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z Organic & Supramolecular Chemistry

Designing of Ferromagnetic 3D Hierarchical Core-ShellFe3O4@NiO/Co3O4 Microspheres Derived from a MOFPrecursor: As an Efficient Catalyst for C-P Cross CouplingReactionArezou Mohammadinezhad and Batool Akhlaghinia*[a]

Herein, we report the synthesis of a 3D hierarchical hetero-structure microsphere through a facile solvothermal and hydro-thermal method followed by an annealing treatment. Theobtained ternary core-shell nanostructured was comprehen-sively characterized by means of variety techniques includingFT-IR, XRD, BET, TEM, FE-SEM, EDX, EDX-mapping, VSM and ICP-OES. It was found that this hierarchical nanostructured couldpromote the C� P cross coupling reaction of a large library offunctional substrates. Benefiting from the unique hierarchicalarchitecture and the synergistic effect among multiple compo-

nents resulted from the coexistence of micro-, meso- andmacropores as well as its small nano-crystallite size (20-71 nm),this novel catalyst revealed superior catalytic performance inthe C� P cross coupling reaction. More importantly, theferromagnetic behavior of 3D hierarchical core-shellFe3O4@NiO/Co3O4 microspheres is responsible for easy separa-tion from the reaction mixture using an external magnetic fieldand its long term-durability under reaction conditions evenafter nine consecutive recycle runs.

Introduction

The construction of a carbon-phosphorus bond is highlyimportant in organic synthesis owing to wider applications oforganophosphorus compounds in medicinal chemistry, coordi-nation chemistry, and organic synthesis.[1–3] Among them,aromatic phosphorus-containing compounds have been ofconstant attention due to interesting applications in biochem-istry and catalysis.[4–6] As a result of their importance, overrecent decades, much attention has been paid to develop newstrategies for their synthesis. Therefore, by introducing catalystswith homogeneous or heterogeneous nature, the manufactur-ing efficiency can be improved by reducing operational costand waste materials. In this context, transition metal-catalyzedC� P bond-forming as a new synthetic strategy has attractedincreasing attention. Transition metal catalysts such as Pd,[7]

Pt,[8] Rh,[9] Mo,[10] Cu[11] and Ni[12] have gained enormousattention as a powerful and straightforward for the directphosphorylation reaction. Undoubtedly, these catalytic systemsrepresent a valuable synthetic route for the preparation oforganophosphorus compounds, nevertheless, some of themstill have limitations and suffer from some drawbacks, includinghazardous organic solvents,[13] expensive noble metalcatalysts,[14] ancillary ligands,[15] unrecyclable metal catalysts[16]

and microwave irradiation.[17] Hence, from a chemical industry

point of view, the emergence of conceptually-new, direct,efficient and practical procedure for the C� P bond formingreaction is criteria. Hereupon, and to the best of our knowl-edge, there is no example of the preparation of organo-phosphorus compounds by a hierarchical metal oxides catalyticsystem.

In recent years, hierarchical nanostructures which com-posed of more than one dimension self-assembly of theprimary structure (nanoparticles, nanorods, nanotubes or nano-sheets) have been investigated with great interest becausediverse properties can be integrated by tailoring the morphol-ogy, composition, and assembling organization of the primarybuilding blocks.[18] The ability to control the assembly config-uration, morphology and size of the building units leads to therealization of multifunctional nano devices.[19] Due to thecomplicated spatial arrangement, the hierarchical architecturesexhibit advantages such as high surface area,[20] large scaleordered arrangement[21] and robustness.[22] Recently, manyefforts have been devoted to the synthesis of hierarchicalstructures with enhanced activity. Being one of the bases of thefunctional materials, hierarchical oxide semiconductor core-shell nanostructures have attracted researchers’ attentionowing to their applications in many fields such as gas sensor,[23]

photocatalysis,[24] lithium ion batteries[25] and so on. Amongstthem, NiO and Co3O4 have possessed increasing considerationdue to their commercial and potential applications in variousfields, including catalysis,[26–35] electrode materials,[36] solid-statesensors[37] and super capacitors.[38]

