Sun-light-driven Suzuki-Miyaura Cross-coupling using a Novel Superparamagnetic Metal-organic Framework Catalyst

A prominent catalytic activity for sun-light driven Suzuki-Miyaura cross-coupling reaction has been shown in the presence of superparamagnetic core-shell metal-organic framework (Fe3O4@Ni-Co-BTC NPs) (with particle size 10–47 nm) and unique bimetallic core-shell nanostructure). Various aryl halides with electron-donating or electron-withdrawing substituents were coupled with different arylboronic acids towards corresponding products at ambient temperature in H2O. The results of photocatalysis demonstrate that Fe3O4@Ni-Co-BTC NPs is a good photocatalyst under sun light irradiation with high efficiency, chemoselectivity and functional group tolerance. This magnetic separable nanostructured catalyst can be reused more than six times with minimal loss of catalytic activity rendering this catalyst long-term stability. The results acquired in the present study can serve as inspiration for the further exploitation of the sun-light-driven cross-coupling reaction under mild conditions.


INTRODUCTION
N modern organic synthesis, Suzuki-Miyaura cross coupling reaction (as one-step pathway) has become arguably the most crucial and fundamental transformation for carbon-carbon bond formation to achieve synthetic building blocks of various natural products and biologically important compounds. [1][2][3][4][5][6] Traditionally, the Suzuki-Miyaura cross-coupling reaction was typically realized mainly using homogeneous or heterogeneous transition metal (Pd, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Ni, [22,23] Co, [22,24] Cu, [22,25,26] Fe [22,27] ) catalytic systems. Nonetheless, the reported processes require relatively high reaction temperature to overcome the activation energy barrier of redox processes (electron transfer step between transition-metal catalysts and organic substances) in C-C coupling reaction. Elevated temperature may reduce the accessible active sites on the solid supports (for the reactants) via the aggregation of metal nanoparticles dispersed on solid supports which damages the recyclability of the catalysts. [28,29] To facilitate the redox cycle, visible light or more preferably natural sunlight as a clean, abundant, and sustainable source of energy, can be utilized to furnish the C-C cross-coupling reaction in the presence of photocatalyst at room temperature. [30][31][32][33][34][35][36][37][38][39] Photocatalysts can convert light energy existing in nature into the required energy for C-C cross-coupling reaction (the photoexcited electrons transfer to substances for initiating C-C coupling reactions). With the development of photocatalytic technology, lightdriven chemical reactions has been attracting much attention as a crucial way to follow environmentally benign, green synthetic and sustainable processes. [40][41][42][43][44][45] In this direction, although a variety of photocatalytic systems have been developed to carry out C-C coupling reactions, studies into visible-light-driven C-C coupling reactions is still considered as an important objective for Suzuki-Miyaura cross coupling reaction. [46][47][48][49][50][51][52][53][54][55] We have been particularly interested in using photocatalyst as a new mean to drive chemical transformations under ambient conditions, with high activity and selectivity. As a part of our ongoing work towards the development of efficient green catalysts, [56,57] very recently, we have reported Fe3O4@Ni-Co-BTC NPs as a highly photoactive magnetic core-shell metal organic framework catalyst for aerobic oxidation of alcohols in green media (water), under base-free conditions and solar light irradiation (Scheme 1). [58] In the present study, we aim to utilize Fe3O4@Ni-Co-BTC NPs in Suzuki-Miyaura cross coupling reaction with visible light at ambient temperature (Scheme 2).

General
All the chemical reagents and solvents were purchased from Merck chemical company and were used as received without any further purification. The purity determination of the products and reaction progress were obtained by TLC on silica gel polygram STL G/UV 254 plates. The melting points of the products were determined with an Electrothermal Type 9100 melting point apparatus. The FT-IR spectra were recorded on an AVATAR 370 FT-IR spectrometer (Therma Nicolet spectrometer, USA) using KBr plates at room temperature in the range between 4000 and 400 cm -1 with a resolution of 4 cm -1 . The NMR spectra were recorded on a Bruker Avance 300 and 400 MHz instrument in CDCl3 as the solvent. Mass spectra were recorded with a CH7A Varianmat Bremen instrument at 70 eV electron impact ionization, in m / z (rel. %). UV-vis. diffuse reflectance (DRS) spectra were obtained in air at room temperature in the 200-800 nm wavelengths range by means of a Shimadzu UV-2550 instrument (Japan). Inductively coupled plasma optical emission spectroscopy (ICP-OES) was carried out on a 76004555 SPECTRO ARCOS ICP-OES analyzer. Core-shell metal-organic framework (Fe3O4@Ni-Co-BTC NPs) was prepared by the method reported in the literature. [58] All the yields refer to the isolated products after purification by thin-layer chromatography /or column chromatography.

