Facile synthesis of nanostructured Ni-Co/ZnO material: An efficient and inexpensive catalyst for Heck reactions under ligand-free conditions

2.1 Materials

The reagents zinc acetate dihydrate [Zn(OAc)₂·2H₂O], sodium hydroxide (NaOH), nickel chloride hexahydrate (NiCl₂·6H₂O), cobalt chloride hexahydrate (CoCl₂·6H₂O) and sodium borohydride (NaBH₄) required for the synthesis of nanocrystalline ZnO and Ni-Co/ZnO were obtained from S.D. Fine-Chem Limited and Loba Chemie Pvt. Ltd. All these reagents are of analytical grade and were used as received without further purification.

2.2 Synthesis of ZnO nanoparticles by sol–gel method

Initially, 0.5 M 250 mL solution of zinc acetate dihydrate, Zn(OAc)₂·2H₂O was taken in a 1000 mL beaker and stirred at room temperature for 10 min. To this solution, 1.0 M NaOH solution was slowly added drop-wise using dropping funnel with continuous magnetic stirring until the pH became 10. The resultant solution was further stirred at room temperature for 10 h. The residue was kept for 36 h at room temperature. Later, the formed gel (residue) was separated by filtration, washed with distilled water till the pH of the filtrate becomes neutral. Then the residue was dried at 120 °C for 10 h. Further, dried residue was calcined at 500 °C in a muffle furnace for 3 h. Then the obtained powder was allowed to cool, grinded in mortar with pestle and again calcined at 500 °C for 3 h in order to ensure complete decomposition of Zn(OH)₂ to ZnO.

2.3 Synthesis of nanostructured Ni-Co/ZnO material

The nanocrystalline Ni-Co/ZnO material was synthesized by facile reduction-impregnation method. In a typical procedure, NiCl₂·6H₂O (0.4 g) and CoCl₂·6H₂O (0.2 g) were dissolved in 30 mL distilled water. To it, NaBH₄ solution (0.3 g NaBH₄ in 30 mL EtOH) was added drop-wise with constant stirring. The resulting solution was stirred at room temperature for 1 h and then at 80 °C for 4 h. The solution is separated from the resulting black residue by filtration. Then residue in the round bottom flask was mixed with water (30 mL), stirred for 20 min and then allowed to settle down and water was separated. The same procedure was repeated using 20 mL ethanol. Afterwards, 30 mL ethanol was added to the obtained black residue followed by slow addition of 1.0 g of as-synthesized nanocrystalline ZnO powder under continuous magnetic stirring. The resulting mixture was stirred at room temperature for 1 h and then refluxed for 4 h. Finally, EtOH was evaporated under vacuum using a rotary evaporator. The obtained powder was dried in an oven at 120 °C for 4 h. Synthesis of nanocrystalline Ni/ZnO and Co/ZnO catalysts were accomplished by applying the same procedure for the sake of comparing the effect of only Ni and Co loading on ZnO nanoparticles on the Heck reaction. For the synthesis of Ni/ZnO, we used 0.4 g of NiCl₂·6H₂O and for the Co/ZnO synthesis; we used 0.4 g of CoCl₂·6H₂O A detailed scheme depicting the synthesis of a nanostructured Ni-Co/ZnO material and its further application in the Heck reaction is shown in Fig. 1.

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Fig. 1

A detailed scheme depicting the synthesis of nanostructured Ni-Co/ZnO material and its further application in the Heck reaction.

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2.4 Procedure for Heck reaction

In a typical Heck reaction, iodobenzene (1.0 mmol, 500 mg), styrene (1.5 mmol), K₂CO₃ (1.5 mmol), Ni-Co/ZnO catalyst (0.4 mmol, 195 mg) and DMF/H₂O (1:1, v/v, 10 mL) were added into a round bottom flask and stirred at 120 °C for 16 h under air atmosphere. The progress of the reaction was checked by thin layer chromatography (TLC). After completion of the reaction, the reaction mixture was cooled to the room temperature, diluted with ethyl acetate and the reaction mass was stirred at room temperature for 20 min to ensure the separation of product from the catalyst. Then the catalyst was separated and again washed with ethyl acetate. The combined centrifuges were mixed with water and the organic phase was separated by extraction, dried over anhydrous Na₂SO₄, filtered, and evaporated. The resultant crude product mass was purified by column chromatography to yield the desired product. The pure organic compounds were analyzed by ¹H NMR, ¹³C NMR, and FTIR spectral techniques.