Therefore, in continuation with research endeavours fromour group on the introduction of heterogeneous nanocatalystfor the C� P bond formation,[39] herein, we report an unprece-dented strategy for the synthesis of ferromagnetic 3D hierarch-

[a] A. Mohammadinezhad, Prof. B. AkhlaghiniaDepartment of Chemistry, Faculty of Science, Ferdowsi University ofMashhad, Mashhad 9177948974, IranE-mail: [email protected]

Supporting information for this article is available on the WWW underhttps://doi.org/10.1002/slct.201903407

Full PapersDOI: 10.1002/slct.201903407

12455ChemistrySelect 2019, 4, 12455–12463 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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http://orcid.org/0000-0002-9940-5045https://doi.org/10.1002/slct.201903407http://crossmark.crossref.org/dialog/?doi=10.1002%2Fslct.201903407&domain=pdf&date_stamp=2019-11-26

ical core-shell Fe3O4@NiO/Co3O4 microspheres via a simpleprocedure which discussed in detail as follows.

Initially, Fe3O4-MAA NPs (I) core was achieved by a one-potsolvothermal method, according to the synthetic processillustrated in Scheme 1. In the following step, a Ni� Co-BTCmetal-organic framework as a viable shell was introduced toencapsulate the magnetic core through a hydrothermalprocedure. Subsequent placing in an electrical furnace at 450°C, leads to obtain a black powder which denoted asferromagnetic 3D hierarchical core-shell Fe3O4@NiO/Co3O4microspheres (III) (Scheme 1).

Results and Discussion

In the first part of this paper, a collection of several techniquesincluding FT-IR, XRD, BET, TEM, FE-SEM, EDX, EDX-mapping,VSM and ICP-OES have been employed to the fully character-ization of ferromagnetic 3D hierarchical core-shell Fe3O4@NiO/Co3O4 microspheres (III). In the second part, in order to evaluatethe efficiency of the aforesaid nanostructured catalyst, thecatalytic activity of ferromagnetic 3D hierarchical core-shellFe3O4@NiO/Co3O4 microspheres was investigated in the C� Pbond formation.

To study the structure of Fe3O4@Ni� Co-BTC NPs (II)precursor and its corresponding 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III), the Fourier transform infrared (FT-IR)spectroscopy was utilized. As can be seen in Figure 1a, themain absorption bands at 1623, 1578, 1444 and 1372 cm� 1

which are indexed to the asymmetric and symmetric stretchingvibrations of coordinated carboxylate groups with Ni or Coions, are in good agreement with the reported literature andcan be assigned to the Ni� Co-BTC framework structure.[40]

Furthermore, the Fe� O vibration of Fe3O4 NPs was observed asa relatively weak absorption band at 593 cm� 1.[41] Aftercalcination treatment at 450 °C (Figure 1b), the characteristicabsorption bands of Fe3O4@Ni� Co-BTC NPs (II) were removed,indicating the decomposition of framework structure. Thecollapse of framework structure leads to formation of 3Dhierarchical Fe3O4@NiO/Co3O4 microspheres (III) which wasconfirmed via attendance of two new absorption bands at 654and 593 cm� 1, relating to Co3O4, NiO and Fe3O4 stretchingvibrations, respectively.[41–43] Moreover, as it is evident from Fig1b, the stretching frequencies of hydroxyl groups on thesurface of Fe3O4 NPs and asymmetric and symmetric stretchingvibrations of Fe–O–H bond, were appeared at 3396, 1125 and864 cm� 1, respectively.[44]

The phase structures of the precursor (Fe3O4@Ni� Co-BTCNPs (II)) and the calcined product (3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III)) were characterized by powder XRD. Asit is evident from Figure 2, all the characteristic peaks at 2θ=17.53 ° (2 2 0), 18.71 ° (1 1 1) and 27.15 ° (2 0 2)[45] which arerelated to the Ni� Co-BTC framework (Figure 2a), removed afterthe calcination process (Figure 2b). Three distinct phases canbe recognized in the XRD pattern of 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) which are related to Fe3O4(Ref. Code: 98–001-7122), NiO (Ref. Code: 98–005-9068) andCo3O4 (Ref. Code: 01–080-1534) structures. No other redundantpeaks are detected, suggesting the thorough formation ofmetal oxides. The crystalline size of 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III) was estimated to be 29 nm usingDebye-Scherrer’s equation.

