nanomaterials Article

Supramolecular Assembly of Gold Nanoparticles on Carbon Nanotubes: Application to the Catalytic Oxidation of Hydroxylamines Nimesh Shah 1,2,† , Pallabita Basu 1,† , Praveen Prakash 2,† , Simon Donck 1,2 , Edmond Gravel 2 , Irishi N. N. Namboothiri 1, * and Eric Doris 2, * 1 2

* †

Department of Chemistry, Indian Institute of Technology, Bombay, Mumbai 400076, India; [email protected] (N.S.); [email protected] (P.B.); [email protected] (S.D.) Alternative Energies and Atomic Energy Commission (CEA), Saclay Institute of Biology and Technology (IBITECS), Department of Bioorganic Chemistry and Isotopic Labeling, 91191 Gif-sur-Yvette, France; [email protected] (P.P.); [email protected] (E.G.) Correspondence: [email protected] (I.N.N.N.); [email protected] (E.D.); Tel.: +91-2225767196 (I.N.N.N.); +33-169088071 (E.D.) These authors contributed equally to this work.

Academic Editors: Hermenegildo García and Sergio Navalón Received: 19 January 2016; Accepted: 16 February 2016; Published: 24 February 2016

Abstract: A supramolecular heterogeneous catalyst was developed by assembly and stabilization of gold nanoparticles on the surface of carbon nanotubes. A layer-by-layer assembly strategy was used and the resulting nanohybrid was involved in the catalytic oxidation of hydroxylamines under mild conditions. The nanohybrid demonstrated high efficiency and selectivity on hydroxylamine substrates. Keywords: carbon nanotubes; gold nanoparticles; nanohybrid; heterogeneous catalysis

1. Introduction Besides stoichiometric approaches that are routinely used for the oxidation of organic substrates [1,2], catalytic processes have also been devised to perform selective oxidation reactions [3]. With heterogeneous catalytic systems, obtained by assembling the metallic catalysts on a solid support, facile reclaim and reuse of the catalytic species can be achieved [4]. Among the various platforms used as supports, allotrope forms of carbon, in particular carbon nanotubes (CNTs), have been shown to provide some key advantages [5]. The latter include chemical, thermal, and mechanical stability, high specific surface area, inertness, and adjustable topography. CNTs are also able to act in a synergistic fashion to enhance the performances of the supported catalytic metal [6,7]. With these features in mind, we sought to develop a catalyst that could catalyze some oxidation reactions under mild conditions of temperature and pressure using low catalytic loadings. Herein, we report the assembly of a CNT-gold nanohybrid catalyst and its application to the selective oxidation of hydroxylamines into either nitroso or azoxy derivatives. 2. Results and Discussion 2.1. Assembly of the AuCNT Nanohybrid Although gold has long been regarded as a poor catalytic metal, its nanosized forms [8], including supported ones [9,10], have recently been shown to be able to catalyze a wide array of chemical transformations. Our CNT-gold catalyst was built using a layer-by-layer approach that was adapted Nanomaterials 2016, 6, 37; doi:10.3390/nano6030037

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2 of 7 from our previous work [11–23]. Carbon nanotubes were first dispersed in an aqueous solution by ultrasonication in the presence of an amphiphilic nitrilotriacetic-diyne (DANTA) surfactant. This surfactant. This first step led to the non-stochastic assembly of the amphiphilic units on the CNT first step led to the non-stochastic assembly of the amphiphilic units on the CNT surface and to surface and to the formation of supramolecular structures with a nanoring-like shape (Figure 1a,c). the formation of supramolecular structures with a nanoring-like shape (Figure 1a,c). This type This type of well-ordered supramolecular assembly on CNTs was first reported in 2003 [24]. of well-ordered supramolecular assembly on CNTs was first reported in 2003 [24]. Amphiphilic Amphiphilic DANTA adsorbed at the surface of the nanotubes by hydrophobic interactions while its DANTA adsorbed at the surface of the nanotubes by hydrophobic interactions while its hydrophilic hydrophilic polar head was pointing toward the aqueous medium. The rings were polymerized by polar head was pointing toward the aqueous medium. The rings were polymerized by ultraviolet ultraviolet (UV) irradiation in a second step. In fact, irradiation of the sample for 6 h at 254 nm led to (UV) irradiation in a second step. In fact, irradiation of the sample for 6 h at 254 nm led to a a topochemical polymerization of the diyne motif incorporated in the hydrophobic part of the starting topochemical polymerization of the diyne motif incorporated in the hydrophobic part of the starting amphiphile [25,26]. Polymerization takes place within individual half-cylinders and strengthens the amphiphile [25,26]. Polymerization takes place within individual half-cylinders and strengthens the cohesion of the assembly. After UV irradiation, the DANTA-decorated nanotubes became resistant cohesion of the assembly. After UV irradiation, the DANTA-decorated nanotubes became resistant to to dialysis against water and to ethanol washes, indicating that the lipid assemblies had been dialysis against water and to ethanol washes, indicating that the lipid assemblies had been polymerized. polymerized. The second layer was thereafter deposited by stirring the suspended nanotubes with a The second layer was thereafter deposited by stirring the suspended nanotubes with a cationic polymer, cationic polymer, poly(diallyldimethylammonium chloride) (PDADMAC), which adsorbed on the poly(diallyldimethylammonium chloride) (PDADMAC), which adsorbed on the nanotube’s surface nanotube’s surface by electrostatic interactions with the primary anionic layer. The double-coated by electrostatic interactions with the primary anionic layer. The double-coated CNTs were then CNTs were then recovered by centrifugation before the final deposition of gold nanoparticles recovered by centrifugation before the final deposition of gold nanoparticles (AuNPs). The latter were (AuNPs). The latter were prepared in parallel by reduction of HAuCl4 in the presence of prepared in parallel by reduction of HAuCl4 in the presence of tetrahydroxymethylphosphonium tetrahydroxymethylphosphonium chloride [27]. The colloidal suspension of gold nanoparticles chloride [27]. The colloidal suspension of gold nanoparticles (AuNPs) was then sequentially added to (AuNPs) was then sequentially added to the coated CNTs in which the polyammonium network the coated CNTs in which the polyammonium network provided robust anchoring and stabilization of provided robust anchoring and stabilization of the metallic nanoparticles (Figure 1b,d). The AuCNT the metallic nanoparticles (Figure 1b,d). The AuCNT nanohybrid was finally suspended in water and nanohybrid was finally suspended in water and used for the catalysis of the reported reactions. used for the catalysis of the reported reactions.

