Spirobifluorene-Based Three-Dimensional Covalent Organic Frameworks with Rigid Topological Channels as Efficient Heterogeneous Catalyst

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2021Spirobifluorene-Based Three-Dimensional Covalent Organic Frameworks with Rigid Topological Channels as Efficient Heterogeneous Catalyst Yamei Liu†, Chenyu Wu†, Qingzhu Sun, Fan Hu, Qingyan Pan, Jing Sun, Yinghua Jin, Zhibo Li, Wei Zhang and Yingjie Zhao Yamei Liu† College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Chenyu Wu† College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052 , Qingzhu Sun College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Fan Hu College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Qingyan Pan College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Jing Sun College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Yinghua Jin Department of Chemistry, University of Colorado, Boulder, CO 80309 , Zhibo Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Wei Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, University of Colorado, Boulder, CO 80309 and Yingjie Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 https://doi.org/10.31635/ccschem.020.202000493 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Although the past decade has witnessed tremendous progress in the development of covalent organic frameworks (COFs), three-dimensional (3D) COFs fabrications and characterizations are still much less studied compared with two-dimensional (2D) COFs, due to their complicated topology structures caused by interpenetration and limited choices of node-building blocks. In this work, we constructed a novel 3D COF ( SP-3D-COF-BPY) successfully using orthogonal dual-planar spirobifluorene and bipyridine as building blocks. Also, we demonstrated the application of 3D COF-supported Pd(II) catalyst in heterogeneous catalysis. A sevenfold interpenetrated dia structure was revealed for SP-3D-COF-BPY by powder X-ray diffraction (PXRD) in conjunction with structural simulation and Pawley refinement. The bipyridine-linked frameworks bear one-dimensional (1D) unobstructed rigid channels and offer a high density of discrete coordination sites for chelating Pd(II) species. Very high loading of Pd(II) (∼15 wt %) was achieved in the asymmetric (as)-prepared Pd(II)@3D-COF-BYP. The generated highly ordered porous channels and easily accessible catalytic sites, such as COF-supported Pd(II) complex, serves as a highly active and stable microporous heterogeneous catalyst in Suzuki–Miyaura coupling reactions. The observed catalytic efficiency was high and the catalyst could be conveniently reused multiple times without any noticeable decay in catalytic performance. Download figure Download PowerPoint Introduction Covalent organic frameworks (COFs) are characteristic topological covalent structures with enormous internal surface area resulting from their nanoporous structures.1–6 Over the past decade, COFs have displayed a large variety of applications.7–12 Besides their traditional advantages in gas storage and separation, COFs recently revealed promising potentials as heterogeneous catalysts, given their highly ordered porous structure and a chemically modifiable internal surface.11,13–16 Owing to the well-defined topological arrangement of active sites, COFs could combine both advantages of small molecule catalysts and traditional heterogeneous catalysts. Indeed, traditional heterogeneous catalysts usually suffer from limited active area, recyclability, stability, leaching and deactivation issues.17–22 By contrast, COF-based heterogeneous catalysts benefit from tremendous internal surface areas, which endow them with much more discrete active sites. More importantly, the highly ordered structures with defined channels facilitate the diffusion of reactants and the precise loading of active metal catalysts. Besides, the visualization of the structure makes it possible to obtain a detailed insight into the relationship between the structure and catalytic activity. Thus, rationally designed COFs could be an excellent alternative to heterogeneous catalysts.23–26 Three-dimensional (3D) COFs exclusively constructed from covalent bonds exhibit superior structural stability and possess unique permanent pores across the topological frameworks.8,15,27–32 Such a high specific surface area and permanent porosity are readily accessible to substrates, making 3D COFs attractive candidates as heterogeneous catalysts, although they have been limited to only a few examples owing to their synthetic challenges.33 One successful example was reported by Qiu's group,34 in which they used dual-linker (DL)-COFs as an acid–base catalyst for one-pot cascade reactions with ∼90% yields. The lack of choices in favorable 3D-building blocks and their relatively complex 3D structures restrict the development of 3D COFs as heterogeneous catalysts.12,35–37 We have employed spirobifluorene ( SP) as a novel type of tetragonal-disphenoid 3D-building block28,38 to construct a new series of 3D COFs ( SP-3D-COFs). Due to the high rigidity and orthogonality of the two intersected fluorene groups in SP, the SP-3D-COFs possess uniform unobstructed one-dimensional (1D) channels. This could be particularly beneficial for heterogeneous catalysis, as these unobstructed channels throughout SP-3D-COFs are more favorable for anchoring noble metal ions, thus maximizing the accessibility of the active sites, compared with the traditional tetraphenylmethane-based 3D COFs. With this