2.5 Characterization of nanostructured materials and organic compounds

Powder X-ray diffractometer (Rigaku MiniFlex-600) with Cu K $\alpha$ ( $\lambda$  = 1.5406 Å) radiation was used to study the crystal structure and phase purity of the prepared nanomaterials. The surface morphology of nanostructured materials was observed with the field emission scanning electron microscope (FESEM, Hitachi S-4800). The elemental composition of Ni and Co in the catalyst was measured by energy-dispersive X-ray spectroscopy (EDX). The UV–Visible diffuse-reflectance spectra (DRS) were recorded using a UV–Vis diffuse-reflectance spectrophotometer (Perkin Elmer, Model Lambda 365). The X-ray photoelectron spectroscopic (XPS) scans were recorded by the Thermo Fisher Scientific instrument UK in order to establish the chemical states of elements in the nanocrystalline Ni-Co/ZnO material. All the XPS spectra were corrected using the adventitious C 1 s peak at 284.8 eV.

The progress of the Heck reactions was monitored by thin layer chromatography using Merck silica gel 60 F₂₅₄ TLC plate. The purification of the desired organic products was accomplished by column chromatography technique using silica gel, 60–120 mesh, and n-hexane and ethyl acetate as an eluent. The ¹H NMR and ¹³C NMR spectra of the Heck reaction products were acquired on a Bruker Ascend 500 NMR spectrometer operating at 500 MHz and 125 MHz, respectively, in CDCl₃ solvent. The ¹H and ¹³C chemical shifts (δ) are reported in ppm relative to Tetramethylsilane (TMS), as the internal standard substance. The coupling constant (J) values are expressed in Hz. FT-IR spectra were acquired on Bruker spectrophotometer.

3.1 X-ray diffraction (XRD)

XRD patterns of pure ZnO, Ni/ZnO, Co/ZnO, and Ni-Co/ZnO samples are displayed in Fig. 2. The prominent diffraction peaks associated with (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2) and (2 0 1) planes were observed for all the synthesized samples. All the diffraction peaks are consistent with the hexagonal wurtzite structure which is identified for pristine ZnO (El-Hilo et al., 2019) and is in agreement with standard JCPDS card no. 36–1451, indicating the polycrystalline nature of all the samples (Fig. 2a–d). No additional crystalline phases of impurities were observed in the XRD patterns of all as-synthesized samples signifying the phase purity of all the samples. Besides, diffraction peaks for Ni and Co species were not detected, as expected. The absence of Ni and Co species demonstrates that Ni and Co ions are incorporated in ZnO lattice without altering the wurtzite structure of ZnO (Ali et al., 2018; Kocyigit and Topkaya, 2019; Vijayaprasath et al., 2016).

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Fig. 2

XRD patterns of: (a) Pure ZnO, (b) Ni/ZnO, (c) Co/ZnO and (d) Ni-Co/ZnO powders. Inset is enlarged view of (1 0 1) plane.

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The crystallite size of all the as-synthesized samples was calculated by Scherrer’s equation using the broadening of (1 0 1) peak of ZnO. The calculated crystallite sizes of powdered samples are shown in Table 1 and it should be noted that all the samples shows nearly the same crystallite size. It has been observed that 2θ values are marginally shifted to lower values for Ni, Co and Ni-Co loaded ZnO samples in comparison to unloaded ZnO indicating effective surface coating over ZnO and/or might be due to partial incorporation of Ni and Co in ZnO crystal lattice as shown in inset of Fig. 2 for (1 0 1) plane. The 2θ values of (1 0 1) plane for pure ZnO, Ni/ZnO, Co/ZnO and Ni-Co/ZnO nanocrystalline samples are around 36.28°, 36.22°, 36.24° and 36.18°, respectively. It should be noted that, the 2θ value of (1 0 1) plane for Ni-Co/ZnO sample was more shifted towards lower value compared to Ni/ZnO and Co/ZnO samples.