Typical procedure for the sun-light-driven Suzuki-Miyaura cross-coupling reaction in the presence of Fe3O4@Ni-Co-BTC NPs
A beaker glassware was charged by a mixture of iodobenzene (1 mmol, 0.203 g), phenylboronic acid (1 mmol, 0.121 g) and KOH (4 mmol, 0.224 g) in water at ambient temperature. Fe3O4@Ni-Co-BTC NPs (II) (0.03 g, 9.6 mol% Ni : 4.47 mol% Co) was then added to the resulting mixture under stirring. The beaker glassware was then placed under the outdoor solar light irradiation between 8 am to 3 pm (July to August 2019, in daily ambient temperature (37-39 °C)). After completion of the reaction (5 h) which was monitored by TLC, the nanostructured catalyst was separated using an external magnetic field, washed with ethanol three times and dried in an oven at 60 °C overnight. The reaction mixture was then extracted with ethyl acetate (5 × 5 mL) and the Step 3 Step 4 Step 5
Scheme 2. Sun-light-driven Suzuki-Miyaura cross-coupling reaction of aryl halides and aryl boronic acids catalyzed by Fe3O4@Ni-Co-BTC NPs (II) combined organic layer was dried over anhydrous Na2SO4. After evaporation of the solvent, the crude product was purified by thin layer chromatography using n-hexane / ethyl acetate (8 : 2) or column chromatography using n-hexane / ethyl acetate (50 : 1) to produce the pure biphenyl (0.145 g, 95 % yield).

Sun-Light-Driven Suzuki-Miyaura Cross-Coupling Reaction in the Presence of Fe3O4@Ni-Co-BTC Nps (II)
At the outset of our investigation, we considered iodobenzene and phenylboronic acid as the model reactants for the sun-light-driven Suzuki-Miyaura cross-coupling reaction at ambient temperature (37-39 °C) with a water bath to exclude the possibility of photothermal effects. To ascertain optimal reaction conditions, a range of reaction parameters (including the nature of the base and its concentration, the solvent, the catalyst loading, as well as the stoichometric ratio between iodobenzene and phenylboronic) were assessed. As this catalytic system is not sensitive to oxygen, the reactions were carried out under air atmosphere. The optimization of the process has been described in detail in Table 1. H2O was first chosen as the solvent for the sunlight-driven model reaction. As the presence of base is essential to activate phenyl boronic acid via enhancing the polarization of the organic ligand and thus facilitating transmetallation, the reaction does not proceed without any base. (Table 1, entries 1, 2) [3] The Fe3O4@Ni-Co-BTC NPs (II) exhibited excellent photocatalytic activity while the reaction did not occur without any catalyst under irradiation, indicating the key catalytic role of Fe3O4@Ni-Co-BTC NPs (II) for driving the Suzuki-Miyaura cross-coupling reaction (Table 1, entries 3, 4). To compare the efficiency of the conversions under different conditions, all reactions were stopped after 5 h and the reaction mixtures were analyzed. A variety of bases were investigated among which, KOH afforded the highest yield of biphenyl product under the given conditions ( Noticeably, without irradiation (dark reaction), the reaction did not proceed to furnish the desired product indicating the significant contribution of irradiation on the catalytic activity of Fe3O4@Ni-Co-BTC NPs (II) ( Table 1, entry 25). To investigate the effect of Fe3O4@Ni-Co-BTC NPs (II) photocatalyst composition, the coupling of iodobenzene with phenylboronic acid was conducted in the presence of Fe3O4@MAA NPs (I), Fe3O4@Ni-BTC NPs, and Fe3O4@Co-BTC NPs under sun-light irradiation (Table 1, entries 26-28). No biaryl product was detected after reacting for 24 h using Fe3O4@MAA NPs (I). Comparatively, the reaction in the presence of Fe3O4@Ni-BTC NPs and Fe3O4@Co-BTC NPs was sluggish and gained lower yield of desired product. To explore the photocatalytic activity of Fe3O4@Ni-BTC NPs, Fe3O4@Co-BTC NPs and Fe3O4@Ni-Co-BTC NPs (II), UV-vis. DRS analysis was performed and its results which discussed previously, [58] are shown