Cu2O Nanoparticle-Catalyzed

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Hierarchical Mesoporous RuO2/Cu2O NanoparticleCatalyzed Oxidative Homo/Hetero Azo-Coupling of Anilines Arijit Saha, Soumen Payra, Balaranjan Selvaratnam, Sumantra Bhattacharya, Sourav Pal, Ranjit T. Koodali, and Subhash Banerjee ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01179 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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ACS Sustainable Chemistry & Engineering

Hierarchical Mesoporous RuO2/Cu2O NanoparticleCatalyzed Oxidative Homo/Hetero Azo-Coupling of Anilines Arijit Saha,a Soumen Payra,a Balaranjan Selvaratnam,b Sumantra Bhattacharya,c Sourav Pal*,d,e Ranjit T. Koodali*,b Subhash Banerjee*a a

Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Koni, Bilaspur, C.G., 495009,

India. b

Department of Chemistry, University of South Dakota, 414E Clark Street, Vermillion, SD

57069, USA. c

Department of Chemistry, National Institute of Technology Sikkim, Barfung Block, Ravangla,

Sikkim, 737139, India. d

Director, Indian Institute of Science Education and Research, Campus Road, Mohanpur,

Kolkata, 741246, India. e

Professor (HAG), Department of Chemistry, Indian Institute of Technology Bombay, Main

Gate Road, Powai, Mumbai, 400076, India.

ACS Paragon Plus Environment

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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*E-mail: [email protected] (S.B.); [email protected] (R.K.); [email protected] (S.P.) Keywords: Mesoporous hierarchical RuO2/Cu2O, Oxidative homo/hetero azo-coupling of anilines, Aromatic azos, DFT calculations, Synergistic effect

Abstract: Herein, we have reported hierarchical mesoporous RuO2/Cu2O nanoparticle catalyzed selective aerobic oxidative homo/hetero azo-coupling of anilines under oxidant/base/additivefree conditions. The synergistic effect of the individual oxides was established by the density functional theory calculations and experimental studies.

Aromatic azo compounds (AAzos) are the imperative structural motifs that frequently show diverse potential applications in developing various organic dyes, food additives, pigments, indicators, therapeutic agents, prodrugs and drug coating materials.1-4 Due to their interesting photophysical properties, they frequently exhibit a variety of applications in electronics and optics.5-6 Additionally, several AAzos have been successfully employed as chemosensors, radical initiators, diagnostic probes, and building blocks of various polymers and natural products.7 Due to their immense importance, much efforts have been dedicated to access these scaffolds. Traditionally, AAzos were prepared by the diazotization of anilines,8 oxidation of anilines9 or hydrazines, reduction of azoxybenzenes and reductive coupling of nitro-aromatics.10-11 Furthermore, direct oxidative coupling of anilines using various oxidants has also represented one of the most straightforward route to prepare AAzos.12-15 However, the major limitations of these protocols are associated with the restriction to homo-coupled products only, lower selectivity and productivity, exploitation of excess toxic oxidants/flammable H2/bio-hazardous CO gases. To overcome these limitations, transition metal catalyzed oxidative coupling of aryl


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amines using CuBr/pyridine/O2,16 AgNO3/calix[4]acetate/air,17 Cu powder/pyridine/aerobic,18 Cu/NH4Br/pyridine/O2,19

CuBr/di-tert-butyldiaziridinone/air,20

and

Cu(II)Fe(III)-

LDH/continuous flow21 have been reported. Although, these catalytic protocols furnished the azo-coupling under mild conditions, however, most of the protocols16-20 have used toxic nitrogenous bases/additives, and use of homogeneous and non-reusable catalyst severely limit the sustainability and scope of these methods.