3.2 Morphological and compositional characterization (FESEM and EDX analysis)

FESEM images of pure ZnO and Ni-Co loaded ZnO nanostructures are shown in Fig. 3. FESEM image corresponding to the pristine ZnO sample shows the formation of elliptical, flower bud-like and irregularly shaped sub-micron sized particle bundles (Fig. 3a). The length of the elliptic and bud-like morphologies varies in the range of 700 to 1500 nm while the thickness is in the range of 200 to 800 nm. The magnified image of a typical bud-like morphology in the inset of Fig. 3a reveals that such morphology is created by the agglomeration of beaded chain like nanostructures of size 30–50 nm. Elliptic and irregular shaped sub-micron sized morphologies also appear to be made up of such beaded chain like nanostructures.

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Fig. 3

FESEM images of (a) pristine ZnO and (b) Ni-Co/ZnO nanostructures.

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FESEM image attributable to Ni-Co loaded ZnO sample as shown in Fig. 3b reveals the presence of agglomerated clusters of nanoparticles over the surfaces of elliptical, flower bud-like and irregularly shaped sub-micron sized particle bundles of ZnO. The magnified image of a typical ellipse like morphology of Ni-Co/ZnO in the inset of Fig. 3b reveals that such morphology is partially covered by the agglomerated nanoparticles of size 10–20 nm (which is speculated to be made up of Ni-Co, indicated by red colored circle and arrows). The distribution of these nanoparticles over ZnO nanostructure is random.

Fig. 4 represents the energy-dispersive X-ray (EDX) spectrum of Ni-Co/ZnO sample. The EDX spectrum reveals that Ni-Co/ZnO sample contains 75.80 wt% zinc, 15.59 wt% oxygen, 4.94 wt% of nickel and 3.67 wt% of cobalt and hence, clearly confirmed that Ni and Co elements were effectively supported over ZnO. The EDX spectrum also discloses that Ni-Co/ZnO contains 3.69 atomic % of Ni and 2.73 atomic % of Co. Inset of Fig. 4 shows the Table corresponding to elemental composition of the sample.

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Fig. 4

FESEM-EDX spectrum of Ni-Co/ZnO catalyst.

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3.3 XPS analysis

X-ray photoelectron spectroscopy (XPS) is a material characterization technique typically used to determine the surface composition and chemical states according to the specific binding energies of various elements on the surface of materials. XPS analysis was performed in order to ascertain the chemical states of the Ni and Co elements in nanocrystalline Ni-Co/ZnO material. Fig. 5 represents the XPS survey spectrum and high resolution scans of different elements present in Ni-Co/ZnO material. Ni, Co, Zn and O elements are observed in the XPS survey spectrum of nanostructured Ni-Co/ZnO (Fig. 5a). In Fig. 5b, the binding energy peaks observed at 1021.3 eV and 1044.5 eV are allied with Zn 2p_3/2 and Zn 2p_1/2 energy states, respectively, signifying Zn²⁺ in ZnO (Mo et al., 2019). The XPS spectrum of O 1 s is depicted in Fig. 5c. The binding energy peak at 530.4 eV is attributed to lattice oxygen of ZnO (Hosseini-Sarvari et al., 2014). Fig. 5d represents the deconvoluted spectrum of Ni 2p core level in which Ni 2p_3/2 and Ni 2p_1/2 energy states are clearly observed at 855.0 eV and 873.1 eV, respectively. The energy difference between these two peaks is 18.1 eV indicating the Ni²⁺ state of Ni in NiO (Malevu et al., 2019). In addition, the Ni 2p deconvoluted spectrum shows the presence of Ni 2p_3/2 peak at binding energy of 856.5 eV which validates the Ni³⁺ state of Ni in the form of Ni₂O₃ (Praveen and Jayakumar, 2019; Yu et al., 2001) and the Ni 2p_1/2 peak at 875.0 eV has been also observed. Moreover, the weak binding energy peak of Ni 2p spectrum at 852.1 eV can be indorsed to the Ni⁰ (Purohit et al., 2017). The other two peaks in the Ni 2p spectrum centered at 861.0 eV and 880.0 eV are attributed to Ni 2p_3/2 and Ni 2p_1/2 satellites, respectively.