Figure 1. (a) Schematic representation of nitrilotriacetic-diyne (DANTA) nanoring polymerization on Figure 1. (a) Schematic representation of nitrilotriacetic-diyne (DANTA) nanoring polymerization on the carbon nanotube (CNT) surface; (b) Schematic representation of the final stages of the AuCNT the carbon nanotube (CNT) surface; (b) Schematic representation of the final stages of the AuCNT synthesis; (c) Transmission electron microscopy (TEM) image of polymerized DANTA nanorings on synthesis; (c) Transmission electron microscopy (TEM) image of polymerized DANTA nanorings on CNT (negative staining); (d) TEM image of the AuCNT final catalyst. CNT (negative staining); (d) TEM image of the AuCNT final catalyst.

2.2. Characterization of the AuCNT Nanohybrid Transmission electron microscopy (TEM, Philips CM12 microscope, Amsterdam, the Netherlands) indicated that the AuNPs were of spherical shape. The average size of the supported nanoparticles was measured using TEM pictures. Statistical measurement indicated that the supported AuNPs had an average diameter of ca. 3 nm (Figure 2a). The metal content of the aqueous AuCNT suspension

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2.2. Characterization of the AuCNT Nanohybrid Transmission electron microscopy (TEM, Philips CM12 microscope, Amsterdam, the Netherlands) indicated AuNPs were of spherical shape. The average size of the supported nanoparticles Nanomaterialsthat 2016,the 6, 37 3 of 8 Nanomaterials 2016, 6, 37 of 7 was measured using TEM pictures. Statistical measurement indicated that the supported AuNPs 3had Nanomaterials2016, 2016,6,6,37 37 Nanomaterials 33ofof77 aqueous AuCNT AuCNT suspension anNanomaterials average diameter Nanomaterials 2016,6,6,37 37of ca. 3 nm (Figure 2a). The metal content of the aqueous 2016, 33ofof77 was determined by inductively coupled plasma mass spectrometry (ICP-MS) ([Au] = 1.2 mM). Finally, determinedbybyinductively inductively coupled plasma spectrometry (ICP-MS) ([Au] = 1.2Finally, mM). was determined coupled plasma massmass spectrometry (ICP-MS) ([Au] = 1.2 mM). was determined byinductively inductively coupled plasma massspectrometry spectrometry (ICP-MS) ([Au] 1.2 mM).Finally, Finally, was determined by inductively coupled plasma mass spectrometry (ICP-MS) ([Au] ===1.2 1.2 mM). Finally, the metallic character of the supported gold nanoparticles was established by X-ray photoelectron was determined by inductively coupled plasma mass spectrometry (ICP-MS) ([Au] 1.2 mM). Finally, was determined by coupled plasma mass (ICP-MS) ([Au] = mM). Finally, the character metallic character of the gold supported gold nanoparticles wasbyestablished by X-ray the metallic of the supported nanoparticles was established X-ray photoelectron the metallic character of the supported gold nanoparticles was established by X-ray photoelectron the metallic character of the supported gold nanoparticles was established by X-ray photoelectron spectroscopy (XPS, VG of ESCALAB 210 spectrometer, Waltham, MA, USA) analysis analysis which showed themetallic metallic(XPS, character ofthe the supported goldnanoparticles nanoparticles was established by X-ray photoelectron the character supported gold was established by X-ray photoelectron photoelectron spectroscopy (XPS, VG 210 Waltham, spectrometer, Waltham, MA, USA) analysis spectroscopy VG ESCALAB 210 ESCALAB spectrometer, MA, USA) which showed spectroscopy (XPS, VG ESCALAB 210 spectrometer, Waltham, MA, USA) analysis which showed spectroscopy (XPS, VG ESCALAB 210 spectrometer, Waltham, MA, USA) analysis which showed characteristic Au 4f-binding energies contributions of Au [28] (Figure 2b). spectroscopy (XPS, VGESCALAB ESCALAB 210spectrometer, spectrometer, Waltham, MA, USA) analysis whichshowed showed spectroscopy (XPS, VG 210 Waltham, MA, USA) analysis which which showedAu characteristic Au 4f-binding energiesofcontributions of Au [28] (Figure 2b). characteristic 4f-binding energies contributions Au [28] (Figure 2b). characteristic Au 4f-binding energies contributions of Au [28] (Figure 2b). characteristic Au 4f-binding energies contributions of Au [28] (Figure 2b). characteristicAu Au4f-binding 4f-bindingenergies energiescontributions contributionsof ofAu Au[28] [28](Figure (Figure2b). 2b). characteristic

