Direct Oxidative Azo Coupling of Anilines Using a Self-Assembled Flower-like CuCo2O4 Material as a Catalyst under Aerobic Conditions.

Herein, we report the synthesis of a self-assembled flower-like CuCo2O4 material by the oxalate decomposition method. The crystalline structure and morphology of the material have been analyzed by powder X-ray diffraction, Raman spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray measurement techniques. The self-assembled flower-like CuCo2O4 material showed remarkable catalytic activity in the direct aerobic oxidative azo coupling of anilines under oxidant and other additive-free reaction conditions. The mechanistic insight of CuCo2O4 in the oxidative azo coupling reaction has been established by density functional theory calculations, which disclosed that the absorption and dissociation of areal oxygen preferentially take place at the Cu site and dissociation of aniline takes place at the Co site. Thus, the Cu and Co sites of CuCo2O4 exert a cooperative effect on the direct oxidative azo coupling reactions through the selective activation of anilines and aerobic oxygen. The CuCo2O4 material was recovered from the reaction mixture and reused for at least eight runs without appreciable loss of catalytic activity.

Introduction

Recently, spinel structures (AB2O4) having binary and ternary mixtures of metal oxides have been established as promising redox catalysts.[1−6] The presence of two mixed valence metal cations offers an opportunity for transporting electrons very easily between multiple transition-metal cations with relatively low energy of activation. Among the spinel structure, spinel cobaltites (MCo2O4), particularly CuCo2O4, are fascinating due to their low cost, nontoxicity, higher stability, higher electronic conductivity, and electrochemical properties. To date, CuCo2O4 has been widely used in the fabrication of supercapacitors,[1] Li-ion batteries,[2] electrodes for oxygen evolution reaction,[3] electrochemical sensors for glucose[4]/acylcholin,[5] and catalytic oxidation of isopropanol.[6] However, the catalytic activities of spinel type MCo2O4 in useful organic transformations have not been investigated. Here, we speculated that the spinel cobaltite will be very effective as a catalyst for oxidation reactions, and we have taken the initiative to explore the catalytic activity of CuCo2O4 in the direct oxidation of anilines to aromatic azos.

Synthesis of aromatic azo compounds is essential as these moieties have found wide industrial applications for the preparation of dyes, pigments, indicators, radical initiators, and additives for food. Azos have also been used as therapeutic, diagnostic, and pro-drug agents as well as building blocks of various polymers and natural products.[7] These molecules have also been frequently applied in electronics and optics.[8] Several strategies have been reported in the literature for the manufacture of azo compounds due their abovementioned widespread applications. The conventional methods include diazotization of anilines,[9] reduction of azoxybenzenes,[7a] reductive coupling of nitroaromatics, etc.[10] Besides these methods, direct oxidative coupling of anilines using stoichiometric oxidants, such as HgO,[12a][12b]/Pb(OAc)4[12]/Mn-based reagents[13]/butyl hypoiodite,[15] is also reported. Later, metal-catalyzed azo coupling reactions using O2 or air as the oxidant have been developed.[15] However, most of these methods have serious drawbacks such as lower yields, longer reaction times (∼24 h), use of very toxic oxidants (HgO, Pb(OAc)4, butyl hypoiodite)[11−14] and flammable (O2 gas) or biohazardous reagents (CO gas), and use of metal salts as homogeneous catalysts in combination of nitrogen-containing base, additives, etc. Thus, development of an efficient and sustainable protocol for direct synthesis of azos by oxidative azo coupling of anilines is considerably anticipated.

In continuation of our previous research on developing sustainable organic transformations using heterogeneous nanocatalysts,[16,17] we have previously synthesized Cu2O–RuO2[18] and Cu0.9Fe0.1@RCAC[18] materials for the oxidative azo coupling of anilines.[17] In this paper, we report the synthesis of self-assembled flower-like CuCo2O4 and its remarkable catalytic efficiency in the direct oxidative azo coupling of anilines (Scheme ).