The textural properties of 3D hierarchical Fe3O4@NiO/Co3O4microspheres (III) obtained from Fe3O4@Ni� Co-BTC NPs (II)precursor were scrutinized via investigating the N2 adsorption-desorption analysis. Figure 3 illustrates the N2 adsorption-desorption isotherms and the corresponding curves of poresize distributions. As shown in Figure 3b, the 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) displays a type IV or Visotherm with an H3-type hysteresis loop at the relative

Scheme 1. The synthetic route for the preparation of 3D hierarchical core-shell Fe3O4@NiO/Co3O4 microspheres (III).

Figure 1. FT-IR spectra of Fe3O4@Ni� Co-BTC NPs (II) (a), 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) (b) and the 9th reused 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) (c).

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pressure range of 1.0-0.1, suggesting the coexistence of micro-pores, mesopores and macro pores. The specific surface area,total pore volume and pore diameter of Fe3O4@Ni� Co-BTC NPs(II) and 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III)were listed in Table 1. The BET surface area of Fe3O4@Ni� Co-BTC NPs (II) and 3D hierarchical Fe3O4@NiO/Co3O4 microspheres

(III) was found to be 12.39 and 32.654 m2 g� 1, respectively. Thehigh surface area of 3D hierarchical Fe3O4@NiO/Co3O4 micro-spheres (III) can be attributed to removing of organic linkers(BTC) after treatment at 450 °C. Barret-Joyner-Halenda (BJH)pore size distribution (inset in Figure 3) reveals that the poredistribution of Fe3O4@Ni� Co-BTC NPs (II) altered from amesopore system to a hierarchical pore system with a wide-range of mesopore and macropore distribution from 1 to100 nm. Noticeably, such a multi-modal porous structure withwell-balanced macro-, meso- and micropores leads to forma-tion an effective medium which may enhance the catalyticactivity of 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III).

The morphology and structure of 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) were studied by the fieldemission scanning electron microscopy (FE-SEM) and trans-mission electron microscopy (TEM) and the results are shown inFigure 4. It had already been approved that the sampleexhibited microsphere structure which are uniformly con-structed with a number of three-dimensional spheres with anaverage size of 22 to 71 nm, according to FE-SEM data(Figure 4a). More detailed observations of the morphology andstructure were obtained by surveying the TEM images (Figure 4c and 4d). TEM images approved that Fe3O4@NiO/Co3O4 NPshave microsphere morphology with a 3D porous nature.Remarkably, the average size of 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III) was estimated to be 20 to 71 nm(according to the particle size distributions inset in Figure 4c)

Figure 2. XRD patterns of Fe3O4@Ni� Co-BTC NPs (II) (a), 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) (b) and the 9

th reused 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) (c).

Table 1. Specific surface area (SBET), total pore volume (Vt) and pore diameter of Fe3O4@Ni� Co-BTC NPs (II) and 3D hierarchical Fe3O4@NiO/Co3O4 microspheres(III).

Pore diameter(nm)a

Vt(cm3 g� 1)

SBET(m2 g� 1)

Samples

3.071 (3.651) 0.031 12.39 Fe3O4@Ni� Co-BTC NPs (II)3.054 (8.846) 0.105 32.654 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III)

a The pore size distributions were calculated using the BJH method of adsorption branch; data in parentheses are calculated from the desorption branch.

Figure 3. The nitrogen adsorption-desorption isotherms of Fe3O4@Ni� Co-BTC NPs (II) (a), 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III) (c) and thecorresponding pore size distribution curves (inset) (b and d).