Figure (a) Size Size distribution distribution obtained obtained from from the the measurement measurement of of 250 gold (b) Figure 2. 2. (a) (a) 250 gold gold particles; particles; (b) (b) X-ray X-ray Figure 2. Size distribution obtained from the measurement of 250 particles; X-ray photoelectron spectroscopy (XPS) analysis (Au 4f core level) of the AuCNT nanohybrid. Figure 2.2. (a) (a) Size distribution distribution obtained(Au from the measurement measurement 250nanohybrid. gold particles; particles; (b) (b) X-ray X-ray Figure Size obtained from the ofof 250 gold photoelectron spectroscopy (XPS) analysis 4f core level) of the AuCNT Figure 2.2. (a) (a) Size distribution distribution obtained(Au from the measurement measurement 250nanohybrid. gold particles; particles; (b) (b) X-ray X-ray Figure Size from the ofof 250 gold photoelectron spectroscopy (XPS) obtained analysis 4f core level) of the AuCNT photoelectronspectroscopy spectroscopy(XPS) (XPS)analysis analysis(Au (Au4f4fcore corelevel) level)ofofthe theAuCNT AuCNTnanohybrid. nanohybrid. photoelectron photoelectronspectroscopy spectroscopy(XPS) (XPS)analysis analysis(Au (Au4f4fcore corelevel) level)ofofthe theAuCNT AuCNTnanohybrid. nanohybrid. photoelectron