Scheme 1. Mesoporous hierarchical nano-RuO2/Cu2O catalyzed azo-coupling of anilines. Recently, few efficient nano-catalytic systems e.g., Au/TiO2/O2,22 Ag/C/KOH/air23 and mesoMn2O3/air balloon24 etc. have also been developed which were also associated with the restriction of using O2 gas/air balloon and strong base as additive (Scheme 1a). Thus, to meet the urgent demand of sustainable development of this transformation, introduction of a novel nanocatalyst is much desired. Very recently, mixed metal oxide nanostructures have been proven to be the promising green catalytic system in clean chemical synthesis.25-26 Essentially, the synergy between two metal cores improves the red-ox properties at their interfaces which facilitate selective oxidative organic transformations.25-29 Herein, we report the fabrication of a novel nanocrystalline mesoporous hierarchical RuO2/Cu2O for selective aerobic oxidative homo/hetero azo-coupling of anilines under oxidant/base/additives-free conditions (Scheme 1b). Initially, the


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mesoporous hierarchical RuO2/Cu2O nanoparticles (NPs) were prepared in aqueous medium under microwave irradiation and then analyzed by various analytical techniques (Figure 1). Ru content in the studied catalyst was found to be 1.783 wt% from ICP analysis. Thus, the remaining is Cu content i.e. 98.217 wt%. All the diffraction peaks in powder X-ray diffraction (XRD) pattern (Figure 1a) of the material were well indexed to (110), (111), (200), (220), (311), (222) planes which indeed indicate the formation of Cu2O nanocrystals (JCPDS file No. 030898).30-33

Figure 1. (a) PXRD pattern; (b) FE-SEM image; (c-e) TEM images showing lattice fringes with lattice spacings for RuO2 and Cu2O (inset: FFT patterns); (f) Nitrogen adsorption and desorption isotherm (inset: pore size distribution curve); (g-h) EDS elemental mapping; (i) EDS spectra of hierarchical mesoporous RuO2/Cu2O NPs. The average crystallite size of 12.7 nm was obtained by applying Debye-Scherrer formula to the (111) diffraction plane which was further confirmed by DLS study (Figure S1a). The absence of any diffraction peak for RuO2 in the XRD pattern could possibly be due to the relatively low Ru contents and high dispersion of RuO2 in the Cu2O matrix. The Field Emission-Scanning Electron Microscopic (FE-SEM) image (Figure 1b) demonstrates the formation of small sized NPs and


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the rough surfaces with alteration of electron density (light and shade) suggests that the material is made of Cu2O NP aggregates to form a hierarchical morphology with an open, bicontinuous and interpenetrating ligament-pore type structure.

30-34

Further, this aggregated architecture of

RuO2/Cu2O NPs has been confirmed from Transmission Electron Microscopic (TEM) study (Figure 1c-e). 30-33 Besides, the lattice fringes with lattice spacing 0.24 nm in Figure 1d is well consistent with Cu2O.

30-33

The presence of RuO2 was spotted (red circle) in a typical TEM

bright-field image (Figure 1c) and confirmed by analyzing the associated lattice fringes with lattice spacing of 0.231 nm responsible for {110} plane of RuO2. 35 The Fast Fourier Transform (FFT) patterns (inset Figure 1c-d) taken from the selected areas (marked by red circles) in the TEM images are indicating the highly crystalline nature of the sample. The magnified TEM image (Figure 1e) shows the irregular growth directions of the (111) facets (marked by black arrows) of Cu2O in the material. The associated lattice fringes with lattice spacing of 0.244, 0.240 and 0.238 nm evidently demonstrate the inhomogeneous aggregation of highly dispersed Cu2O nanocrystals to generate porous hierarchical matrices in the interior. 30-33 The BrunauerEmmett-Teller (BET) surface area of RuO2/Cu2O was found to be 5.7 m2/g (Figure 1f). The much lower surface area than the solid (> 10 m2/g) or hollow (> 20 m2/g) nanostructures confirms the formation of hierarchical architecture.33 However, this value is much higher than previously reported ones,11a which is expected and due to small size of the NPs. Conversely, the Barrett-Joyner-Halenda (BJH) model revealed that the sample have mesopores with pore size of 19 Å (inset, Figure 1f) and total pore volume of 0.02cm3/g (Figure S1b). The Energy Dispersive X-ray Spectroscopic (EDS) elemental mapping (Figure 1g-h) of the nanomaterial indicates good intermingling of the respective oxides. Also, the EDS analysis discloses the high purity of the sample (Figure 1i). As well, the Thermo Gravimetric Analysis (TGA) indicates its high thermal