Scheme 1

CuCo2O4-Catalyzed Azo Coupling of Anilines

Results and Discussion

Initially, we synthesized CuCo2O4 by the oxalate decomposition method.[18] Typically, 20 mmol of Co(NO3)2·6H2O and 10 mmol of Cu(NO3)2·3H2O were dissolved in 10 mL of conc. HNO3, and ammonia was added drop by drop until pH 6–7. Then oxalic acid (2 wt %) was added into the resulting solution resulting in a pink-colored precipitate. The precipitate was washed with distilled water and dried in an oven at 60 °C and heated at 350 °C in a muffle furnace for 3 h in a crucible to produce CuCo2O4 as a black material. The formation of the nanostructured CuCo2O4 material was analyzed by powder X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDX) measurement techniques. Figure a shows the XRD pattern of CuCo2O4 powder. The diffraction peaks are observed at 2θ values of 18.77, 31.78, 35.86, 38.76, 44.72, 53.95, 58.60, 66.15, 68.32, and 75.42°, which were assigned to the diffraction (111), (220), (311), (222), (400), (422), (511), (440), (531), and (533) lattice planes of CuCo2O4, respectively. These diffraction peaks are within the statistical limits of counts indicating the polycrystalline cubic spinel phase of CuCo2O4 (JCPDS Card No. 01–1155).

Figure 1

(a) Powder XRD pattern and (b) Raman spectrum of the CuCo2O4 material.

The crystallite sizes (D) on the orientations of (311) and (222) have been estimated using the Debye–Scherrer formula, which is given by the relation[19]D = 0.9λ/βcosθ, where λ (i.e., 1.54 Å) is the wavelength of the incident X-ray source and β is the full width at half-maximum (FWHM). The D values along the direction of the (311) and (222) planes have been found to be 27.83 and 23.25 nm, respectively.[20] In order to get further confirmation on phase formation, Raman spectroscopy measurement has been performed. Figure b shows the Raman spectrum of the CuCo2O4 powder sample. The two Raman peaks approximately at 187 and 460 cm–1 were present corresponding to the Raman active modes of F2g, and Eg, respectively.[21] These Raman modes are in good agreement with that of the CuCo2O4 crystalline phase.

In order to have morphological insights on the synthesized CuCo2O4 material, FESEM measurement has been performed using a 15 keV electron beam. Figure a,b shows the FESEM images of the CuCo2O4 material. The formation of CuCo2O4 microsized rods along with some sheets has been observed in the FESEM image, and these are assembled in different orientations to form a flower-like morphology.

Figure 2

(a,b) FESEM images, (c) EDX spectrum, and (d) TEM image of the CuCo2O4 material.

The average lengths of the microrods and microsheets are found to be around 965 nm and 1.4 μm, respectively. The collected EDX spectrum is shown in Figure c, illustrating that the presence of dispersive peaks corresponds to the elements C, O, Cu, Co, and Pt (i.e., due to Pt coating during SEM measurement).Hence, the presence of these elements confirms the purity of the sample.[22] Apart from the FESEM study, the TEM measurement has also been carried out to confirm the morphology of CuCo2O4 powder. Figure d shows the TEM image of CuCo2O4 powder, which clearly indicates the formation of self-assembled flower-like morphology.

Next, the catalytic activity of freshly prepared and well-characterized CuCo2O4 has been investigated in the direct oxidative azo coupling of anilines leading to aromatic azo compounds. Initially, we have tested the oxidative azo coupling of p-toluidine as a model reaction under aerobic conditions. When a mixture of p-toluidine (1 mmol) and CuCo2O4 (20 mol %, 50 mg) was heated at 85 °C in 4 mL of acetonitrile (MeCN) under air flow conditions, consequently, an excellent amount of (E)-1,2-di-p-tolyldiazene (2b, 95%) is obtained. Next, the effects of different reaction parameters have been investigated to optimize the reaction conditions, which are summarized in Table . The high solvent selectivity could possibly be due to the elevated solubility of areal oxygen in acetonitrile.[22b] It is clearly observed from Table that the mixed oxide CuCo2O4 is superior to only Co2O3 (entry 5, Table ) and CuO (entry 6, Table ). Moreover, to demonstrate the role of Cu in CuCo2O4 in the azo coupling reaction, we have performed the reaction using only Co3O4. It is observed that mixed oxides of Co(II) and Co(III), i.e., CoCo2O4, could produce a very less amount of product (20%), which confirms that Cu(II) plays a significant role in the oxidative azo coupling reaction (entry 7, Table ). The reaction conditions as depicted in entry 1 in Table are considered as optimum reaction conditions.