in Fig. 1. The band gap energies of Fe3O4@Ni-BTC NPs, Fe3O4@Co-BTC NPs and Fe3O4@Ni-Co-BTC NPs (II), calculated by the Kubelka-Munke equation were estimated to be 2.5, 3.2 and 2.3 eV respectively. According to the obtained results, it could be concluded that higher photocatalytic activity of Fe3O4@Ni-Co-BTC NPs (II) in comparison to Fe3O4@Ni-BTC NPs and Fe3O4@Co-BTC NPs could be related to its calculated band gap energy (2.3 eV). Moreover, due to high band gap of Fe3O4@Co-BTC NPs (3.2 eV), high energy (such as UV light) is necessary to activate it. As sunlight contains low percentage of UV light (~ 14 %), Fe3O4@Co-BTC NPs exhibit lower photocatalytic activity under sun light irradiation.  (a) All reactions were performed by using 1 / 1 molar ratio of iodobenzene/phenylbronic acid at ambient temperature (37-39 °C) and solar light irradiation. (b) The mol% result was obtained from ICP-OES analysis. On the basis of the optimized reaction conditions (Table 1, entry 22), we then determined the scope and limitations of the sun-light-driven Suzuki-Miyaura cross-coupling reaction of a series of differently substituted aryl halides (Cl, Br and I) with different arylboronic acids in the presence of Fe3O4@Ni-Co-BTC NPs (II) ( Table 2). The yield and conversion time of the sun-light-driven Suzuki-Miyaura cross-coupling reaction depend on the type of the aryl halides, the positions and electron-donating or electron-withdrawing nature of the substituents on the reactants. In comparison, aryl chlorides show little reactivity and lower conversion than similar aryl bromides and aryl iodides ( Table 2, entries 13 and 15 vs. entries 8, 9 and entries 1 and 3). As illustrated in Table 2, electron-poor aryl halides (Table 2, entries 2-4, 9 and 14, 15) show higher reactivity than electron-rich ones (Table 2, entries 5, 6, 10 and 17-19). Moreover, due to steric hindrance orthosubstituted aryl halide shows poor reactivity than p-substituted one (Table 2, entry 18 vs. entry 19). Furtheremore, to evaluate the selectivity of the catalyst in the sun-lightdriven Suzuki-Miyaura cross-coupling reaction, the competing reactions of aryl iodide and aryl chloride were studied using the dihalogenated aryl derivative as substrate (Table 2, entry 4). According to the obtained result (Table 2, entry 4), the conclusion could be derived that the aforesaid nanostructured catalyst catalyzed the sun-light-driven Suzuki-Miyaura cross-coupling reaction in a chemoselective manner. Driven by the above results, we then tried to extend the catalytic scope and generality of sun-lightdriven Suzuki-Miyaura cross-coupling reaction system towards various aryl boronic acids at the same reaction conditions. As shown in Table 2, aryl boronic acids with electron-releasing and electron-withdrawing substituents smoothly react with aryl halides to form the corresponding coupling products in excellent yields (Table 2, entries 20-25). Comparatively, the electron-rich aryl boronic acids reacted faster than those with electron-poor aromatic rings (Table 2, entries 20 vs. 23, 21 vs. 24 and 22 vs. 25). Finally, to show the merit of the current protocol in the generality and versatility substrate scope, 2-bromothiophene as a heteroaryl halide and 2-bromonaphthalene as a model of sterically hindered aryl halide were efficiently reacted with phenyl boronic acid to give the corresponding cross coupled products under the same reaction conditions (Table 2, entries 11, 12).
All the cross-coupled products were known and isolated and purified by thin-layer chromatography or column chromatography as solid products. The obtained products were initially identified in detail by comparison of their melting points as well as mass spectra with those reported previously or with authentic samples. In addition, the structures of some selected products were effectively corroborated by surveying their FT-IR, high-field 1 H NMR and 13 C NMR spectral data (see Supporting Information file).