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stability of the studied nano-catalyst (Figure S1f). X-ray Photoelectron Spectroscopic (XPS) study was performed to probe the surface composition of the material. The doublet peak located at the binding energies of 932.21 eV and 952.03 eV in Figure 2a are attributed to the Cu 2p3/2 and Cu 2p1/2 respectively of Cu2O.36 The other doublet peak appears at binding energies of 933.81 eV and 953.64 eV and the presence of shake-up satellite peaks indicate the existence of CuO.14a But, no diffraction peaks corresponding to CuO were observed in the powder XRD pattern (Figure 1a) of the RuO2/Cu2O NPs. Meanwhile, the Cu LMM Auger peak (Figure S1d) at the binding energy of 570 eV reveals the presence of Cu2O only on the surface of the material. Thus, powder XRD and XPS studies clearly demonstrate the existence of Cu2O predominantly on the surface of the material along with a very less amount of CuO may be due to the surface oxidation. However, the peaks corresponding to CuO have not been observed in the XPS Cu LMM Auger and powder XRD studies which further confirmed the presence of very low content of CuO.

Figure 2. High-resolution XPS spectrum of meso-RuO2/Cu2O NPs.


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Besides, the Ru 3d5/2 and 3d3/2 peaks (Figure 2b) at the binding energies of 281.75 eV and 285.18 eV respectively strongly implies that Ru is in +4 oxidation state, i.e., RuO2.37 Nonetheless, it is difficult to analyze the Ru 3d spectra perfectly due to the overlapping with C1s (ca. 284.60 eV). Further, the Ru 3p spectra (Figure 2c) with the binding energies of 462.47 eV (3p3/2) and 485.54 eV (3p5/2) confirms the formation of RuO2. 37 The XPS O1s peaks (Figure 2d) centered at 529.06 eV, 530.52 eV, 530.68 eV and 932.38 eV can be ascribed to the lattice oxygen of CuO, lattice oxygen of Cu2O, lattice oxygen of RuO2 and surface-adsorbed oxygen species respectively. 35-37 Next, we have investigated the catalytic performance of the studied nanomaterial for the synthesis of AAzo via oxidative coupling of aniline (1a) as the model substrate. To our delight, meso-RuO2/Cu2O NPs (50 mg; 1.783 wt % of Ru) produced excellent yield of (E)-1,2diphenyldiazene (2aa, 94%) in acetonitrile at refluxing conditions under open atmosphere. The high solvent selectivity could possible due to the elevated solubility of areal oxygen in acetonitrile.38-39 Besides, several Ru and Cu based nanocatalysts were employed in this transformation to compare the catalytic efficacy of the studied catalyst. For the optimization of reaction conditions, we have used 50 mg of the supported Ru-catalyst (Ru content ~ 1.8-2 wt%) unless otherwise stated. Among them, RuO2/Cu2O was found to be much more productive and selective and hence, was proven as the best choice for the synthesis of AAzos. Notably, a rapid decrease of yield of 2aa was accounted upon decreasing the reacting temperature from 85 oC to 65 oC under similar conditions. As expected, similar reaction without loading of any catalyst did not result in any desired product (2aa) even after 24 h. The study on the optimization of reaction conditions is presented in Table 1.


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Table 1. Optimization of reaction conditions for azo-coupling of aryl aminesa

Entry

Catalyst

Solvent

Time (h)

Yield (%)

1 No catalyst MeCN 24 00 2 RuO2/Cu2O NPs MeCN 16 94 3 RuO2/CuO NPs MeCN 24 60 4 RuO2/Cu NPs MeCN 24 68 5 RuO2/rGO NPs MeCN 24 27 6 RuO2/SiO2 NPs MeCN 24 12 7 RuO2/SiO2/CoFe2O4 NPs MeCN 24 18 8 RuO2/CoFe2O4 NPs MeCN 24 25 9 RuO2/NiFe2O4 NPs MeCN 24 12 10 RuO2/CuFe2O4 NPs MeCN 24 20 11 RuO2/Fe3O4 NPs MeCN 24 15 12 RuO2/NiO NPs MeCN 24 00 13 Ru/Co NPs MeCN 24 08 14 Ru/Fe NPs MeCN 24 00 b 15 Cu2O NPs MeCN 24 57 16c RuO2 NPs MeCN 24 31 c 17 Ru NPs MeCN 24 05 d 18 Cu NPs MeCN 24 34 19 RuO2-Cu2O/SiO2 NPs MeCN 24 36 e 20 RuO2/Cu2O NPs Dioxane 24 21 e 21 RuO2/Cu2O NPs DMF 24 00 e 23 RuO2/Cu2O NPs DMSO 24 05 e 24 RuO2/Cu2O NPs PhMe 24 28 e 25 RuO2/Cu2O NPs THF 24 00 e 26 RuO2/Cu2O NPs DCE 24 11 27 RuO2/Cu2O NPs [PMIm]Br 24 16 f 28 RuO2/Cu2O NPs MeCN 24 42 d 29 RuO2/Cu2O NPs MeCN 24 70 g 30 RuO2/Cu2O NPs MeCN 24 94 a Unless otherwise stated, all reactions were performed with aniline (1.0 mmol), catalyst (50 mg), solvent (4 mL) at 85 oC under open atmosphere. b 45 mg catalyst was used. c 2 mg catalyst was used. d 40 mg catalyst was used. e Reactions were carried out at 110 °C. f The reaction was carried out at 65 °C. g 60 mg catalyst was used.