Table 1

Optimization of Reaction Conditions for Azo Coupling of p-Toluidinea

entry	catalyst	temperature	solvent	time (h)	yield (%)
1	CuCo2O4	85 °C	MeCN	10	95
2	CuCo2O4	100 °C	PhMe	10	85
3	CuCo2O4	100 °C	DMF	24	00
4	CuCo2O4	85 °C	DCE	12	11
5	Co2O3	85 °C	MeCN	12	trace
6	nano-CuO	85 °C	MeCN	10	47
7	CoCo2O4	85 °C	MeCN	10	20
8	CuFe2O4	85 °C	MeCN	10	15
9	CuCo2O4	60 °C	MeCN	12	60
10	CuCo2O4	85 °C	MeCN	24	70b
11	CuCo2O4	85 °C	MeCN	24	96c
12	none	85 °C	MeCN	24	NR

Unless otherwise stated, all reactions were performed with toluidine (1.0 mmol), catalyst (20 mol %, 50 mg), and solvent (4 mL) at 85 °C under an open atmosphere.

12 mg of catalyst was used.

75 mg of catalyst was used.

Interestingly, a significant decrease in yield has been observed when Co(III) in CuCo2O4 is replaced by Fe(III), i.e., by using CuFe2O4 (entry 8, Table ). It is worth mentioning here that upon changing the reaction condition from refluxing to 60 °C reaction, a decrease of yield of azo is observed under similar conditions (entry 9, Table ). Next, the amount of CuCo2O4 has been optimized (entries 9 and 10, Table ), and it is observed that a 20 mol % catalyst is sufficient to push the reaction forward. In a control experiment, the reaction under catalyst-free conditions did not result in the desired azobenzene even after 24 h (entry 12, Table ).

Next, the scope of the CuCo2O4-catalyzed direct oxidative azo coupling reaction has been explored under optimized reaction conditions for the selective synthesis of azos. A variety of aromatic amines smoothly participated in the CuCo2O4-catalyzed oxidative azo coupling reaction to produce homocoupled azo compounds under optimized reaction conditions in good to excellent yields (85–95%). The results are summarized in Table .

Table 2

Scope of CuCo2O4-Catalyzed Aerobic Oxidative Homocoupling of Aryl aminesa

1 (1 mmol) and 50 mg of CuCo2O4 were heated at 85 °C in MeCN (5 mL) in air flow using an air pump (2.0 mL/s flow rate), unless otherwise stated.

It is observed that the monosubstituted aromatic amines with electron-donating groups (2b–h) produced higher yields of azo compounds than disubstituted aromatic amines (2i–j). The lower isolated yield could possibly be due to the steric effect. All the reactions have been performed under air under refluxing conditions. The reactions listed in Table for the synthesis of azos are very clean and high yielding (81–98%). After the reactions, the catalyst was filtered using filter paper, washed thoroughly with ethyl acetate (2 × 1 mL), dried in a hot-air oven, and reused for subsequent runs. The combined organic layers have been evaporated under reduced pressure and purified by column chromatography over silica gel using ethyl acetate/pet ether (1:9) as eluting solvent. All the azos, listed in Table , are known in the literature and were identified by 1H NMR studies. The spectra are given in the Supporting Information.

The recyclability of CuCo2O4 has been tested for the oxidative azo coupling of aniline as model reaction (2a, Table ) in 2-fold scaled-up conditions. It was observed that the catalyst can be recycled up to 8 times without significant loss in catalytic activity. However, a slow but steady loss in the yield of the azobenzene (2a) is observed (Figure ). This decrease in yield may possibly be due to the loss of catalyst during the recycling process. Interestingly, the phases of CuCo2O4 remained intact even after the 8th cycle as confirmed from the powder XRD and FESEM studies of the recycled catalyst, as shown in Figure .

Figure 3

Recyclability of the CuCo2O4 material for the synthesis of azobenzene (2a) as model reaction.

Figure 4

(a) Powder XRD and (b) FESEM image of reused CuCo2O4.