2.3. Oxidation of of Hydroxylamines Hydroxylamines with Nanohybrid 2.3. Oxidation with the the AuCNT AuCNT Nanohybrid Nanohybrid 2.3. Oxidation of Hydroxylamines with the AuCNT Nanohybrid 2.3. Oxidation of Hydroxylamines with the AuCNT Nanohybrid 2.3.With Oxidation Hydroxylamines withthe theAuCNT AuCNTNanohybrid Nanohybrid 2.3. Oxidation ofofHydroxylamines with nanohybrid the gold-based gold-based With the the gold-based nanohybrid in in hand, hand, we we investigated investigated its its potential potential in in the the aerobic aerobic oxidation oxidation With the gold-based nanohybrid in hand, we investigated its potential in the aerobic oxidation With the gold-based nanohybrid in hand, we investigated its potential in the aerobic oxidation of hydroxylamines. In the latter transformation, we expected the formation of the corresponding hydroxylamines. In the latter transformation, we expected the formation of the corresponding nitroso With the gold-based nanohybrid in hand, we investigated its potential in the aerobic oxidation With the gold-based nanohybrid in hand, we investigated its potential in the aerobic of hydroxylamines. In the latter transformation, we expected the formation of the correspondingoxidation nitroso of hydroxylamines. In the latter transformation, we expected the formation of the corresponding nitroso of hydroxylamines. In the latter transformation, we expected the formation of the corresponding nitroso derivatives [29]. The nanohybrid-catalyzed oxidation of tert-butyl hydroxylamine derivatives [29]. The nanohybrid-catalyzed oxidation of tert-butyl hydroxylamine (1a) in CHCl 3nitroso /H 2in O ofhydroxylamines. hydroxylamines. Inthe thelatter lattertransformation, transformation, weexpected expected theformation formation ofthe thecorresponding corresponding nitroso of we the of derivatives [29]. The In nanohybrid-catalyzed oxidation of tert-butyl hydroxylamine (1a) in CHCl(1a) 3nitroso /H 2O derivatives [29]. The nanohybrid-catalyzed oxidation of tert-butyl hydroxylamine (1a) in CHCl 3use /H 2O derivatives [29]. The nanohybrid-catalyzed oxidation of tert-butyl hydroxylamine (1a) in CHCl 3/H 2O CHCl /H O afforded tert-butyl nitroso compound 1b in 81% yield after 12 h of reaction (Table 1, afforded tert-butyl nitroso compound 1b in 81% yield after 12 h of reaction (Table 1, Entry 1). The 3 tert-butyl 2 [29]. derivatives [29].The Thenanohybrid-catalyzed nanohybrid-catalyzed oxidation oftert-butyl tert-butyl hydroxylamine (1a)inin CHCl 3/H 2O derivatives oxidation of hydroxylamine CHCl 2O afforded nitroso compound 1b in 81% yield after 12 h of reaction (Table 1, (1a) Entry 1). The3/H use afforded tert-butyl nitroso compound 1b in 81% yield after 12 h of reaction (Table 1, Entry 1). The use afforded tert-butyl nitroso compound 1b in 81% yield after 12 h of reaction (Table 1, Entry 1). The use Entry 1). The use ofnitroso a binary solvent mixture permitted us tohof increase the(Table rate of1,1, the reaction and ofafforded binary solvent mixture permitted usinin to81% increase the rate of the reaction and afforded more afforded tert-butyl nitroso compound 1b 81% yieldthe after 12 h of reaction (Table Entry 1).aaThe The use tert-butyl compound 1b yield after 12 of reaction Entry 1). use of aa binary solvent mixture permitted us to increase rate the reaction and afforded more of a binary solvent mixture permitted us to increase the rate of the reaction and afforded a more of a binary solvent mixture permitted us to increase the rate of the reaction and afforded a more afforded a more selective transformation. The reaction of N-cyclohexylhydroxylamine (2a) also cleanly selective transformation. The reaction of N-cyclohexylhydroxylamine (2a) also cleanly produced ofaabinary binary solventmixture mixture permittedofus us toincrease increasethe therate rateof ofthe thereaction reaction and afforded more of solvent to afforded aamore selective transformation. The permitted reaction N-cyclohexylhydroxylamine (2a) alsoand cleanly produced selectivenitrosocyclohexane transformation. The reaction of N-cyclohexylhydroxylamine (2a) also cleanly produced produced selective transformation. The reaction of N-cyclohexylhydroxylamine (2a) also cleanly produced produced (2b) in 83%of yield (Entry is noteworthy thatalso noisomerization isomerization of nitrosocyclohexane (2b) in inThe 83% yield (Entry 2). It It2).is is Itnoteworthy noteworthy that no isomerization selective transformation. The reaction of N-cyclohexylhydroxylamine (2a) also cleanly produced selective transformation. reaction N-cyclohexylhydroxylamine (2a) cleanly nitrosocyclohexane (2b) 83% yield (Entry 2). that no of nitrosocyclohexane (2b) in 83% yield (Entry 2). It is noteworthy that no isomerization of nitrosocyclohexane (2b) in 83% yield (Entry 2). It is noteworthy that no isomerization of nitrosocyclohexane (2b) into the corresponding cyclohexanone oxime was detected. This result is to be nitrosocyclohexane (2b) into the corresponding cyclohexanone oxime was detected. This result is toof nitrosocyclohexane (2b) in 83% yield (Entry (Entry 2). ItIt isis noteworthy noteworthy that no no isomerization isomerization of nitrosocyclohexane (2b) yield 2). nitrosocyclohexane (2b) intoin the83% corresponding cyclohexanone oxime was that detected. This result is to nitrosocyclohexane (2b)into into the corresponding cyclohexanone oximewas was detected.when Thisresult resultis isto to nitrosocyclohexane (2b) into the corresponding cyclohexanone oxime was detected. This result isis to noted since tautomerization ofthe nitroso compounds toto observed working benitrosocyclohexane noted since tautomerization of nitroso compounds tooximes oximesisis isclassically classically observed when working nitrosocyclohexane (2b) into the corresponding cyclohexanone oxime was detected. This result to (2b) corresponding cyclohexanone oxime detected. This be noted since tautomerization of nitroso compounds oximes classically observed when working be noted since tautomerization of nitroso compounds to oximes is classically observed when working be noted since tautomerization of nitroso compounds to oximes is classically observed when working with substrates carrying Cα-protons. The reaction of aliphatic substrates thus satisfactorily provided with substrates carrying Cα-protons. benoted noted sincecarrying tautomerization ofnitroso nitroso compounds oximes classically observedwhen when working be since tautomerization of compounds totooximes isisclassically working with substrates Cα-protons. The reaction of aliphatic substrates thus observed satisfactorily provided with substrates carrying Cα-protons. The reaction of aliphatic substrates thus satisfactorily provided with substrates carrying Cα-protons. The reaction of aliphatic substrates thus satisfactorily provided access to nitroso compounds in high yields, but aromatic hydroxylamines did not behave similarly. nitroso compounds in high yields, but aromatic hydroxylamines did not behave similarly. withsubstrates substrates carryingCα-protons. Cα-protons. Thereaction reaction ofaliphatic aliphatic substratesthus thusnot satisfactorily provided with carrying The of substrates satisfactorily provided access to nitroso compounds in high yields, but aromatic hydroxylamines did behave similarly. accessto tonitroso nitrosocompounds compoundsin inhigh highyields, yields,but butaromatic aromatichydroxylamines hydroxylaminesdid didnot notbehave behavesimilarly. similarly. access to nitroso compounds in high yields, but aromatic hydroxylamines did not behave similarly. access to nitroso compounds in high yields, but aromatic hydroxylamines did not behave similarly. access [a] [a]. Table Table1. 1. Oxidation Oxidation of of various various hydroxylamines hydroxylamineswith withAuCNT AuCNT [a] Table 1. Oxidation of various hydroxylamines with AuCNT . [a] [a] Table1.1.Oxidation Oxidationofofvarious varioushydroxylamines hydroxylamineswith withAuCNT AuCNT[a] Table .. Table1.1.Oxidation Oxidationofofvarious varioushydroxylamines hydroxylamineswith withAuCNT AuCNT [a]. . Table

H R N OH

Entry Entry Entry Entry Entry Entry Entry 1 1 11 1 11 2 22 22 22 3

a

AuCNT

Substrate a Substrate aaa Substrate Substrate Substrate Substrate Substrate a aa

oxidized product b Product b Product Productbb Product bb Product Product Product bb

Yield (%) Yield (%) Yield (%) Yield (%) Yield (%) Yield (%) Yield (%) [b] 81 [b][b] [b] 81 8181 [b] [b] 81 [b] 8181 83 [b] [b] [b] 83 83 [b][b] [b] 83 8383 [b] 83

33 3 33

96 [c] [c] [c] 96[c] 96 [c] 96 [c] 9696

4

98 [c] [c] [c]

Entry Entry Entry Nanomaterials 2016, 6, 37

Substrate Substrate Substrateaaa

Product Product Productbb b

111

Yield Yield (%) Yield(%) (%) [b] [b] 81 81 81 [b]

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Table 1. Cont.