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The scope of this methodology was then explored using the optimized reaction conditions. It was observed that the meso-RuO2/Cu2O NPs were efficiently furnished the oxidation of a variety of structurally diverse aryl amines to produce desired AAzo compounds in good to excellent yields (75-100%) within 14-18 h as shown in Table 2. Table 2. Substrates scope for RuO2/Cu2O catalyzed aerobic oxidative homo-coupling of aryl aminesa

Cl

Cl N

Cl N Cl

2kk, 16h, 88%

a

1 (1 mmol) and meso-RuO2/Cu2O NPs (50 mg) were refluxed in MeCN (4 mL) at 85 oC at open atmosphere, unless otherwise stated.


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Conspicuously, electron rich mono-substituted aryl amines showed admirable reactivity with higher isolated yields (>90%). Besides, di-substituted aryl amines (1k-m) were also selectively oxidized; albeit with lower yields (80-88%) that may be due to steric hindrance. The studied catalyst is also effective in large scale synthesis of AAzos and good yield (83%, 1.5 g) of 2aa was obtained even after 10 fold scaled-up (10 mmol scale) under the standard reaction conditions. In-situ UV-Visible and kinetic study (Figure 3) have been performed to show the reaction profile in different interval of time that clearly indicates the full conversion of 1b to 2bb.

0.0

(b)

-0.1 -0.2

In [A]t/[A]0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.3

-1

K = 0.64 h 2 R = 0.9968

-0.4 -0.5 -0.6 -0.7 -0.8 -0.9 6

7

8

9

10

11

12

Time (h)

Figure 3. (a) In-situ UV-Visible study and (b) kinetic study of meso-RuO2/Cu2O NPs catalyzed azo-coupling of aniline.


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Further, diversity of this protocol has been demonstrated by investigating the feasibility of the formation of asymmetric AAzos under standard reaction conditions. It is noteworthy to mention that asymmetric AAzos are industrially important and synthetically challenging compounds as these moieties usually achieved via diazotization of electron poor anilines followed by the coupling with electron rich arenes.22 To our delight, meso-RuO2/Cu2O hierarchical NPs efficiently catalyzed oxidative cross-coupling of different aryl amines, producing asymmetric AAzos in moderate to good yields (55-72%, Table 3). However, it is noteworthy to mention that some amounts of symmetrical AAzos were also produced during synthesis of unsymmetrical AAzos via homo-coupling of respective anilines. The formation of mixture of products was confirmed from 1H NMR (See SI-5, S23-S25) and HRMS (See SI-6, S26-S27) spectroscopic studies of crude products. Table 3. Substrates Scope for RuO2/Cu2O catalyzed aerobic oxidative cross-coupling of aryl aminesa

N

N OMe

2ab, 16h, 67%

a

1 (0.5 mmol each) and meso-RuO2/Cu2O NPs (50 mg) were refluxed in MeCN (4 mL) at 85 oC at open atmosphere, unless otherwise stated.


