Nickel-metalated porous organic polymer for Suzuki–Miyaura cross-coupling reaction

A new Ni(ii)-α-diimine-based porous organic polymer, namely Ni(ii)-α-diimine-POP, was constructed in high yield via the Sonogashira coupling reaction between the metallo-building block of Ni(ii)-α-diimine and 1,3,5-triethynylbenzene. Besides the high thermal and chemical stability, the obtained Ni(ii)-α-diimine-POP can be a highly active reusable heterogeneous catalyst to promote the Suzuki–Miyaura coupling reaction. The obtained results indicate that the Ni(ii)-α-diimine-POP herein is a promising sustainable alternative to the Pd-based catalysts for catalysing the C–C formation in a heterogeneous way.


Introduction
Homogeneous metal catalysts play a leading role in C-C bondforming reactions. 1a Due to the increasing environmental issues and need for sustainable development, the metal-involving heterogeneous catalysts for C-C coupling such as in the Suzuki-Miyaura cross coupling reaction have drawn more and more attention during the past several decades. Among them, the precious palladium-based catalysts have been the protagonist. 1b To date, various solid carriers such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), carbon-based nanomaterials (CNMs), silica, and so on were employed to support precious Pd complex, 2 Pd NP 3 and Pdbimetallic alloy 4 for the fabrication of Pd heterogeneous catalysts. In contrast, the rst row transition metals such as Ni are rarely used to fabricate the solid carrier-supported heterogeneous catalysts. Compared to Pd, Ni is more reactive, low-cost and earth-abundant. As a sustainable alternative to Pd, the Nibased catalysts, especially solid carrier supported Ni metal catalysts, for the C-C cross-coupling reactions are extremely appealing. 5 The a-diimine-based metal catalysts have gained remarkable attention because of their easy synthesis, air stability, and high catalytic activity. 6 For example, a-diimine Ni(II) complexes have been widely used in polymerization, 7 dry CO 2 reforming of methane, 8 chemical bond activation, 9 bimetallic catalysis, 10 reductive cross-coupling reaction, 11 and reductive amination. 12 On the other hand, porous organic polymers (POPs), as a typical class of porous organic materials, have been broadly applied in the eld of gas storage and separation, 13 drug delivery, 14 water treatment 15 and heterogeneous catalysis. 16 On account of their high surface area, controllable porosity and ability to be functionalized, both crystalline and amorphous POPs, such as covalent organic frameworks (COFs), 17 hypercrosslinked polymers (HCP), 18 porous aromatic frameworks (PAFs), 19 polymers of intrinsic microporosity (PIMs), 20 conjugated microporous polymers (CMP), 21 are an promising class of carriers to upload active catalytic species. So far, the metal nanoparticle (M NP) loaded and metalated POPs by postsynthetic approach have been the main theme in the fabrication of POP-supported metal-solid catalysts. 22 In principle, the POP-based and metal-involved catalytic materials could also be prepared by in situ one-pot assembly of metal-containing building blocks. By doing so, the POPs with high-density and evenly distributed metal catalytic sites would be generated. So far, the metalated POPs obtained in this way, however, are very rarely reported. 23 In this contribution, we report, the rst of its kind, a Ni(II)-POP which was generated from the metallo-building block of Ni(II) a-diimine and 1,3,5-triethynylbenzene via Sonogashira cross-coupling reaction under solvothermal conditions. The obtained Ni(II)-a-diimine-POP can be a highly active and reusable heterogeneous catalyst to promote Suzuki-Miyaura crosscoupling reactions.