222 Entry

[b] [b] 83 83 83 [b]

Substrate a

Product b

Yield (%)

[c][c][c] 96 9696 96 [c]

3 33 3

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98 98[c][c]

[c] [c] 9898

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Table1. 1.cont. cont. Table Table 1. cont. TableTable 1. cont.1. cont.

5

5 5 55

[c] 92[c][c] 92 [c] 92 92

5

[c] 92 [c] 92

6

6

66 6 66

7

7 7

777

94 [c]

[c] 94 [c] 94[c][c] 94 [c] 94

94 NR NR NR

NR NR NR

[c] --[c][c] ----

[c] [c] --– [c] --

[a]Conditions: Conditions:N-hydroxylamine N-hydroxylamine(0.1 (0.1mmol), mmol),AuCNT AuCNT(0.5 (0.5mol mol%), %),KK K22CO 2CO3 (0.2 mmol), room temperature [a] [a] Conditions: N-hydroxylamine (0.1 mmol), AuCNT (0.5 mol %), CO33(0.2 (0.2mmol), mmol),room roomtemperature temperature [a] Conditions: N-hydroxylamine (0.1AuCNT mmol), AuCNT (0.5 mol %),3mmol), K 2CO3room (0.2 roomunder temperature Conditions: N-hydroxylamine (0.1 mmol), (0.5 mol2mol %), K%), CO temperature [a][a]Conditions: N-hydroxylamine (0.1 mmol), AuCNT (0.5 KReaction CO (0.2 mmol), room 2[c] 32(0.2 under air. [b] Reaction time 12 h, CHCl 3/H2O (1:1, mL). time 2h, h,mmol), CHCl3temperature 3 (2 mL). under air. [b] Reaction time 12 h, CHCl 33/H 22O (1:1, 2 mL). [c] Reaction time 2 CHCl mL). under air. [b] Reaction time 12 h, CHCl /H O (1:1, 2 mL). [c] Reaction time 2 h, CHCl 3(2 (2 mL). air. [b] Reaction time 12 h, CHCl (1:1, 2 mL). time[c] 2 h,Reaction CHCl3 (2time mL).2 h, CHCl3 (2 mL). 3 /H12 2 Oh, under [b] Reaction CHCl 3/H[c] 2OReaction (1:1, 2 mL). under air. [b] air. Reaction time 12time h, CHCl 3/H2O (1:1, 2 mL). [c] Reaction time 2 h, CHCl3 (2 mL).

For example, example,the the reaction reactionof of phenylhydroxylamine phenylhydroxylamine (3a) (3a) with with AuCNT AuCNT in inCHCl CHCl333gave gaveaaa dimeric dimeric For For example, the reaction of phenylhydroxylamine (3a) with AuCNT in CHCl gave dimeric For example, the reaction of phenylhydroxylamine (3a) with AuCNT in CHCl 3 gave a dimeric For example, theform reaction ofazoxy phenylhydroxylamine (3a) with AuCNT inisCHCl 3 gave athat dimeric product in the of the derivative 3b in 2 h only (Entry 3). It noteworthy the binary For example, the reaction phenylhydroxylamine with AuCNT dimeric product in form of derivative hh (Entry 3). isis noteworthy that the product inthe the form ofthe theofazoxy azoxy derivative 3b 3bin in222(3a) honly only (Entry 3).ItIt Itin isCHCl noteworthy that thebinary binary 3 gave a product in the form the azoxy derivative only (Entry noteworthy that the binary product in thesystem form of the of azoxy derivative 3b incase 23bh in only (Entry 3). It is3).noteworthy that the binary solvent was not required in the of aromatic hydroxylamine substrates. A possible product in the form of thenot azoxy derivative in 2of only (Entry 3). It is noteworthy that solvent system was required in case hydroxylamine substrates. A possible solvent system was not required in the the3b case ofharomatic aromatic hydroxylamine substrates. A the possible solvent system was not required the case of aromatic hydroxylamine substrates. A possible solvent system was not required inobserved theincase of aromatic hydroxylamine substrates. A possible mechanism for the conversion in entry 3 (formation of the azoxy compound) could involve binary solvent system was not required in the in case of aromatic hydroxylamine substrates. A could possible mechanism for the conversion observed entry 3 (formation of the azoxy compound) involve mechanism for the conversion observed in entry 3 (formation of the azoxy compound) could involve mechanism the conversion observed in 3entry 3 (formation thenitroso azoxy compound) could involve mechanism for thefor conversion observed in entry (formation of the of azoxy compound) could3a’ involve the oxidation of phenylhydroxylamine (3a) into the corresponding derivative (Scheme 1). mechanism for the conversion observed in entry (formation of the azoxy compound) could involve the of (3a) the nitroso derivative 3a’ (Scheme theoxidation oxidation ofphenylhydroxylamine phenylhydroxylamine (3a)3into into thecorresponding corresponding nitroso derivative 3a’ (Scheme1). 1). the oxidation of phenylhydroxylamine (3a) into the corresponding nitroso derivative 3a’ (Scheme 1). the oxidation of phenylhydroxylamine (3a)itsinto the corresponding nitroso derivative 3a’ (Scheme 1). (3a) As 3a’ accumulates in the solution, condensation with the unreacted phenylhydroxylamine the oxidation of phenylhydroxylamine (3a) the corresponding derivative 3a’ (Scheme 1). (3a) As in its condensation with unreacted phenylhydroxylamine As 3a’ 3a’ accumulates accumulates in the the solution, solution, itsinto condensation with the thenitroso unreacted phenylhydroxylamine (3a) 3a’ accumulates the solution, its condensation with the unreacted phenylhydroxylamine (3a) As 3a’As accumulates in thein solution, itsderivative condensation withthe the unreacted phenylhydroxylamine leads to the formation of azoxy 3b, after elimination of a molecule ofwater. water. (3a) As 3a’leads accumulates in the solution, its condensation with the unreactedof phenylhydroxylamine (3a) to the formation of azoxy derivative 3b, after the elimination a molecule of leads to the formation of azoxy derivative 3b, after the elimination of a molecule of water. leads the formation of azoxy derivative 3b,the after the elimination of a molecule of water. leads to the to formation of azoxy derivative 3b, after elimination of a molecule of water. leads to the formation of azoxy derivative 3b, after the elimination of a molecule of water.