The present CuCo2O4-catalyzed oxidative azo coupling protocol is also applicable in large-scale synthesis of azos. The reaction of oxidative azo coupling of p-toluidine has been scaled-up to 10-folds (10 mmol scale), and good yield (80%) of 2b has been obtained under the standard reaction conditions.

Finally, we have presented a comparative study of the present method of synthesizing (E)-1,2-di-p-tolyldiazene by oxidative azo coupling of p-toluidine to previously reported methods (Table ).

Table 3

Comparative Study of the Present Method with Respect to Previously Reported Catalysts/Reagents for the Oxidative Azo Coupling of p-Toluidine

entry	catalyst	oxidant/additive	conditions	time	yield (%)	ref
1	Cu0.9Fe0.1@RCAC	air flowa	MeCN, 80 °C, 10 W LED	10 h	95	(17b)
2	RuO2/Cu2O NPs	open air	MeCN, 85 °C	16 h	94	(17a)
3	CuBr	pyridineb	PhMe, 60 °C	20 h	96	(15a)
4	Cu powder	pyridineb	DCM, 120 °C	20 h	91	(15c)
5	Au/TiO2	O2	PhMe, 100 °C	3 h	98	(15g)
6	CuCl	air flow	MeCN, RT	10 h	85	(15j)
7	meso-Mn2O3	air balloon	PhMe, 110 °C	7 h	99	(15i)
8	MnO2@g-C3N4	air flow	MeCN, RT, 20 W domestic bulb	12 h	91	(15k)
9	CuCo2O4	air flowa	MeCN, 85 °C	10 h	95	this work

Air flow using an air pump (flow rate = 2.0 mL/s).

50 bar and 0.1 mL/min flow rate.

This study clearly demonstrated that the present method produced comparable yield in a shorter reaction time. The present method also avoided the use of inflammable O2 gas and toxic nitrogenous base like pyridine.

In order to get mechanistic details of dissociation of the oxygen molecule over the CuCo2O4 material, the mechanism of dissociation of the oxygen molecule over the CuCo2O4 surface at the molecular level has been studied. To enlighten the same, we have applied the Density Functional Theory (DFT) method as implemented in the Gaussian 09 software suite (revision A.01).[23] To take care of the exchange correlation parameters, we have employed the Perdew–Burke–Ernzerhof (PBE)[24] functional, with a variant of Generalized Gradient Approximation (GGA). However, the incorporation of the basis set is not straightforward. We have used the 6-31G(d,p) basis set for the nontransitional elements and the Los Alamos National Laboratory 2 Double-Zeta basis set for Cu and Co. All the calculations are performed in the gas phase to minimize the intermolecular interaction. Therefore, to perform DFT calculations, the initial structure of CuCo2O4 is obtained from the crystal structure as reported by Bertaut and Delorme.[25] From the periodic structure, we have constructed a dimer of the same species by maintaining the stoichiometry to model the spinel-shaped structure of CuCo2O4. Since all the metal ions have incompletely filled d-orbitals, hence, different spin states of the system are expected. Thus, to obtain the equilibrium geometry at the ground state with the most stable spin state, we have optimized Cu2Co4O8 at various spin states and the geometry with a particular spin state possessing minimum energy having highest stability and considered for further calculations. Their relative energies are compiled in Table S1, and the optimized structure of Cu2Co4O8 is shown in Figure .

Figure 5

Optimized structure of the Cu2Co4O8 system (where cream, sky blue, and red colors indicate copper, cobalt, and oxygen atoms, respectively).

Our calculations suggest that the Cu2Co4O8 molecule with the septet spin state is most stable. Further, the dimer is allowed to interact with the optimized ground state oxygen (triplet spin state) as well as the aniline (singlet spin state) molecule. Herein, the oxygen molecule can have two possible sites of interaction, viz., the Cu site and Co site. Like the optimization of the most stable dimer of CuCo2O4, here, we have also applied the strategy of optimizing the spin states of the oxygen–Cu2Co4O8 adduct and aniline Cu2Co4O8 adduct for both structures. When the oxygen molecule is closer to the Cu site, we represent the adduct as CuO, and when the oxygen molecule is closer to the Co site, we denote the adduct as CoO. In the same way of nomenclature, Cuaniline and Coaniline indicate that aniline is closer to the Cu site and Co site, respectively. The relative energies of all four possible structures with different spin states are compiled in Table (energies of the CuO composite at different spin states), Table S3 (energies of the CoO composite at different spin states), Table S4 (energies of the Cuaniline composite at different spin states), and Table S5 (energies of the Coaniline composite at different spin states). From these four tables, we have filtered the triplet state of all the species due to their maximum stability. The optimized geometries of all four species are shown in Figure .