Results and discussion
Structural and morphological characterization Ni(II)-a-diimine-POP was successfully synthesized through a Sonogashira coupling reaction following the route outlined in  Scheme 1. The diiodine-substituted ligand A was prepared as the bright yellow crystalline solids by double Schiff-base condensation between 4-iodo-2,6-diisopropylbenzenamine and butane-2,3-dione in good yield. The metallo-building block B was synthesized as the brick-red crystalline solids by metalation of A with DME(NiBr 2 ) in moderate yield. Besides routine characterizations, the molecular structure of A was further determined by the X-ray single crystal analysis (CCDC 1906672, Fig. S1, Tables S1 and S2, ESI †). Aer combination of B and 1,3,5-triethynylbenzene, the Ni(II)-a-diimine-POP was generated through Sonogashira cross-coupling reaction under solvothermal conditions (toluene, 80 C, 72 h, 86%). Aer the reaction, the resulting precipitate was treated by Soxhlet extraction with CH 2 Cl 2 , methanol and acetone to remove any possible residues. Aer dried in vacuo at 110 C for 12 h, the target POP was obtained as deep brown solids (Scheme 1, inset). The obtained POP was the irregular granular particle which was well evidenced by the scanning electron microscopy (SEM, Scheme 1, inset). IR spectra (Fig. 1a) showed that the characteristic peaks at 3278 cm À1 attributed to the n C-H (-C^C-H) in 1,3,5-triethynylbenzene basically disappeared in Ni(II)-a-diimine-POP aer coupling reaction, meanwhile the band at 2109 cm À1 that corresponded to the -C^Cin 1,3,5-triethynylbenzene moved to 2206 cm À1 , indicating that the precursors of B and 1,3,5-triethynylbenzene in POP were successfully connected to each other via covalent C-C bond. Furthermore, the solid-state 13 C NMR also supported this POP formation. As indicated in Fig. 1b, the peak for the aliphatic carbon atoms of -CH 3 and -CH(CH 3 ) 2 groups was located at 19.03 ppm. 24 The broad signals centred at 74.58 and 132.40 ppm were respectively ascribed to the bridging -C^Cand phenyl moieties. 25 The signal at 163.62 ppm was associated with carbon atom in the imine group. 26 The TGA trace indicated that the obtained Ni(II)-a-diimine-POP remained intact till temperature over 250 C, implying its good thermal stability (Fig. 2a). Notably, no Ni NP was generated during POP formation process, which demonstrated by the high-resolution transmission electron microscopy (HRTEM, Fig. 2b). The existed Ni species valence was demonstrated by the X-ray photoelectron spectroscopy (XPS) measurement. As shown in Fig. 3a, the observation of 2p 3/2 and 2p 1/2 peaks at 855.97 and 873.28 eV conrmed that the nickel species in Ni(II)-a-diimine-POP existed as Ni(II). 27 The other observed doublet was attributed to intensive shake-up satellites that always occurs for Ni(II) during acquiring XPS. 27 On the other hand, the single N 1s peak in B (399.03 eV) and Ni(II)-a-diimine-POP (398.96 eV) shied to higher binding energy compared to that of A (398.88 eV), suggesting that Ni(II) species in B and Ni(II)-a-diimine-POP was chelated by N donors (Table 1). 28 Inductively coupled plasma (ICP) analysis showed that the nickel content in Ni(II)-a-diimine-POP was up to 7.6 wt% (calcd 8.3 wt%).
As mentioned above, the Ni(II)-a-diimine-POP herein was prepared by metallo-building block via in situ one-pot approach, so it should feature the uniform texture. As shown in Fig. 3b, the C, N, Ni and Br species evenly distributed in the POP matrix, as evidenced by the SEM-energy dispersive X-ray (EDX) mapping.
The crystalline nature of the Ni(II)-a-diimine-POP was characterized by PXRD measurement (Fig. S2, ESI †). A observed broad peak from 20 to 25 suggested its amorphous nature. The N 2 sorption analysis at 77 K was used to measure its specic  surface area with architectural rigidity and permanent porosity (Fig. 3c). Brunauer-Emmett-Teller (BET) analysis of Ni(II)-adiimine-POP indicated its surface area was up to 265.3 m 2 g À1 . Also, the pore size of the POP was determined based on the N 2 sorption isotherm by employing the nonlocal density functional theory (NLDFT) method, and a series of sharp and broad peaks were observed. It was found to be mostly mesoporous having the major contribution of the pore width at ca. 2.7 nm (Fig. S2, ESI †).