Scheme1.1. 1.Postulated Postulatedmechanism mechanismfor forthe theformation formationof ofazoxy azoxycompounds. compounds. Scheme Scheme Postulated mechanism for the formation of azoxy compounds. Scheme 1. Postulated mechanism the formation of compounds. azoxy compounds. Scheme 1. Postulated mechanism for thefor formation of azoxy

The same same type type of of reactivity reactivity was was also also evidenced evidenced for for other other aromatic aromatic substrates substrates such such as as The The same type of reactivity was also evidenced for other aromatic substrates such as The same type of reactivity was also evidenced for other aromatic substrates such as The same type of reactivity was also evidenced for other aromatic substrates such as 4-chlorophenylhydroxylamine (4a,Entry Entry 4) and4-fluorophenylhydroxylamine 4-fluorophenylhydroxylamine (5a,Entry Entry 5).Indeed, Indeed, The same type of reactivity (4a, was also 4) evidenced for other aromatic substrates such as 4-chlorophenylhydroxylamine and (5a, 5). 4-chlorophenylhydroxylamine (4a, Entry 4) and 4-fluorophenylhydroxylamine (5a, Entry 5). Indeed, 4-chlorophenylhydroxylamine (4a, Entry 4) and 4-fluorophenylhydroxylamine (5a, Entry 5). Indeed, 4-chlorophenylhydroxylamine (4a, Entry 4) and 4-fluorophenylhydroxylamine (5a, Entry 5). Indeed, we detected the formation of azoxy dimers 4b (98% yield) and 5b (92% yield) starting from 4-chlorophenylhydroxylamine (4a,of Entry 4) and 4-fluorophenylhydroxylamine (5a, Entry 5).starting Indeed, from we azoxy dimers 4b yield) we detected detected the the formation formation of azoxy dimers 4b (98% (98% yield) yield) and and 5b 5b (92% (92% yield) starting from we detected the formation of azoxy dimers 4b (98% yield) and 5b (92% yield) starting from we detected the formation of azoxy dimers 4b (98% yield) and 5b (92% yield) starting from compounds 4a and 5a, respectively. The transformation worked equally well on we detected the formation azoxy dimers 4b (98% and 5b (92% yield) equally starting compounds 4a respectively. The transformation worked well compounds 4a and and of5a, 5a, respectively. The yield) transformation worked equally from well on on compounds 4a and 5a, respectively. The transformation worked equally well on compounds andrespectively. 5a, (6a) respectively. The equally well hindered on benzylhydroxylamine in 94% 94% yield yield transformation (Entry 6)equally but worked failed on the more more compounds 4a 4a and 5a, transformation worked6) well on benzylhydroxylamine benzylhydroxylamine (6a) in (Entry but failed on the hindered benzylhydroxylamine (6a) The in 94% yield (Entry 6) but failed on the more hindered benzylhydroxylamine (6a) in 94% yield (Entry 6) but failed on the more hindered benzylhydroxylamine (6a) in 94% yield (Entry 6) but failed on the more hindered 2,6-dimethylphenylhydroxylamine (7a, Entry 7) which remained unaffected. (6a) in 94% yield (Entry 6) but failed on the more hindered 2,6-dimethylphenylhydroxylamine 2,6-dimethylphenylhydroxylamine (7a, Entry 7) remained unaffected. 2,6-dimethylphenylhydroxylamine (7a, Entry 7)which which remained unaffected. 2,6-dimethylphenylhydroxylamine (7a, Entry 7) which remained unaffected. 2,6-dimethylphenylhydroxylamine (7a, Entry 7) which remained unaffected. (7a, Entry 7) which remained unaffected. 2.3.1.Recycling Recyclingof ofthe theAuCNT AuCNTNanohybrid. Nanohybrid. 2.3.1. 2.3.1. Recycling of the AuCNT Nanohybrid. 2.3.1. Recycling the AuCNT Nanohybrid. 2.3.1. Recycling of the of AuCNT Nanohybrid. Toassess assessthe therecyclability recyclabilityof ofthe theAuCNT AuCNTnanohybrid, nanohybrid,multiple multipleoxidation oxidationcycles cycleswere werecarried carriedout out To To assess the recyclability of the AuCNT nanohybrid, multiple oxidation cycles were carried out To assess the recyclability of the AuCNT nanohybrid, multiple oxidation cycles were carried out To the recyclability ofsame the AuCNT multiple oxidation cycles werewas carried out the byassess successive reuse of of the the samplenanohybrid, of catalyst. catalyst. A A classical oxidation reaction set using using by successive reuse same of classical oxidation reaction was the by successive reuse of the same sample sample of catalyst. A classical oxidation reaction was set set using the by successive reuse of the same sample of catalyst. A classical oxidation reaction was set using the by successive reuse of thedescribed same sample of catalyst. A classical oxidation reaction was set using the After general procedure above which was applied to N-phenylhydroxylamine (3a). general procedure described above which was applied to N-phenylhydroxylamine (3a). After general procedure described above which was applied to N-phenylhydroxylamine (3a). After general procedure described above was applied to the N-phenylhydroxylamine (3a). general procedure described above whichwhich wascentrifugation, applied to N-phenylhydroxylamine (3a). After completion, the catalyst was recovered by and supernatantwas wasworked worked up.After The completion, the catalyst was recovered by centrifugation, the up. The completion, the catalyst was recovered by centrifugation, and and the supernatant supernatant was worked up. The completion, the catalyst was recovered by centrifugation, and the supernatant was worked up. The completion, the catalyst was recovered by centrifugation, and the supernatant was workedreactions. up. The This catalyst was washed with tetrahydrofuran (THF) and reused in subsequent oxidation catalyst catalystwas waswashed washedwith withtetrahydrofuran tetrahydrofuran(THF) (THF)and andreused reusedin insubsequent subsequentoxidation oxidationreactions. reactions.This This catalyst was washed with tetrahydrofuran (THF) and reused in subsequent oxidation reactions. This