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Further, to shed light on the catalytic role of meso-RuO2/Cu2O NPs in this transformation, we have performed Density Functional Theory (DFT) calculations using Vienna ab initio simulation package (VASP) and and Gaussian 09 software suit (Revision A.01) (ESI 4).40 Here, we have chosen (111) surface of Cu2O (Figure S18) for our study as (111) surface is the most active surface among different surfaces of Cu2O.41 Besides, we have taken a small cluster of Ru2O4 (Figure S19). Our calculations suggest that the binding energy of aniline on Cu2O (111) surface and Ru2O4 are -32.5 kcal/mol and -13.3 kcal/mol respectively (Figure 4a-b). This phenomenon clearly demonstrates that the oxidation of aniline is promoted at Cu2O (111) surface. On the other hand, adsorption energies of molecular oxygen on Cu2O (111) surface and Ru2O4 are -37.3 kcal/mol and -52.5 kcal/mol respectively (Figure 4c-d). This observation clearly reveals that the binding of oxygen is more favored on Ru2O4 site. Further, the adsorption energy of -11.4 kcal/mol indicates favorable interaction of Ru2O4 on Cu2O surface (Figure 4e). To get a deeper insight into the dissociation of oxygen on ruthenium oxide, we have performed theoretical calculations based on density functional theory at the molecular level using g09. The equilibrium bond length of oxygen molecule is 1.22 Å, while interacting with Ru2O4, a significant elongation along O-O bond is observed, (∆dO-O =0.14 Å). Thus, we propose that the spin state of oxygen is changed from triplet to singlet on Ru2O4. This is further confirmed by the spin multiplicity of the composite system (A) which is triplet in the ground state. The BSSE corrected (counter poise correction) binding energy between oxygen and Ru2O4 in A, is calculated to be -5.7 kcal/mol. Next, we have investigated the dissociation of oxygen over Ru2O4 using transition state modeling (Figure S25). The free energy barrier for such process is computed to be 6.6 kcal/mol which indicates a facile dissociation of the oxygen on Ru2O4. The O-O bond length in TS is 1.75 Å and both the forming Ru-O bond distances become 1.85Å. Thus, the lower energy barrier for oxygen


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dissociation is attributed to the strong Ru-O interactions in the transition state. Finally, the dissociation of oxygen leads to the formation of product Ru2O6 (P) and the reaction is computed to be exergonic by 39.8 kcal/mol which proves the facile dissociation of oxygen on RuO2 surface. The HOMO of oxygen molecule is energetically closer to the LUMO of Ru2O4; hence electron from HOMO of oxygen molecule might transfer into the LUMO of Ru2O4.

Figure 4. Binding of aniline on (a) Cu2O (111) surface and (b) Ru2O4; binding of oxygen on (c) Cu2O (111) surface and (d) Ru2O4; (e) binding of Ru2O4 on Cu2O (111) surface. Cream, red, sea green, grey and blue colours represent Cu, O, Ru, C and N respectively. The theoretical results were further corroborated by set of experimental studies as shown in Scheme 2. The formation of product (2aa) was sluggish under closed vessel or N2 atmosphere compared to O2 atmosphere (Eq. 1). This underscores the participation of areal oxygen. Besides, the reaction does not initiate in the presence of radical quencher, TEMPO (Eq. 1) and indicates that the formation of the products is through the radical pathway, which was further supported by the Electron Paramagnetic Resonance (EPR) study (Fig. 5). Again, the reaction with Cu2O and RuO2 NPs individually were failed to produce good yields of 2aa (Eq. 2). This clearly indicates the powerful synergistic effect of composite RuO2/Cu2O NPs. Taking into account these results, we can rationalize that the reaction is taking place at the interfaces of two oxide layers at the surface of the catalyst. Further, fast oxidation of hydrazine into 2aa (Eq. 3) indicates the absence of any intermediate during the reaction.


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Scheme 2. Control experiments for mechanistic insight. Figure 5 insights facile activation of areal oxygen by meso-RuO2/Cu2O NPs. Single pick was observed when the catalyst was dispersed in MeCN. Interestingly, one new pick (g = ~ 2.04) was appeared once it was refluxed in MeCN under open air. Literature survey42-44 revealed that the new pick is the characteristic signal of superoxide radicals adsorbed on the surface of the catalyst. These superoxide radicals are forming via activation of areal oxygen which indeed confirmed when the catalyst was dispersed in MeCN under oxygen atmosphere.