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which is very close to the particle size determined using XRDand FE-SEM data.

The type of elements in the structure of 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) was studied via the energy-dispersive X-ray (EDX) technique. Surveying the EDX spectrumconfirmed the existence of Fe, Co, O and Ni elements in the 3Dhierarchical Fe3O4@NiO/Co3O4 microspheres (III) composition(Figure 5).

The existence of Fe, Co, O and Ni elements in the 3Dhierarchical Fe3O4@NiO/Co3O4 microspheres (III) compositionwas further confirmed through investigating the EDX-mappingdata which obtained by scanning the FE-SEM image. Theobtained results are in a good agreement with the EDX data interms of existing elements in the 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III) composition.

The magnetic property of 3D hierarchical Fe3O4@NiO/Co3O4microspheres (III) was investigated by the vibrating samplemagnetometer (VSM) technique. The field dependent magnet-ization curve is plotted in Figure 7. As can be seen from

Figure 7, the 3D hierarchical Fe3O4@NiO/Co3O4 microspheres(III) exhibits a ferromagnetic behavior. The maximum magnet-ization, remnant magnetization and coercivity were estimatedto be 10.23 emug� 1, 3.77 emug� 1 and 900 Oe, respectively.

Figure 4. FE-SEM image (a), TEM images (c and d) and particle size distributions (inset Figure 4c) of 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III) and FE-SEM image of the 9th reused 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III) (b).

Figure 5. EDX spectrum of 3D hierarchical Fe3O4@NiO/Co3O4 microspheres(III).

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Catalytic performance of 3D hierarchical Fe3O4@NiO/Co3O4microspheres (III) in the C-P cross coupling reaction

After full characterization of 3D hierarchical Fe3O4@NiO/Co3O4microspheres (III), the catalytic performance of this novelnanostructured catalyst was investigated in the C� P crosscoupling reaction (Scheme 2). The results are discussed in detailas follows.

Having the full characterized catalyst in hand, our attentionwas next focused on evaluating the catalytic activity of 3Dhierarchical Fe3O4@NiO/Co3O4 microspheres (III) in the C� Pcross coupling reaction and studying how different parameterscould affect its outcome. In that way and to obtain the bestreaction conditions, the C� P cross coupling reaction wasperformed between iodobenzene and diethylphosphite as amodel reaction. The effects of various parameters such assolvent, temperature, base, molar ratio of iodobenzene/diethylphosphite, the molar ratio of iodobenzene/ base and theamount of catalyst were studied on the model reaction. Theresults were summarized in Table 2. The first stage of theoptimization process involved a series of experiments whichcan be used to evaluate the role of base and catalyst in theC� P cross coupling reaction. To this end, the model reactionwas performed in the absence of base or catalyst (Table 2,entries 1–3). Based on the obtained results, no cross-coupledproduct was observed in these conditions even after a longperiod of time, indicating the essential role of base and catalystin the C� P cross coupling reaction (Table 2, entries 1–3).Subsequent experiments were done via surveying of commonsolvent including DMF, DMSO, THF, n-hexane, CH3CN, EtOH,H2O and solvent free conditions. Amongst them, EtOH wasfound to be the best solvent furnishing the cross-coupledproduct in a reasonable isolated yield (Table 2, entries 4–11). Todetect the suitable temperature, the model C� P cross couplingreaction was carried out at different temperatures (Table 2,entries 9 and 12–14). After a few attempts, it was found thatthe model reaction takes place in a sensible yield at refluxconditions (Table 2, entry 9). We also explored the effect of thebase system on the model reaction (Table 2, entries 15–22).Gratifyingly, KOH was found to be highly efficient for the C� Pcross coupling reaction (Table 2, entry 18). Another importantaspect that we consider was the effect of variable amounts ofcatalyst, the molar ratio of iodobenzene/ diethylphosphite andthe molar ratio of iodobenzene/ base. In principle, empiricalexamines clearly demonstrated that the cross coupling reactionproceeded very effectively using 1/2.1 molar ratio of iodoben-

Figure 6. FE-SEM image of information collection area and EDX-mapping of3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III).