Figure 6

Optimized structures of the (A) CuO, (B) CoO, (C) Cuaniline, (D) Coaniline systems. Cream, sky blue, red, deep blue, gray, and white colors indicate copper, cobalt, oxygen, nitrogen, carbon, and hydrogen atoms, respectively.

With these filtered composite systems, we studied their interaction energies, and based on their extent of interaction, binding energies may also vary. From the Basis Set Superposition Exchange corrected (Counter Poise correction)[26] binding energy values, we can say that the greater the binding energy, the higher the extent of stability of that particular adduct. The adsorption energies of molecular oxygen on Cu and Co sites are −10.76 and −6.03 kcal/mol, respectively. On the other hand, adsorption energies of aniline on Cu and Co sites are −17.23 and −24.94 kcal/mol, respectively. These results clearly indicate that the molecular oxygen or aniline bind selectively with Cu and Co sites. Particularly, aniline shows a greater tendency to bind at the Co site, and the oxygen molecule prefers to bind at the Cu site rather than the Co site. Hence, we can say that the oxygen molecule prefers the Cu site and dissociation of the oxygen molecule may take place from the Cu site. The BSSE-corrected energies are given in Table S6. In the same way, we have optimized the complex containing both aniline and oxygen molecule. Herein, we have found that the structure with triplet spin is the most stable state. Their energy differences are tabulated in Table S7. The BSSE-corrected energy for the most stable complex shows that the structure is stable. The structure is shown in Figure .

Figure 7

Optimized structure of the CoCu2O4–aniline–oxygen complex system in the triplet spin state. Cream, sky blue, red, deep blue, gray, and white colors indicate copper, cobalt, oxygen, nitrogen, carbon, and hydrogen atoms, respectively.

Finally, a set of control experiments have been performed to investigate the mechanistic insights into CuCo2O4-catalyzed oxidative azo coupling of aniline. The results are presented in Scheme . When the optimized reaction for the synthesis of 2a has been performed under a N2 atmosphere, a trace amount of product is formed (eq 1). Again, under a closed vessel, only 20% of 2a was isolated (eq 2). The same reaction under an O2 atmosphere produced 95% of 2a within 6 h (eq 3). These experiments indicate the active involvement of areal oxygen. Further, in the presence of a radical quencher, TEMPO (eq 4), the reaction does not initiate, which indicates that CuCo2O4-catalyzed oxidative azo coupling proceeds via the radical pathway. Again, the reactions with only CuO and only Co2O3 have failed to produce good yields of 2a (entries 5–6, Table ). Even only cobaltite (Co3O4) was also found to be less effective than CuCo2O4 (entry 7, Table ). This clearly indicates the cooperative effect of Cu and Co of the CuCo2O4 material on the oxidative coupling.

Scheme 2

Few Control Experiments for the Investigation of the Reaction Mechanism

Based on experimental and theoretical studies, a plausible mechanism for the CuCo2O4-catalyzed oxidative azo coupling of aniline is presented in Scheme . Initially, aerobic dioxygen decomposes on the surface of CuCo2O4 preferably by the interaction with the copper site. Next, aniline absorbs on the surface of the catalyst and interacts with cobalt of CuCo2O4 to form an aniline radical cation, which reacts with another molecule of aniline to produce a hydrazobenzene radical cation by removal of a water molecule from the surface of the catalyst. Next, the hydrazobenzene radical cation accepts an electron and changes into hydrazobenzene and subsequently into azobenzene by again removing a water molecule from the surface of the catalyst.