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2.3.1. Recycling of the AuCNT Nanohybrid To assess the recyclability of the AuCNT nanohybrid, multiple oxidation cycles were carried out by successive reuse of the same sample of catalyst. A classical oxidation reaction was set using the general procedure described above which was applied to N-phenylhydroxylamine (3a). After completion, the catalyst was recovered by centrifugation, and the supernatant was worked up. The catalyst was washed with tetrahydrofuran (THF) and reused in subsequent oxidation reactions. This process was repeated over five consecutive cycles and showed no significant decrease in yields of oxidized azo product 3b (Table 2). After the fifth run, TEM analysis showed no major alteration of the nanohybrid morphology. The use of non-supported AuNPs provided lower yields of product and the catalyst could not be recycled. Nanomaterials 2016, 6, 37 5 of 7 Table 2. Recycling of AuCNT for the oxidation of N-phenylhydroxylamine. Table 2. Recycling of AuCNT for the oxidation of N-phenylhydroxylamine.

Entry Entry 11 22 33 44 5 5

AuCNT AuCNT fresh fresh 1st 1st reuse reuse reuse 2nd2nd reuse 3rd reuse 3rd reuse 4th reuse 4th reuse

Yield Yield(%) (%) 9696 9393 9191 9393 94 94

2.3.2. Is Is AuCNT AuCNT aa Heterogeneous Heterogeneous Catalyst? Catalyst? 2.3.2. The involvement involvement of of the the nanohybrid nanohybrid in in the the oxidation oxidation process process was was demonstrated demonstrated by by the the following following The experiment. A standard oxidation reaction of N-phenylhydroxylamine (3a) was run and, after 45 min, min, experiment. A standard oxidation reaction of N-phenylhydroxylamine (3a) was run and, after 45 split into into two. At this this stage, stage, approximately approximately 50% 50% conversion conversion of of 3a 3a into into the the corresponding azoxy split two. At corresponding azoxy 1 H-NMR (Bruker Avance DPX 400 MHz spectrometer, Billerica, MA, derivative 3b was detected by 1 derivative 3b was detected by H-NMR (Bruker Avance DPX 400 MHz spectrometer, Billerica, MA, USA). The The AuCNT AuCNTcatalyst catalystwas wasremoved removedbybycentrifugation centrifugation one sample, whereas it was in USA). in in one sample, whereas it was left left in the the other. reactions further stirred for additional before analysis. While the reaction other. BothBoth reactions werewere further stirred for additional timetime before analysis. While the reaction was was nearly quantitative (ca. 95% yield) in the AuCNT-containing sample, no further conversion nearly quantitative (ca. 95% yield) in the AuCNT-containing sample, no further conversion was was detected in the absence nanohybrid.These Theseobservations observationsconfirmed confirmedthat that oxidation oxidation of of detected in the absence of of thethenanohybrid. N-phenylhydroxylamine (3a) (3a) occurred occurred atatthe thesurface surfaceofofthe theAuCNT AuCNTnanohybrid nanohybridwhich whichacted acted N-phenylhydroxylamine asas a a solid catalyst [30] and whose performances are comparable to that of a previously developed solid catalyst [30] and whose performances are comparable to that of a previously developed rhodium-carbon nanotube rhodium-carbon nanotube catalytic catalytic system system [23]. [23]. 3. Experimental Section 3. Experimental Section 3.1. Assembly Assembly of of the the Nanohybrid Nanohybrid 3.1. Amphiphilic DANTA DANTA (20 mg) was was dissolved dissolved in in 22 mL mL of of 25 25 mM mM pH pH 88 aqueous aqueous Tris-buffer Tris-buffer before before Amphiphilic (20 mg) multiwalled carbon nanotubes (50 mg) were added. The dispersion was sonicated and the multiwalled carbon nanotubes (50 mg) were added. The dispersion was sonicated and the stable stable suspension transferred into two tubes. The tubes were centrifuged at 5000ˆ g and the supernatants suspension transferred into two tubes. The tubes were centrifuged at 5000×g and the supernatants were The latter for 45 45 min. The supernatant was discarded were collected. collected. The latter were were centrifuged centrifuged at at 15,000ˆ 15,000×gg for min. The supernatant was discarded and the the pellets pellets taken for 45 45 min. min. The The pellets pellets were were finally finally and taken in in buffer buffer and and centrifuged centrifuged again again at at 15,000ˆ 15,000×gg for resuspended in 1.5 mL of buffer and submitted to UV irradiation at 254 nm for 6 h. resuspended in 1.5 mL of buffer and submitted to UV irradiation at 254 nm for 6 h. After polymerization, polymerization, the the buffer buffer volume volume was was adjusted adjusted to to 1.5 1.5 mL. mL. The The suspension suspension was was stirred stirred in in After the presence presence of of the the cationic cationic polymer polymer PDADMAC water solution) h. The The ensuing ensuing the PDADMAC (700 (700 µL µ L of of aa 20% 20% water solution) for for 11 h. centrifugation at 15,000ˆ g for 30 min permitted to get rid of the polymer in excess. The pellets were centrifugation at 15,000×g for 30 min permitted to get rid of the polymer in excess. The pellets were taken in 2 mL of buffer. This operation was repeated twice using the buffer solution and two more taken in 2 mL of buffer. This operation was repeated twice using the buffer solution and two more times using using pure pure water. times water.