Catalytic properties
Next, we examined the catalytic activity of Ni(II)-a-diimine-POP for the Suzuki-Miyaura cross coupling reactions, in which the iodobenzene and phenylboronic acid coupling was chosen as the model reaction (Fig. S3, ESI †). As shown in Table 2, the solvent screening revealed that toluene is the best one among the other solvents such as N,N-dimethylformamide (DMF), iso-propyl alcohol (IPA), dioxane, acetone, tetrahydrofuran (THF), and CH 3 CN that we tested (Table 2, entries 1 to 7). In addition, various bases, including K 3 PO 4 $3H 2 O, K 2 CO 3 , triethylamine (TEA), 1,8diazabicyclo[5.4.0]undec-7-ene (DBU), pyridine, and piperidine, were used to performed the reaction, and we found that K 3 PO 4 $3H 2 O was better than the other inorganic and organic bases ( Table 2, entries 8-13). Furthermore, when the reaction was carried out with less catalyst loading, 2 (Table 2, entry 14), 3 (Table 2, entry 15) or 4 mol% (Table 2, entry 16) instead of 5 mol%, the coupled product was obtained in signicantly lower 45-89% yields. On the other hand, the reaction temperature appeared to be crucial to the catalytic efficiency. As indicated in Table 2, the catalytic activity of Ni(II)-a-diimine-POP was signicantly diminished at lower temperature (from r.t. to 80 C, Table  2, entries 17-19). Also, shortening reaction time of 4-6 h would also lead to the signicantly reduced 58-79% yields under the given reaction conditions (Table 2, entries 20-21). We did not observe any coupled product formation in the absence of the nickel source (Table 2, entry 22), further conrming the loaded Ni served as the catalytic active sites.    (Fig. S4, ESI).
To verify the heterogeneous nature of this POP-based catalyst, the hot leaching test was conducted. As shown in Fig. 4a, no further reaction occurred aer ignition of the reaction at 4.0 h when it reached the yield of ca. 65%, at which point the heterogeneous particles were centrifuged out of the reaction mixture, and the reaction was performed for additional 4 h. GC analysis indicated that the yield of the reaction did not change during the prolonged reaction time, indicating that the Ni(II)-adiimine-POP exhibited a typical heterogeneous catalyst nature herein.
Once we conrmed the heterogeneous nature of the catalyst, we also tested its recyclability. Aer each catalytic run, the solid catalyst was separated by centrifugation, washed with toluene (3 Â 2 mL), CH 3 CN (3 Â 2 mL), and dried at 110 C for 12 h in vacuum. The recycling catalytic runs were conducted by combining the recovered catalyst with inorganic base, iodobenzene, and phenylboronic acid in toluene. As shown in Fig. 4b, the solid catalyst of Ni(II)-a-diimine-POP still showed excellent activity and the cross-coupling yield was even up to 90% aer ve catalytic cycles (Fig. S5, ESI †). Aer multiple catalytic cycles, ICP measurement demonstrated that the Ni content in Ni(II)-a-diimine-POP was 6.3 wt%, suggesting a 17% Ni leaching occurred aer ve catalytic runs. This catalyst loss might be responsible for this slight yield decrease. On the other hand, no valence change for Ni species was observed (Table 1), implying that the Ni species in POP was stable during the reusable processes under the given conditions. In addition, the POP morphology and elemental distribution were well maintained aer the recycle (Fig. 3b and S6 †).
To test the available scope of this catalyst, we performed coupling reactions of phenylboronic acid with a series of substituted aryl halides under optimized conditions (Table 3). It is noteworthy that the catalytic system was tolerant to a wide range of functional groups such as -CF 3 , -NO 2 , -COCH 3 , -CH 3 , -OCH 3 , -CO 2 Me, and -CN at different substituted positions. Aryl bromides with both electron-donating and electronwithdrawing groups at para-, meta-or ortho-substituted position afforded cross-coupling products of i-viii (Table 3, entries 1-9) with good-to-excellent yields (90-98%). Compared to aryl bromides, the aryl iodides for the cross-coupling reaction were more active. As shown in Table 3 (entries 10-15), the yields for the target coupled products of i, iv, v and viii-x based on iodobenzene and para-or meta-substituted aryl iodides were more than 99%. Meanwhile, the ortho-substituted iodobenzene provided the products of xi and xii in slightly lower 90-92% yields (Table 3, entries 16 and 17). On the other hand, the crossing-coupling reactions between aryl iodide and phenylboronic acids with both electron-donating and electronwithdrawing groups at para-, meta-or ortho-substituted position still afforded excellent 98 to >99% yields (Table 3, entries 18-22, compounds iii, v, vii, xiii and xiv). The slightly lower 90% yield for iv from para-methoxyphenylboronic acid with aryl iodide was also observed (Table 3, entry 23). However, the chlorobenzene and phenylboronic acid coupling gave a low 26% yield of i in 12 h (Table 3, entry 24), indicating that the Ni(II)-adiimine-POP cannot signicantly activate the ArCl-based Suzuki-Miyaura cross-coupling reactions.
exhibited an impressive catalytic performance. On the other hand, we believe the Ni-catalysed Suzuki-Miyaura crosscoupling reaction herein went through the same mechanism as the reported one, [33][34][35] which should involve the initial oxidative addition of the aryl halide to a Ni(0) species, followed by trans-metalation and subsequent reductive elimination step to afford the expected coupling product, meanwhile the catalytically active nickel(0) species was regenerated (Fig. S8, ESI †).