Scheme 3

Plausible Mechanism of CuCo2O4-Catalyzed Homocoupling of Aniline

Conclusions

In conclusion, we have fabricated flower-like assembled CuCo2O4 microsized rods by the oxalate decomposition method, which were analyzed by powder XRD, FESEM, TEM, EDX, and Raman spectroscopic techniques. The flower-like CuCo2O4 material showed remarkable catalytic efficiency in the direct oxidative azo coupling of anilines under oxidant and other additive-free reaction conditions. All the reactions are very clean and high yielding (81–98%). The mixed spinel CuCo2O4 has demonstrated superior catalytic activity to its individual metal oxides, namely, CuO and Co2O3. The catalyst has been recovered from the reaction mixture and reused for at least eight runs without significant loss in the catalytic activity. The interaction of areal oxygen and aniline with Co and Cu sites of CuCo2O4 has been established by density functional theory calculations. The density functional theory calculations indicate that the oxygen molecule preferably dissociates on the Cu site and aniline dissociates on the Co site, and thus, Cu and Co provides a cooperative effect on the oxidative azo coupling of anilines. Moreover, this is the first application of CuCo2O4 in organic transformation, and we believe that the material will find more applications in organic synthesis.

Experimental Section

General Information

All the chemicals and solvents were purchased from Merck, Sigma Aldrich, TCI Chemicals, Sd-Fine, and HIMEDIA (India). All chemicals were used as received from suppliers. The solvents were distilled before use without any further purification, and double distilled water was used throughout the experiment. All the reactions have been carried out under an open air atmosphere in oven-dried glassware. Precoated Merck 60F 254 silica gel plates with 0.25 mm breadth were used for thin layer chromatography (TLC), and ethyl acetate and petroleum ether were used as eluting solvent. The 1H NMR spectra of the synthesized compounds were recorded using Bruker 400 MHz and JEOL 400 MHz spectrometers. The chemical shifts were recorded to the center of solvent resonance at CDCl3 (δ = 7.26; 1H) for 1H NMR studies.[27,28] The chemical shifts of NMR studies are depicted as s (singlet), d (doublet), t (triplet), and m (multiplet), and J (Hz) is denoted for coupling constants.

Method for the Preparation of Nano-CuCO2O4 (by the Oxalate Decomposition Method)

A mixture of 10 mmol of Cu(NO3)2·3H2O and 20 mmol of Co(NO3)2·6H2O was dissolved in 10 mL of conc. nitric acid with stirring, and NH3 was added dropwise until the pH of the resulting solution reached 6–7. The pink-colored precipitate was formed after adding 2 wt % oxalic acid. The precipitate was filtered, washed with distilled water, and dried overnight in an oven at 80 °C. Then the dried material was annealed in a muffle furnace at 350 °C for 3 h, cooled to room temperature, and ground to get black nano-CuCo2O4, which was characterized by various spectroscopic and analytical techniques.

Method for the Preparation of Nano-Co3O4 (by the Hydrothermal Method)

First, 25 mL of 0.10 M Co(NO3)2 and 25 mL of 0.20 M of NaOH solution were prepared in distilled water separately in 50 mL beakers, and then both solutions were mixed. A precipitate was formed, which was transferred into a 100 mL beaker. The beaker was placed in a water bath, and the setup was heated to 120 °C for 4 h and allowed to cool naturally. The obtained brown-colored solid product was thoroughly washed with a water/methanol mixture and dried in an oven at 80 °C for 12 h. The solid product was ground and was annealed in a muffle furnace at 450 °C for 4 h. Finally, the black solid was ground to obtain nano-Co3O4 and was characterized by various spectroscopic and analytical techniques.[29]

General Experimental Procedure for Nano-CuCo2O4-Catalyzed Oxidation of Anilines to Azobenzenes (Entries 2a–l, Table )

An R. B. flask (50 mL) was filled with a mixture of aniline (1.0 mmol), toluene (2 mL), and nano-CuCo2O4 (20 mol %, 50 mg). The mixture was stirred with a magnetic bar at 85 °C under air flow (2.0 mL/s flow rate) conditions. After completion of the reaction (TLC-monitored), nano-CuCo2O4 NPs were separated by filtration. The filtrate was extracted with ethyl acetate (5 mL) and washed with distilled water. The organic layer was dried over anhydrous Na2SO4, filtered, and concentrated on a rotatory evaporator, and finally, the resulting solution was purified by column chromatography over silica gel (60–120 mesh) using a mixture of petroleum ether and ethyl acetate (95:5) as an eluting solvent to get the pure azobenzenes 2a–l.