Figure 7. Magnetization curve of 3D hierarchical Fe3O4@NiO/Co3O4 micro-spheres (III).

Scheme 2. C� P cross coupling reaction in the presence of 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III).

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zene/ diethylphosphite, 1/3 molar ratio of iodobenzene/ baseand 0.005 g (1.6 mol% Co: 1.4 mol% Ni) of 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) (Table 2, entries 23–30).Not surprisingly, no product was detected in the absence ofcatalyst which signified the essential role of catalyst in the C� Pcross coupling reaction (Table 2, entry 31). Finally, the catalyticefficiency of 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III)was further confirmed through performing a series of con-trolled experiments in the presence of Fe3O4-MAA NPs (I),Fe3O4@Ni� Co-BTC NPs (II), Fe3O4@NiO NPs and Fe3O4@Co3O4NPs leads to a significant decrease of the reaction yield(Table 2, entries 32–35). Moreover, the catalytic activity of NiO/Co3O4 NPs was evaluated on the model reaction. As it is evidentfrom Table 2, the pure and magnetic catalysts exhibit the samecatalytic activity (Table 2, entries 29 and 36).

To show the generality of this synthetic method, theoptimized C� P cross coupling reaction was extended on adiversity of substrate scope. It consisted of a variety of arylhalides and aryl boronic acids with electron-withdrawing andelectron-donating substituents which reacted with diethylphos-phite/ or triethylphosphite to generate the desired products.The results are presented in Table 3. As expected, due to lowerC� I bond strength vs C� Br and C� Cl bond, the aryl iodide wascoupled more successfully than aryl bromide and aryl chloride(leaving ability of halogens (C� I > C� Br > C� Cl)) (Table 3,entry 1 vs entries 7 and 12). Comparatively, aryl halides bearingelectron-deficient substituents produced the desired productsmore quickly than those containing electron-donating groups(Table 3, entries 2–6, 8–9 and 13–14). Thereafter, the generalityof the present study was further extended by the C� P cross

Table 2. Optimization of reaction conditions of C� P cross coupling reaction.

Entry Catalyst (mol% ratioof Co:Ni)a

Molar ratio of Iodobenzene/Diethylphosphite

Molar ratio ofIodobenzene/Base

Base Solvent Temperature(°C)

Time(h)

IsolatedYield (%)

1 - 1/2 1/0 - DMF 100 24 -2 - 1/2 1/2 K2CO3 DMF 100 24 -3 0.96 : 0.84 1/2 1/0 - DMF 100 24 -4 0.96 : 0.84 1/2 1/2 K2CO3 DMF 100 11 355 0.96 : 0.84 1/2 1/2 K2CO3 DMSO 100 10 456 0.96 : 0.84 1/2 1/2 K2CO3 THF 100 13 407 0.96 : 0.84 1/2 1/2 K2CO3 n-