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Fig. 5

(a) XPS survey spectrum of nanostructured Ni-Co/ZnO; (b) Zn 2p, (c) O 1 s, (d) Ni 2p and (e) Co 2p XPS spectra of Ni-Co/ZnO nanostructure.

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The deconvolution spectrum of Co 2p core level is depicted in Fig. 5e in which the binding energy peaks of Co 2p_3/2 and Co 2p_1/2 are located at 780.2 eV and 795.8 eV, respectively, which can be fitted to Co³⁺ species (Li et al., 2018a,b; Wu et al., 2018). The observed energy difference between Co 2p_3/2 and Co 2p_1/2 is 15.6 eV. In addition, the peaks at 782.0 eV and 797.9 were corresponding to the Co 2p_3/2 and Co 2p_1/2 energy levels, respectively, which could be ascribed to Co²⁺ species (Li et al., 2018a,b). The Co 2p binding energy peaks observed at 780.2 eV and 795.8 eV are associated with Co₃O₄ (Chen et al., 2007; Liao et al., 2006). The other two peaks in the Co 2p scan centered at 785.7 eV and 802.1 eV are allied to the Co 2p_3/2 and Co 2p_1/2 satellites of Co species.

3.4 UV–Vis DRS analysis

Optical properties of pure ZnO, Ni/ZnO, Co/ZnO and Ni-Co/ZnO nanocrystalline samples were investigated using Ultraviolet–visible diffuse reflectance spectra (UV–Vis DRS). Fig. 6a displays the absorbance versus wavelength graph of unloaded and Ni, Co and Ni-Co loaded ZnO samples. The absorption spectra divulge that there is red shift of the light absorption edge of Ni/ZnO, Co/ZnO and Ni-Co/ZnO materials as compared to pure ZnO. The absorption edge of bare ZnO is at around 395 nm with corresponding band gap of 3.11 eV. By loading with Ni and Co elements, the absorption region gets altered, might be due to partial incorporation of Ni and Co elements in to the ZnO lattice. The metal Ni, Co and Ni-Co loaded ZnO samples exhibited a tailed absorption in the visible light (>400 nm) region.

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Fig. 6

(a) UV–Vis DRS spectrum and (b) Tauc’s plot of unloaded ZnO and metal-loaded ZnO samples.

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The optical band gap energy values of the as-synthesized nanostructures were investigated using the absorption band-edge according to Tauc’s plot (Fig. 6b). The Tauc plot shows that there was a decrease in band gap energy for all metal loaded ZnO samples. The plot of (αhν)² versus photon energy (hν) in eV between valence and conduction bands shows that pure ZnO displayed an energy threshold in the UV region corresponding to an estimated bulk energy band gap of 3.11 eV, while the band gap narrowing was observed for Ni, Co, and Ni-Co loaded ZnO materials. The calculated band gap values are 3.11, 3.01, 2.97 and 2.94 eV for unloaded ZnO, Ni/ZnO, Co/ZnO, Ni-Co/ZnO samples, respectively. The Tauc plot suggests that the incorporation of Ni, Co and Ni-Co in ZnO shows marginal red shift in absorption edge to longer wavelength as well as increase in absorption in entire visible region. The red shift in band gap might be due to the sp-d exchange interactions between the band electrons of ZnO and the localized d-electrons of Ni and Co ions in metal loaded ZnO samples (Kocyigit and Topkaya, 2019).