Materials and measurements
All chemicals and solvents were at least of analytic grade and employed as received without further purication. The elemental analysis was conducted on a PerkinElmer Model 2400 analyzer. MS spectra were obtained by Bruker maxis ultra-high resolution-TOF MS system. NMR data were collected using an AM-400 spectrometer. 13 C CP/MAS NMR experiments were performed on Agilent 600 DD2 spectrometer at a resonance frequency of 150.15 MHz. 13 C NMR spectra were recorded on a spinning rate of 15 kHz with a 4 mm probe at room temperature. 13 C CP/MAS experiments were performed with a delay time of 5 s and a contact time of 1 ms. Infrared spectra were obtained in the 400-4000 cm À1 range using a Bruker ALPHA FT-IR spectrometer. Powder X-ray diffraction (PXRD) measurements were performed at 293 K on a D8 ADVANCE diffractometer (Cu Ka, l ¼ 1.5406Å). ICP analysis was performed on an IRIS InterpidII XSP and NU AttoM. XPS spectra were obtained from PHI Versaprobe II. Thermogravimetric analyses were carried out on a TA Instrument Q5 simultaneous TGA under owing nitrogen at a heating rate of 10 C min À1 . HRTEM (high resolution transmission electron microscopy) analysis was performed on a JEOL 2100 Electron Microscope at an operating voltage of 200 kV. The scanning electron microscopy (SEM) micrographs were recorded on a Gemini Zeiss Supra TM scanning electron microscope equipped with energy-dispersive X-ray detector (EDX). The elemental analysis was conducted on a PerkinElmer Model 2400 analyzer. The crystal data were obtained by Agilent SuperNova X-ray single crystal diffractometer.

Synthesis of B 30,31
A mixture of A (1.1 mmol, 0.722 g) and DME(NiBr 2 ) 32 (1 mmol, 0.308 g) in 50 mL dry dichloromethane was charged in a 100 mL Schlenk ask. Aer stirred at room temperature for 2 day in N 2 , the reaction system was ltered though a pad of Celite. The resulting solids were further washed with anhydrous diethyl ether, and then dried in vacuo to afford complex B as brick-red solid (0.59 g, 68.  General procedure for the Suzuki-Miyaura cross-coupling reaction between iodobenzene and arylboronic acid A mixture of iodobenzene (1.0 mmol, 116 mL), phenylboronic acid (1.1 mmol, 0.134 g), K 3 PO 4 $3H 2 O (2 mmol, 0.533 g) and Ni(II)-a-diimine-POP (52 mg) in 2 mL toluene was stirred at 100 C for 8 or 12 h in N 2 to afford the corresponding product. Yield was determined by the GC analysis.
General procedure for the Suzuki-Miyaura cross-coupling reaction between bromobenzene and arylboronic acid A mixture of bromobenzene (1.0 mmol, 104 mL), phenylboronic acid (1.1 mmol, 0.134 g), K 3 PO 4 $3H 2 O (2 mmol, 0.533 g) and Ni(II)-a-diimine-POP (52 mg) in 2 mL toluene was stirred at 100 C for 8 h or 12 h in N 2 to afford the corresponding product. Yield was determined by the GC analysis.
General procedures for the recycle of Ni(II)-a-diimine-POP Aer each catalytic run, the solid catalyst was recovered by centrifugation, washed with toluene (3 Â 2 mL), acetonitrile (3 Â 2 mL) and dried at 110 C for 12 h in vacuum and then was reused for the next catalytic run under the same reaction conditions.

Conclusions
In summary, we report herein a new Ni(II)-a-diimine decorated porous organic polymer Ni(II)-a-diimine-POP by assembly of metallo-building block and its polymerized partner via in situ one-pot approach. The resulting polymer is porous, solvent-and thermal-stable. More importantly, the obtained Ni(II)-a-diimine-POP can highly promote the Suzuki-Miyaura crosscoupling reaction in a heterogeneous way with excellent yields and a reasonable scope. The catalyst could be reused at least 5 times without signicant loss of the catalytic activity (>90% yield). We expect the presented approach to be viable for the construction of many more new metalated POP-based heterogeneous catalytic materials for various organic transformations.

Conflicts of interest
There are no conicts to declare.