To postulate a suitable mechanism for the sun-lightdriven Suzuki-Miyaura cross coupling reaction, a series of control experiments were performed. To this end, the model optimized reaction (reaction between iodobenzene and phenyl boronic acid) was carried out in the presence of a hole scavenger (triethanolamine (TEOA)) and radicaltrapping agent (benzoquinone (BQ)). Before the quenching experiments, we first examined the possibility of processing reaction in the absence of light source to confirm the role of electron-hole pairs. As illustrated in Table 1 (entry 25), performing the reaction in the absence of light source leads to the desired product in 20 % yield. It can be concluded that light has an essential role in this reaction. In the second step of our investigations, we turned our attention to study the reaction manner in the presence of scavengers. Upon light irradiation, hole-electron pairs can be generated via charge separation state (Scheme 3). When TEOA and BQ were introduced separately into the reaction system, the obtained yields are almost the same as the dark conditions suggesting that photo-generated holes and radicals are completely quenched. These studies suggest that the photo-generated holes are essential for carrying out the Suzuki-Miyaura cross coupling reaction. Based on the above experiments result and according to the previous studies on the photo-catalyzed Suzuki-Miyaura cross coupling reaction, [53,59] it is proposed that the mechanism initiated by attacking the photo-generated electrons to iodobenzene towards the cleavage of C-I bond. In the meantime, the positive hole activates C-B bond of phenyl boronic acid which is transferred into an electronically negative B(OH)3species in basic media. The remaining step is consisting of reductive elimination and transmetalation which leads to the formation of the desired coupled product.  From both economic and environmental perspectives, the superiority of the heterogeneous catalysts lies in their lifetime and their level of reusability in the processes of the reactions and separation. To investigate the recyclability of Fe3O4@Ni-Co-BTC NPs (II), the sun-light-driven Suzuki-Miyaura cross-coupling reaction of iodobenzene and phenylbronic acid was carried out in the presence of Fe3O4@Ni-Co-BTC NPs (II) as the catalyst in water as the sole solvent. After completion of each reaction run, the catalyst was separated using an external magnetic field, washed with ethanol three times and dried in an oven at 60 °C overnight. The above test reaction was performed again under the same conditions utilizing the recovered catalyst. Figure 2 demonstrates a slight loss of performance after six consecutive runs, suggesting excellent reusability and outstanding stability of Fe3O4@Ni-Co-BTC NPs (II) without noticeable deactivation. According to the obtained data from ICP-OES analysis, 1.49 and 1.48 mmol of cobalt as well as 3.2 and 3.2 mmol of nickel were anchored onto 1.000 g of fresh and sixth reused Fe3O4@Ni-Co-BTC NPs (II) respectively.
Here, to ascertain the stability of Fe3O4@Ni-Co-BTC NPs (II), we employed FT-IR spectroscopy before and after the sun-light-driven Suzuki-Miyaura cross-coupling reaction (Figure 3). Figure 3a is identical to Figure 3b. We found that there are no changes in intensities, shapes and frequencies of absorption bands.
The catalytic activity of various photocatalysts in the literature was compared with Fe3O4@Ni-Co-BTC NPs (II) in the Suzuki-Miyaura cross-coupling reaction of iodobenzene with phenylbronic acid as shown in Table 3. The comparison reveals that Fe3O4@Ni-Co-BTC NPs (II) catalyzed Suzuki-Miyaura cross-coupling reaction under natural sun light irradiation without applying any other light sources (Table 3, entries 1-6 and entries 8-10). Also, compared with the other catalysts, Fe3O4@Ni-Co-BTC NPs (II) can be easily separated by an external magnetic bar from the reaction mixture (Table 3, entries 1-7 and entries 9, 10) and showed high reusability as well (Table 3, entries 3-8 and entry10). The present methodology exhibits more efficiency in green media (Table 3, entries 4, 5, 10) without using expensive Pd catalysts (Table 3, entries 1-7 and entries 9, 10).

CONCLUSION
In conclusion, the previously designed and characterized Fe3O4@Ni-Co-BTC NPs (as a heterogeneous nanostructured catalyst with particle size (10-47 nm) and unique bimetallic core-shell nanostructure) efficiently activated aryl halides for Suzuki-Miyaura cross-coupling reaction under the sun light irradiation. So the sun-light-driven cross-coupling reaction proceeded in green solvent at room temperature with high chemoselectivity. The findings of the present study demonstrated the excellent photocatalytic performance of Fe3O4@Ni-Co-BTC NPs in the coupling of differently substituted aryl halides (Cl, Br and I) with different arylboronic acids under sustainable and mild process conditions. Fe3O4@Ni-Co-BTC NPs shows excellent chemical and catalytic stability compared to the other catalysts considered in this study. Given the magnetic nature, the photocatalyst showed great potential to be separated very simple by means of an external magnetic field and reused six times with only a slight decrease in its catalytic activity. Such impressive catalytic activity, operational simplicity, mild reaction conditions, high product yield, and high reusability in addition to the utilization of sunlight as the energy source and environmental-friendly solvent are the notable advantages of the present procedure and make Fe3O4@Ni-Co-BTC NPs a significant green, sustainable, cost-efficient and industrially viable photocatalyst as well. It can be envisioned that this novel genre of photocatalyst could be applied to other classic cross coupling reactions and also, the present strategy can be used as a potential tool for the direct utilization of sunlight for attaining industrially crucial catalytic reactions that were happened under the restricted conditions previously.