HexaneReflux 10 25

8 0.96 : 0.84 1/2 1/2 K2CO3 CH3CN Reflux 11 309 0.96 : 0.84 1/2 1/2 K2CO3 EtOH Reflux 9 5510 0.96 : 0.84 1/2 1/2 K2CO3 H2O Reflux 13 Trace11 0.96 : 0.84 1/2 1/2 K2CO3 - 100 24 -12 0.96 : 0.84 1/2 1/2 K2CO3 EtOH 70 9 4513 0.96 : 0.84 1/2 1/2 K2CO3 EtOH 60 10 3014 0.96 : 0.84 1/2 1/2 K2CO3 EtOH R.T 24 Trace15 0.96 : 0.84 1/2 1/2 Li2CO3 EtOH Reflux 10 3016 0.96 : 0.84 1/2 1/2 NaHCO3 EtOH Reflux 13 3517 0.96 : 0.84 1/2 1/2 K3PO4 EtOH Reflux 9 5518 0.96 : 0.84 1/2 1/2 KOH EtOH Reflux 7 7519 0.96 : 0.84 1/2 1/2 TETAb EtOH Reflux 8 6020 0.96 : 0.84 1/2 1/2 NEt3 EtOH Reflux 7 6521 0.96 : 0.84 1/2 1/2 WERSAc - 78 24 Trace22 0.96 : 0.84 1/2 1/2 WEPP d - 78 24 Trace23 0.96 : 0.84 1/1.9 1/2 KOH EtOH Reflux 7.30 7024 0.96 : 0.84 1/2.1 1/2 KOH EtOH Reflux 6 8025 0.96 : 0.84 1/2.2 1/2 KOH EtOH Reflux 6 8026 0.96 : 0.84 1/2.1 1/3 KOH EtOH Reflux 5 8527 0.96 : 0.84 1/2.1 1/4 KOH EtOH Reflux 5 8528 1.28 : 1.12 1/2.1 1/3 KOH EtOH Reflux 4 9029 1.6 : 1.4 1/2.1 1/3 KOH EtOH Reflux 3 9530 1.92 : 1.68 1/2.1 1/3 KOH EtOH Reflux 3 9531 - 1/2.1 1/3 KOH EtOH Reflux 24 -32e 0.005 (g) 1/2.1 1/3 KOH EtOH Reflux 24 Trace33f 1.6 : 1.4 1/2.1 1/3 KOH EtOH Reflux 5 8034g 1.4 1/2.1 1/3 KOH EtOH Reflux 6.30 6535h 1.6 1/2.1 1/3 KOH EtOH Reflux 5 7036i 1.6 : 1.4 1/2.1 1/3 KOH EtOH Reflux 3 95

a The mol% result was obtained from ICP-OES analysis. b Triethylenetetramine. c water extract of rice straw ash. d water extract of papaya plant. e Reaction wasperformed in the presence of Fe3O4@MAA NPs (I). f Reaction was performed in the presence of Fe3O4@Ni� Co-BTC NPs (II). g Reaction was performed in thepresence of Fe3O4@NiO NPs.

h Reaction was performed in the presence of Fe3O4@Co3O4 NPs.i Reaction was performed in the presence of NiO/Co3O4 NPs.

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coupling reaction of various aryl boronic acid with diethylphos-phite/ or triehtylphosphite (Table 3, entries 15–17). As it isillustrated in Table 3, the electron-poor aryl boronic acidsreacted faster than those with electron-rich aromatic rings(Table 3, entries 16 and 17). Table 3 obviously demonstratesthat 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III) is notonly a highly efficient nanostructured catalyst to catalyze thereaction between a wide range of aryl halides and aryl boronicacids with diethylphosphite, but also is very impressive toaccelerate the C� P cross coupling reaction of the above-mentioned substrates with triethylphosphite.

The progress of the reactions was monitored by theformation of the desired products on TLC. All of the synthe-sized products were known and isolated as oil. Their structureswere confirmed via surveying their high field 1HNMR and13CNMR spectral data (See supporting information file).

In accordance with the previously reported mechanism inthe literature,[39] a proposed catalytic cycle has been depictedin Scheme 3. The cross coupling reaction is initiated uponoxidative addition of aryl halide /aryl boronic acid to the activesites of catalyst (Ni and Co) which afforded intermediate (I). In

the following step, the deprotonation process under basicconditions is responsible for the formation of intermediate (II)via coordination of phosphite substrate to intermediate (I).Finally, intermediate (II) on a reductive elimination mannergives the desired product with regeneration of the catalystduring the catalytic reaction process. To investigate whetherNiO or Co3O4 act as a catalyst promoter in this reaction, themodel C� P cross coupling reaction was in turn performed inthe presence of Fe3O4@NiO NPs/ or Fe3O4@Co3O4 NPs under theoptimized reaction conditions. As it is evident from the resultswhich summarized in Table 2, lower yields were gained usingFe3O4@NiO NPs /or Fe3O4@Co3O4 NPs as catalyst in comparisonto 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (Table 2,entries 34 and 35 vs entry 29). Moreover, according to Table 2,the cross-coupled product was produced with a satisfactoryyield in the presence of Fe3O4@Co3O4 NPs whereas under thesame reaction conditions, lower yield was observed in thepresence of Fe3O4@NiO NPs. Therefore, based on the obtainedresults, it could be concluded that coexistence of Ni and Co inthe MOF structure is critical to enhance the catalytic activityand accelerate the C� P cross coupling reaction.