3.5 FTIR analysis

The Fourier transform infrared (FTIR) spectra of pure ZnO, Ni/ZnO, Co/ZnO and Ni-Co/ZnO samples are measured in the range of 4000 to 400 cm⁻¹ and are presented in Fig. 7. There is no extra peak observed in the range of 3300 to 3700 cm⁻¹ (Fig. 7a) indicating the absence of monodentate O—H group which is from water (surface free or bonded). The absence of O—H peak at 3300 to 3700 cm⁻¹ indicates that the complete formation of oxides. Spectral peaks at 1737 to 1218 cm⁻¹ are related to C⚌O, C—C, C—O and C—H stretching vibrations (Hassanpour et al., 2019; Xiong et al., 2009). In FTIR spectra, the sharp peaks positioned at around 497 cm⁻¹ and 567 cm⁻¹ are attributed to the formation of Zn—O bond. Distinctive IR peaks less than 1000 cm⁻¹ are very important to examine the presence of Zn—O, Ni—ZnO, Co—ZnO and Ni—Co—Zn—O bonds. The FT-IR absorption bands observed around 649, 634, 653 and 650 cm⁻¹ can be ascribed to the stretching modes of Zn—O, Ni—Zn—O, Co—Zn—O and Ni—Co—Zn—O nanostructured samples, respectively (Vijayaprasath et al., 2016), as shown in Fig. 7b.

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Fig. 7

FTIR spectra: (a) Pure ZnO and metal-loaded ZnO samples and (b) Enlarged view of the wavenumber (below 1500 cm⁻¹).

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3.6 Catalytic application of nanostructured Ni-Co/ZnO material

After a successful synthesis and characterization of nanocrystalline Ni/ZnO, Co/ZnO, and Ni—Co/ZnO materials, their catalytic performances were studied to accomplish the Heck reactions. The screening reactions were performed using iodobenzene and styrene as model substrates (Scheme 1).

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Scheme 1

Model Heck reaction.

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For optimization of the reaction conditions, effects of the catalyst, solvent, base and temperature on the Heck coupling reaction of iodobenzene and styrene were studied and the summary of optimization is given in Table 2. The reaction did not occur with pure ZnO. When the reaction was carried out with Ni/ZnO in N, N-dimethylformamide (DMF) using NEt₃, the 64% yield was observed at 90 °C in 16 h (entry 2, Table 2). Using a DMF/H₂O (1:1, v/v) solvent and K₂CO₃ as a base for Ni/ZnO and Co/ZnO catalyzed Heck reactions, yields of 70% and 66% were obtained, respectively at 90 °C (entries 3–4, Table 2). However, when reaction was carried out at 120 °C for Ni/ZnO catalyst using DMF and NEt₃, a yield of 82% was obtained (entry 5, Table 2) and 74% yield for Co/ZnO (entry 6, Table 2). This means that with an increase in reaction temperature, an improvement in yield was observed.

Subsequently, we performed Heck reactions using nanocrystalline Ni and Co bimetallic supported ZnO material (Ni-Co/ZnO catalyst) and observed good results compared to monometallic Ni and Co supported ZnO catalysts. Ni-Co/ZnO provided a yield of 88% using DMF and NEt₃ (entry 7, Table 2), while using DMF/H₂O and K₂CO₃, a yield of 92% was obtained (entry 8, Table 2). For Na₂CO₃ as a base, a yield of 88% for the Heck reaction using Ni-Co/ZnO was obtained (entry 9, Table 2). The role of water as a co-solvent in the carbon–carbon cross coupling reactions is highly noticed (Sherwood et al., 2019). The use of water as a polar co-solvent can induce the homogeneity of inorganic base (i.e. K₂CO₃) and reaction mixture which, in turn, can enhance the rate of reaction. The results indicated that good yield results have been obtained for inorganic bases in DMF/H₂O solvent system.