The final pellets were resuspended in 1 mL of water. Then 50 μL of the latter suspension was transferred to Eppendorf® tubes (×20). To each tube was added 1 mL of a 1 mM colloid suspension of the gold nanoparticles [27] and the mixture was vortex-stirred at room temperature for 1 min every 30 min (during 4 h). The suspension was then centrifuged at 3000×g for 5 min. The supernatant was discarded and 1 mL of a fresh gold colloid suspension was added. The same process was repeated

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The final pellets were resuspended in 1 mL of water. Then 50 µL of the latter suspension was transferred to Eppendorf® tubes (ˆ20). To each tube was added 1 mL of a 1 mM colloid suspension of the gold nanoparticles [27] and the mixture was vortex-stirred at room temperature for 1 min every 30 min (during 4 h). The suspension was then centrifuged at 3000ˆ g for 5 min. The supernatant was discarded and 1 mL of a fresh gold colloid suspension was added. The same process was repeated two more times. The pellets were washed three times by centrifugation/redispersion in water. The 20 pellets were combined and 4 mL of water was finally added. 3.2. Procedure for the Oxidation of Hydroxylamines A typical procedure is given for the oxidation of N-cyclohexylhydroxylamine 2a. Under air, to a stirred solution of N-cyclohexylhydroxylamine hydrochloride (2a.HCl, 0.1 mmol) in 2 mL of CHCl3 /H2 O (1:1) was added K2 CO3 (2 equivalents) and 0.5 mol % of the suspension of the AuCNT catalyst. The reaction mixture was stirred at room temperature for 12 h. The aqueous layer was extracted with CHCl3 . The combined organic layer was dried over anhydrous Na2 SO4 , filtered, and concentrated under vacuum. The crude residue was purified by column chromatography to afford nitroso-cyclohexyl amine 2b in 83% yield. 1 H-NMR (400 MHz, CDCl3 ) δ (ppm) 5.07 (triplet of triplet, J = 3.8 Hz, J = 11.6 Hz, 1H), 1.97–1.94 (multiplet, 2H), 1.89–1.86 (multiplet, 2H), 1.69–1.65 (multiplet, 2H), 1.43–1.31 (multiplet, 2H), 1.28–1.20 (multiplet, 2H); 13 C-NMR (100 MHz, CDCl3 ) δ (ppm) 65.6, 28.2 (2C), 25.0, 24.5 (2C). 4. Conclusions A nanohybrid catalyst was produced by the layer-by-layer supramolecular assembly of gold nanoparticles on carbon nanotubes. The gold-based nanohybrid (AuCNT) was employed in the oxidation of hydroxylamines to provide straightforward access to the corresponding oxidized products in good to excellent yields. The transformation led either to nitroso derivatives in the case of aliphatic hydroxylamines or azoxy derivatives in the case of aromatic/benzylic hydroxylamines. Selectivity, low catalyst loading and mild reaction conditions (e.g., room temperature, open air) are the salient features of the AuCNT methodology. Acknowledgments: Supports from the Indo-French Centre for the Promotion of Advanced Research (IFCPAR)/Centre Franco-Indien pour la Promotion de la Recherche Avancée (CEFIPRA) (Project No. 4705-1) and from the CEA-Enhanced Eurotalents program are gratefully acknowledged. The TEM-team platform (CEA, IBITECS) is acknowledged for help with TEM images. The “Service de Chimie Bioorganique et de Marquage” belongs to the Laboratory of Excellence in Research on Medication and Innovative Therapeutics (ANR-10-LABX-0033-LERMIT). Author Contributions: N.S., P.B., P.P. and S.D. performed the experiments. E.G., I.N.N.N. and E.D. designed the experiments and wrote the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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