Considering economic and environmental factors, andencouraged by the importance of reusability of heterogeneouscatalysts from an industrial point of views, the long term-durability test was performed. To this end, a nine-cycle experi-ments was done under optimized reaction conditions. Therecycling efficiency has been plotted in Figure 8 and theobtained results are discussed in details. Briefly, after each run,the nanostructured catalyst was separated using an externalmagnetic field, washed with EtOH (3 × 10 mL) and dried at 70°C overnight for subsequent recycle run. As it is evident fromFigure 8, a negligible decrease in the yield and conversion wasobserved even after 9 consecutive recycle runs, indicating thesupreme catalytic activity and long term-durability of 3D

Table 3. Scope and functional group tolerance of 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) catalyzed the C� P cross coupling

reaction.

Entry Ar X Time (h) HP(O)(OEt)2/ P(OEt)3

Product Isolatedyield (%)

1 C6H5 I 3/2:15 1 a 95/952 4-

OMeC6H4I 5:30/4 2 b 80/85

3 4-MeC6H4

I 5:15/3:30 3 c 80/90

4 4-CNC6H4

I 4:30/4 4 d 85/90

5 4-CHOC6H4

I 5/4:30 5 e 70/80

6 4-NO2C6H4

I 4/3:30 6 f 95/95

7 C6H5 Br 4/3 1a 80/858 4-

NH2C6H4Br 6:30/6 7g 70/70

9 4-CNC6H4

Br 6/5:30 4d 70/75

10 1-naphthyl

Br 8/7:15 8h 55/65

11 2-thienyl Br 6:30/6 9i 65/7012 C6H5 Cl 6/4:30 1a 70/8013 4-

NH2C6H4Cl 8:40/7:50 7g 55/60

14 4-CNC6H4

Cl 7:30/6:45 4d 70/70

15 C6H5 B(OH)2

4:30/3:45 1a 85/85

16 4-ClC6H4 B(OH)2

5:15/4:30 10j 85/85

17 4-MeC6H4

B(OH)2

7/6:15 3c 75/85

Scheme 3. Proposed mechanism for the C� P cross coupling reaction in thepresence of 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III).

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hierarchical Fe3O4@NiO/Co3O4 microspheres (III) in the C� Pcross coupling reaction.

Furthermore, to confirm the stability of 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) after 9 cycles in the C� Pcross coupling reaction, any structural changes of the aforesaidnanostructured catalyst were investigated by several techni-ques including FT-IR, XRD, FE-SEM and ICP-OES.

It is clearly evident from FT-IR spectrum of the 9th reused3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III) that fre-quencies, intensities and shapes of absorption bands are wellpreserved (Figure 1c). Moreover, it is interesting to note thatthe crystalline structure of the 9th reused 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) was not changed, accord-ing to the XRD pattern illustrated in Figure 2c. Additionally,surveying the FE-SEM image of the 9th reused 3D hierarchicalFe3O4@NiO/Co3O4 microspheres (III) noticeably exhibited thatno changes in the particle size, dispersion or morphology wasobserved (Figure 4b).

As a final point, in order to estimate the exact amount ofmetal ions (Co and Ni) in the fresh and the 9th reused 3Dhierarchical Fe3O4@NiO/Co3O4 microspheres (III), ICP-OES tech-nique was applied. The obtained results showed that 3.2 and3.2 mmol of cobalt as well as 2.8 and 2.7 mmol of nickel areexisted in 1.000 g of the fresh and the 9th reused nano-structured catalyst, respectively.