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(a) Dispersed in MeCN

(b) Refluxed in MeCN under open air

EPR Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(c) Dispersed in MeCN under oxygen

0

2000

4000

6000

8000 10000 12000 14000

Magnetic Field (G)

Figure 5. EPR evidenced activation of areal oxygen by meso-RuO2/Cu2O NPs. In view of the above findings, a plausible mechanistic pathway for mesoporous RuO2/Cu2O NPs catalyzed oxidative coupling of anilines is outlined in Scheme 3.


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Scheme 3. Plausible mechanism for the RuO2/Cu2O-catalyzed azo-coupling of aniline. Finally, to examine the heterogeneity of the studied catalyst, hot filtration test was performed under optimal conditions. The catalyst was separated from reaction mixture under hot condition after a reaction time of 12 h by ultra centrifugation. The filtrate was kept for next 5 h under same reaction conditions. Only 47% of (E)-1,2-diphenyldiazene (2aa) was formed after 12 h without further improvement (Figure 6a).


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100

94%

(a)

Complete run Filtrate recovered after 10 h

90 80 70

Yield (%)

60

51%

51%

50

47%

40 30 20 10 0 0

2

4

6

8

10

12

14

16

Time (h) 94

90

92

91

91

88

(b) 86

85 80

80

100 90 80

70

70

60

60

50

50

40

40

30 20

16

16

18

18

18

20

20

24

Time (h)

100

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30 20 10

10

0

0 1

2

3

4

5

6

7

8

Run

Figure 6. (a) Leaching study; (b) reusability study of the meso-RuO2/Cu2O NPs in azo-coupling of aniline. Thus, the catalyst is highly stable and no such leaching of metal contents was observed which was eventually evidenced from the ICP analysis of the filtrate. Further, to check the reusability in the oxidative coupling of aniline (1a), mesoporous RuO2/Cu2O hierarchical nanostructures was recovered from the reaction system by centrifugation, washed with acetonitrile and dried well


17


prior to reuse. To our delight, the catalyst could be reused at least eight times without significant loss of its catalytic efficiency (Figure 6b). However, the little loss in yield (14%) and the increase of reaction time (16 h to 24 h) attributed to slow conversion of Cu2O to CuO due to aerial oxidation during the reactions and recycling process which was evident from the powder XRD

30

40

50

60

{113} CuO

{311} CuO

{220} Cu2O

{202} CuO

{202} CuO

{111} CuO

Reused meso-RuO2/Cu2O NPs

{200} Cu2O

{111} Cu2O

{002} CuO

analysis (Figure 7) of recovered catalyst after 8th cycle.

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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70

2θ (degree)

Figure 7. Powder XRD pattern of reused meso-RuO2/Cu2O NPs. In conclusion, we have reported a novel hierarchical mesoporous nano-RuO2/Cu2O for sustainable production of symmetrical/unsymmetrical azoarenes. Catalytic role and the synergistic effect of the studied catalyst have been well demonstrated by DFT calculations and experimental studies. DFT calculated adsorption energies clearly demonstrate that aniline will preferably binds on Cu2O (111) surface whereas, the binding and dissociation of oxygen is more favoured on RuO2 surface. The productivity, selectivity and sustainability are the major significant advancement for this protocol for synthesis of AAzos. We believe that the studied hierarchical meso-RuO2/Cu2O NPs will find many useful applications in catalysis and organic transformations.


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ASSOCIATED CONTENT Supporting Information. Preparation and characterizations of the catalysts, synthetic procedure of hierarchical meso-RuO2/Cu2O NPs catalyzed oxidative azo-coupling of aryl amines, detailed DFT study, representative NMR copies and representative HRMS copies will be available in the ssupporting information. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S.B.); [email protected] (R.K.); [email protected] (S.P.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We

are

pleased

to

acknowledge

funding

agency

CCOST,

Raipur

(ENDT

No

2096/CCOST/1I[RP/2017). Special thanks to Prof. R. Balamurugan and his research group and UGC Networking Resource Centre, School of Chemistry, University of Hyderabad for analytical facilities. REFERENCES 1

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Table of Contents

Here, smart hierarchical mesoporous RuO2/Cu2O nanoparticle-catalyzed selective aerobic oxidative homo/hetero azo-coupling of anilines has been demonstrated. A synergistic effect of the individual oxides was established by the density functional theory calculations and experimental studies.


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Cu2O Nanoparticle-Catalyzed

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