In addition, we have studied the Heck reaction using toluene and CH₃CN solvents; however, lower yields were observed (entries 10–11, Table 2). CH₃CN is less polar solvent than DMF, but due to its characteristic polarity, it could offer a conversion of 54% of desired product. It has been reported that the combination of amine base and less polar solvent such as toluene provide a convenient route for the formation of homogeneous solution in the Heck reaction (Sherwood et al., 2019). Therefore, here toluene could offer a yield of 58% using a nanostructured Ni-Co/ZnO catalyst in presence of triethylamine base (i.e. NEt₃). Furthermore, the boiling point of toluene is 110.6 °C; somewhat high to enhance the reaction rate. Heck reaction was also performed using morpholine as a base, but no product formation was observed (entry 12, Table 2). Morpholine is an unhindered secondary amine. The Heck reaction proceeds through the formation of $\pi$ -allylic complexes which are susceptible to undergo nucleophilic attack with a secondary amine such as morpholine. This leads to the formation of tertiary allylic amine as a major product instead of desired alkene product (Heck, 2004). Therefore, morpholine could not give the desired alkene product.

Thus, we observed better results for the Heck reaction using nanostructured Ni-Co/ZnO material as compared to Ni/ZnO and Co/ZnO. The increase in catalytic activity for bimetallic Ni and Co supported ZnO nanomaterial can be attributed to important synergistic and cooperative interactions between individual metallic components (structural and electron tuning) which modify the surface electronic properties. Additionally, the stability of the catalyst can be enhanced by the presence of a second counterpart (Rai et al., 2016a,b). Kaur et al. (2018) reported that the two metal components of bimetallic nanoparticles shows improved bifunctional performance and also provide extra properties viz. strength, functionality, and stability to the nanostructured material.

The mmol concentration of Ni-Co/ZnO catalyst was also optimized to obtain the highest yield. For 0.1 mmol, a trace yield was observed (entry 1, Table 3). For 0.2 mmol of catalyst, the yield observed was 38% (entry 2, Table 3). A further increase in the mmol of Ni-Co/ZnO catalyst from 0.3 mmol to 0.4 mmol resulted in an increase in the product yield from 72% to 92% (entries 3–4, Table 3). By increasing the mmol of catalyst from 0.4 to 0.5, the almost similar yield was obtained (entry 5, Table 3). Therefore, 0.4 mmol of Ni-Co/ZnO catalyst was selected and further Heck reactions were performed using 0.4 mmol of nanostructured Ni-Co/ZnO.

Using the optimized reaction conditions, a variety of structurally different aryl halides were used in the Heck reaction using styrene (Scheme 2) to obtain the desired carbon–carbon coupled products (entries 1–10, Table 4). It is observed that aryl iodides (iodobenzene and 4-iodotoluene, entries 1 and 3, Table 4) can contribute more in the Heck reactions compared to aryl bromides (bromobenzene and 4-bromotoluene, entries 2 and 4, Table 4). The results showed that the aryl iodides containing electron-rich substituents get converted to the desired products with more yields (entries 5–6, Table 4) than aryl iodides with electron-withdrawing substituents (entries 7–8, Table 4). In addition, the Heck reaction using 3-iodopyridine was carried out; however, lower yield of 74% was observed (entry 9, Table 4). We have also investigated the role of Ni-Co/ZnO catalyst with respect to chlorobenzene (entry 10, Table 4), but observed negligible formation of the product. This might be due to the poor reactivity of chloro group in such type of carbon–carbon bond forming reactions (Heck, 2004; Zhou et al., 2007).

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Scheme 2

General Heck reaction catalyzed by nanostructured Ni-Co/ZnO material.

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Table 4 Heck reaction of different aryl halides with styrene catalyzed by nanostructured Ni-Co/ZnO.^a.

Entry	Aryl halide	Styrene	Product	Yieldb (%)
1				92
2				76
3				84
4				72
5				88
6				91
7				83
8				81
9				74
10				trace

a Reaction conditions: Aryl halide (1.0 mmol), styrene (1.5 mmol), Ni-Co/ZnO (0.4 mmol), K₂CO₃ (1.5 mmol), 16 h, DMF/H₂O (1:1, v/v), 120 °C, air atmosphere.

b Isolated yield.

Thus, Ni-Co/ZnO catalyzed Heck reaction generated the corresponding cross-coupled products with moderate to good yields (72% to 92%) at 120 °C using a polar DMF/H₂O solvent system and K₂CO₃ as a base in 16 h, under ligand-free conditions and in an air atmosphere.