According to the obtained results from FT-IR, XRD, FE-SEMand ICP-OES techniques, now, there is no doubt that 3Dhierarchical Fe3O4@NiO/Co3O4 microspheres (III) is completelystable in terms of functional groups, crystalline structure andmorphology even after nine consecutive recycle runs.

The catalytic efficiency of 3D hierarchical Fe3O4@NiO/Co3O4microspheres (III) in the C� P cross coupling reaction wasconfirmed through comparing the performance of the afore-said nanostructured catalyst with some of the previouslyreported catalysts in the literature. Although, all of the listedcatalysts in Table 4 afford the desired product, however, thecurrent protocol is much superior to almost all of them in termsof the reaction time (entries 1 and 4–6), temperature (entries 2and 4–5), solvent (entries 1 and 3) as well as using expensivecatalysts (entries 1–6).

Conclusions

In summary, we have skillfully synthesized a ferromagnetic 3Dhierarchical core-shell Fe3O4@NiO/Co3O4 microspheres as ahigh-performance catalyst for the C� P cross coupling reaction.Structural characterizations including FT-IR, XRD, BET, TEM, FE-SEM, EDX, EDX-mapping, VSM and ICP-OES techniques suggestthat this heterostructure catalyst exhibits coexistence of micro-,meso- and macropores and nano-size crystalline structure (20-71 nm, according to the XRD, TEM and FE-SEM data). As a resultof its unique structure, the as-synthesized hierarchical bimet-allic catalyst exhibited excellent activity in the C� P crosscoupling reaction towards the desired products. Diverse arylhalides and aryl boronic acids containing electron-withdrawingand electron-donating substituents were converted to thecorresponding products in good to high yields. Notably, thisnovel catalyst remains quite stable during reaction conditionsand can be efficiently separated using an external magneticfield and reused for at least nine recycle runs. This synthesizednanostructured catalyst could be a highly promising substitutefor the expensive and toxic noble metal in the field of crosscoupling reactions. This work can pave the way to additionaladvances into designing of various catalysts using cobalt andnickel as a readily available, low-cost and non-toxic metals.

Supporting Information Summary

Experimental section, 1 HNMR and 13CNMR of products weredescribed.

Figure 8. Reusability of 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III) inthe C� P cross coupling reaction under optimized reaction conditions.

Table 4. Comparison of the catalytic activity of 3D hierarchical Fe3O4@NiO/Co3O4 microspheres (III) with some literature precedents in C� P cross

coupling reaction.

Entry Catalyst Solvent Temperature(°C)

Time (h)/Isolatedyield (%)

Ref.

1 Pd(OAc)2/dpe-phos a

THF 25 16/86 46

2 Pd-2-ATP b-γ-Fe2O3@SiO2

Solventfree

100 1.5/97 47

3 Pd(OAc)2/dppfc THF 68 3/81 48

4 Pd-DABCO-γ-Fe2O3

H2O 100 4/82 49

5 Pd-BIP d-γ-Fe2O3@SiO2

- 100 6/73 50

6 Cyclopalladatedferrocenylimines

iPrOH Reflux 16/99 51

7 3D hierarchicalFe3O4@NiO/Co3O4 micro-spheres (III)

EtOH Reflux 3/95 Presentstudy

a Dpephos= (Oxydi-2,1-phenylene)bis(diphenylphosphine). b Aminothio-phenol. c 1,1’-bis(diphenylphosphino)ferrocene. d Bis(imino) pyridines.

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Acknowledgements

The authors gratefully acknowledge the partial support of thisstudy by Ferdowsi University of Mashhad Research Council (Grantno. p/3/43367).

Conflict of Interest

The authors declare no conflict of interest.

Keywords: Fe3O4@NiO/Co3O4 microspheres · C� P crosscoupling reaction · heterogeneous catalysta · hierarchicalarchitecture

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Submitted: September 11, 2019Accepted: October 27, 2019

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