of II to give hydroxylamine benzene (III) did not happen without Ni.
A major strength of this reaction protocol is its recyclability, its selectivity when using halonitroarenes as substrates and its ability to perform the reduction in aqueous medium, which is not trivial for Ni based catalysts.The absence of arylhydroxylamine during reaction is an additional strong point, because accumulation of arylhydroxylamine may lead to rapid exothermic decomposition.[42] By comparing Ni-pol catalytic system with the commercial available Raney Ni , [43] it is apparent that the latter dramatically deactivates after few runs and it needs harsh conditions (102°C and 1.62 MPa of H 2 ) to obtain high yields.Raney Ni/NaBH 4 system has been recently proposed, but no studies on its recyclability have been reported.[44] Table 4

Conclusions
In conclusion, the polymeric catalyst Ni-pol (based on a cheap metal, i.e.Ni) efficiently catalyzed the transfer hydrogenation reaction of aromatic nitro compounds with NaBH 4 under sustainable conditions (aqueous medium and room temperature), thus avoiding the use of the commonly employed noble metals (Pd, Pt).STEM analyses showed that the active species were metallic Ni nanocubes ranging from 28 to 70 nm in side size, stabilized by the polymeric support.Ni-pol was stable in aqueous medium and could be reused for at least five cycles keeping the same activity and selectivity.These occurrences are not trivial for catalysts supporting Ni nanoparticles, because the latter have been found water-sensitive in similar catalytic systems.
Finally, Ni-pol catalytic system was selective towards haloanilines in the reduction of halonitrobenzenes, avoiding the formation of hydro-dehalogenation products.To the best of our knowledge Ni-pol system is the second example reported to date of Ni based reusable catalyst, selective in the hydrogenation of halonitrobenzenes to haloanilines.
NaBH 4 were stirred at room temperature under nitrogen.bYield determined by GLC with the internal standard (biphenyl) method.c Catalyst recovered from the reaction of the previous entry.

a
Reaction Conditions: 0.5 mmol of nitroarene, 10.0 mmol of NaBH 4 and Ni-pol (9.3•10 -3 mmol of Ni) were stirred in 2.5 mL of double deionized water and 2.5 mL of diethyl ether at room temperature for the appropriate amount of time.b Yield determined by GLC with the internal standard (biphenyl) method.Isolated yield after column chromatography in parenthesis.

Figure 2 .
Figure 2. Selectivity of Ni-pol towards 4-bromoaniline over five cycles.Reaction conditions: 0.5 mmol of 4-bromonitrobenzene, 10.2 mg of Ni-pol (Ni% w = 5.35), 10 mmol of NaBH 4 in H 2 O/Et 2 O Due to the absence of steric hindrance, 2,4-dinitrotoluene reacted faster (entry 7) than its isomer reported in entry 6 It is noteworthy that o-and p-diaminobenzene are notoriously important in the industry of azo dyes.[35]

Table 3
Transfer hydrogenation reaction of nitroarenes catalyzed by Ni-pol a [29]lays the reaction conditions employed in catalytic systems based on recent examples of Ni supported catalysts in the hydrogenation of nitroarenes.It is apparent that either high temperatures and H 2 pressures or high NaBH 4 /substrate molar ratio are needed to get good yield.In addition, to the best of our knowledge Ni-pol system is the second example reported to date[29]of Ni based reusable catalyst, selective in the hydrogenation of halonitrobenzenes to haloanilines.

Table 1 :
Preliminary catalytic tests for the reduction of nitrobenzene a Reduction of various halonitroarenes catalyzed by Ni-pol a a Reaction conditions: 0.5 mmol of nitrobenzene, 10.2 mg of Ni-pol (Ni% w = 5.35) or 17.5 mg of b Yield determined by GLC with the internal standard (biphenyl) method.cCatalystrecoveredfrom the reaction of the previous entry.Table2aReactionConditions: 0.5 mmol of nitroarene, 10.0 mmol of NaBH 4 and Ni-pol (9.3•10 -3 mmol of Ni) were stirred in 2.5 mL of double deionized water and 2.5 mL of diethyl ether at room temperature for the appropriate amount of time.bYielddetermined by GLC with the internal standard (biphenyl) method.Isolated yield after column chromatography in parenthesis.

Table 3
Transfer hydrogenation reaction of nitroarenes catalyzed by Ni-pol a bYield determined by GLC with the internal standard (biphenyl) method.Isolated yield after column chromatography in parenthesis.

Table 4 :
reaction conditions for Ni-based catalyst reported