In this work, we have performed Heck reaction under phosphine ligand-free conditions and in such reaction condition, there is absence of strongly bound neutral ligands. In order to facilitate the Heck reaction, phosphine-free catalytic systems can avail N, N-dimethylformamide (DMF) as a polar solvent with an aqueous medium (Beletskaya and Cheprakov, 2000). It has been also reported that the more polar DMF solvent promotes the oxidative addition and olefin insertion steps during the Heck reaction mechanism (Lin et al., 2004). The role of DMF has been noticed as a reducing agent as well as a solvent in many reactions (Carpenter et al., 2012; Jeong et al., 2009; Rai et al., 2016a,b), and broadly used for the reduction of metal ions (Bastakoti et al., 2015). Furthermore, DMF has sufficiently high boiling point (153 °C) to increase the rate of reaction (Sherwood et al., 2019). Therefore, in this work, we have performed Heck reactions using polar DMF solvent in combination with water (i.e. DMF/H₂O solvent system).

The plausible mechanism of the reaction between aryl halide and styrene over nanostructured Ni-Co/ZnO catalyst should be similar to the homogeneous Heck type mechanism (Scheme S1, ESI). In the first step (i.e. oxidative addition), nanostructured Ni-Co/ZnO catalyst (1) could inset in the aryl and halide bond to generate intermediate (2). In next step, intermediate (2) reacts with styrene and forms a $\pi$ complex (3). In migratory insertion step, the styrene from the $\pi$ complex (3) undergoes insertion with catalyst and aryl ring and produces intermediate (4). Next step is β hydride elimination step in which there is removal of hydrogen with the formation of $\pi$ complex (5) on the surface of nanocatalyst. The $\pi$ complex (5) then undergoes decomposition with removal of alkene product. The catalyst could regenerate in the final reductive elimination step by K₂CO₃ base. Here, we have assumed that the reduction of cationic Ni and Co nanoparticles supported by ZnO to active metallic Ni and Co could take place by the DMF solvent under the aqueous and basic reaction medium and this could complete the catalytic cycle of the Heck reaction.

The ¹H NMR, ¹³C NMR and FT-IR spectral data of as-synthesized Heck reaction products are well matched to the desired products (see ESI). In addition, the observed coupling constant value (J = 16.5 Hz) for the olefin (alkene) protons clearly confirmed the formation of trans-substituted alkenes (Kaur et al., 2018) and not the cis isomer. In general, cis hydrogen atoms show the coupling constant value approximately of 10 Hz, while trans hydrogen atoms shows the coupling constant value of around 16 Hz (Pavia et al., 2008).

3.7 Recyclability study of the catalyst

The recyclability of catalyst is an essential factor from an economic and eco-friendly point of view. Consequently, the recyclability of Ni-Co/ZnO catalyst was checked in the Heck reaction between iodobenzene and styrene under the optimized reaction conditions. At the end of each reaction, the reaction mixture was allowed to cool to room temperature, the catalyst was separated, washed with ethyl acetate, dried at 100 °C for 4 h and reused. As shown in Table 5, the catalyst was reused five times without significant loss of catalytic activity and a yield of 76% was obtained after the fifth run for the heterogeneous catalyst.

3.8 Comparison of catalytic activities

We compared the catalytic activity of nanostructured Ni-Co/ZnO catalyst with some of the previously reported catalysts in the Heck reaction of iodobenzene and styrene, and the results are presented in Table 6. Obviously, some of the reaction conditions are mild and some reported methods shows better results. However, few of the reported methods required long reaction time, higher temperature and use of noble Pd or Au metals. The significant advantages of the present work are use of inexpensive and efficient Ni-Co/ZnO catalyst in the Heck reaction, heterogeneous catalysis, environmentally friendly method, minimization of metal contamination, and palladium and phosphine ligand-free conditions. Ni-Co/ZnO catalyzed Heck reaction offered the corresponding product with good yield. Moreover, the nanostructured Ni-Co/ZnO catalyst was synthesized by facile reduction-impregnation method.