consideration, we introduced the 2,2′-bipyridine ( BPY) units (commonly used as metal chelating ligands) as linkers for designing a new SP-3D-COF ( SP-3D-COF-BPY). The characterization of the product by powder X-ray diffraction (PXRD) analysis revealed distinct features of unobstructed rigid channels and confirmed a sevenfold interpenetrated ( dia-c7) structure of SP-3D-COF-BPY. Subsequently, by a simple one-step soaking process at room temperature, the discrete bipyridine units could chelate Pd(II) ions. Due to the high accessibility of the active sites through the channels, the Pd(II)@SP-3D-COF-BPY exhibited superior catalytic efficiency over those with similar structures such as the Pd(II)-loaded SP-3D-COF-BPH (without bipyridine as the chelation group). We also demonstrated the superiority of Pd(II) ions, compared with reduced palladium nanoparticles (PdNPs) when each of them was deposited in the same COF in catalyzing Suzuki–Miyaura coupling reactions. In short, Pd(II)@SP-3D-COF-BPY as a highly ordered nanoporous catalyst demonstrated substantially improved performance and stability in catalyzing Suzuki–Miyaura coupling reactions than its other counterparts. Experimental Methods Materials Organic solvents including dichloromethane, ethanol, petroleum ether, acetone, tetrahydrofuran (THF), mesitylene, benzyl alcohol, acetic acid, N,N-dimethylformamide (DMF), N,N-dimethylsulfoxide (DMSO), 9,9′-spirobifluorene, 4-phthalaldehyde, 1-iodo-4-nitrobenzene, phenylboronic acid, 4-bromobiphenyl, 1-bromo-4-iodobenzene, 1-bromo-4-nitrobenzene, 2-bromonaphthalene, 4-iodoanisole, 4-iodotoluene, palladium diacetate, 3-pyridinecarboxaldehyde, and isoamyl nitrite were purchased from Adamas-Beta (Shanghai, China) and used as received. Fuming nitric acid and acetic anhydride were purchased from Sinopharm (Beijing, China) and used as received. Palladium on activated carbon was purchased from Acros Organics (Chile, China) and used as received. Hydrogen stored in a high-pressure gas cylinder was ordered from Dehai Gas (Qingdao, China). Synthesis of SP-3D-COF-BPY The synthesis of SP-3D-COF-BPY is illustrated in Scheme 1. The orthorhombic building block A (9,9′-spirobi[fluorene]-3,3′,6,6′-tetraamine) was synthesized as described previously.39 Compound D containing bipyridine was used as the linker. The synthesis of SP-3D-COF-BPY through the imine condensation of A (15 mg, 0.04 mmol) and D [3,3′-bipyridine]-6,6′-dicarbaldehyde (17 mg, 0.08 mmol) was performed in a mixture of phenylmethanol (134 μL), mesitylene (266 μL), and 6 M acetic acid (40 μL) in a decompressive glass tube at 120 °C for 3 days. Subsequently, the crystalline precipitates were isolated via filtering, followed by washing with THF before it was dried at 80 °C under vacuum for 12 h. SP-3D-COF-BPY was obtained as a yellow crystalline powder, which appeared insoluble in most common solvents. Scheme 1 | (a) Synthesis of SP-3D-COF-BPY, SP-3D-COF-BPH, and the corresponding model compound C. (b) Schematic representation of Pd(II) or PdNPs impregnation to the SP-3D-COF-BPY. Download figure Download PowerPoint Synthesis of Pd(II)@SP-3D-COF-BPY Palladium acetate (5.0 mg, 0.022 mmol) and SP-3D-COF-BPY (5 mg) were dispersed in 2 mL of dichloromethane. The suspension was stirred for 3 days at room temperature. The residue was isolated by filtration then washed with dichloromethane. The resulting powder was subjected to Soxhlet extraction for 1 day in dichloromethane and dried under vacuum at 80 °C for 12 h to yield Pd(II)@SP-3D-COF-BPY as a brown powder. Synthesis of [email protected] A well-dispersed suspension of SP-3D-COF-BPY (4 mg) in methanol (3 mL) was mixed with a solution of K2PdCl4 (4 mg, 0.012 mmol) in H2O (2 mL) for 3 h at room temperature. The mixture was brought to dryness under vacuum with stirring to deposit metal precursors in SP-3D-COF-BPY support. The residue was dispersed in methanol and H2O (5 mL, MeOH/H2O, v/v = 3∶2). A solution of NaBH4 in methanol (0.25 M, 2 mL) was added dropwise, stirring the mixture for 2 days at room temperature. The product was collected by centrifugation, washed three times with ethanol and dichloromethane, and dried under vacuum until further use. Suzuki–Miyaura coupling reaction catalyzed by Pd(II) @SP-3D-COF-BPY In a typical one-pot of Suzuki–Miyaura coupling reaction, phenylboronic acid (92 mg, 0.75 mmol), aryl halide (0.5 mmol), potassium carbonate (138 mg, 1.0 mmol), and Pd(II)@SP-3D-COF-BPY (2 mg, 0.5 mol %) were dispersed in 1 mL of p-xylene. The suspension was heated to 70 °C for 2 h. The reaction progress was monitored by thin-layer chromatography (TLC) until completion. The resulting product was filtered and purified by chromatographic column. Model compound C For comparison, model compound C was also prepared under the same condition except for that monofunctional compound B (2-pyridinecarboxaldehyde) was used instead of D. 9,9′-Spirobifluorene-3,3′,6,6′-tetraamine (10 mg, 0.026 mmol) and 3-pyridinecarboxaldehyde (28 mg, 0.26 mmol) were added to ethanol (1.5 mL) and chloroform (1.5 mL). The mixture was refluxed overnight and then dropped into petroleum ether. The precipitate was centrifuged and washed with petroleum ether to obtain a light yellow solid (18 mg; 95% yield) product, characterized as follows: Proton nuclear magnetic resonance (1H NMR; 600 MHz, CDCl3, δ): 9.11 (s, 4H), 8.77 (d, J = 61.2 Hz, 4H), 8.64 (s, 4H), 8.35 (d, J = 7.7 Hz, 4H), 7.76 (t, J = 18.1 Hz, 4H), 7.47 (s, 4H),7.07 (d, J = 8.4 Hz, 4H), 6.86 (d, J = 8.4 Hz, 4H). Carbon nuclear magnetic resonance (13C NMR; 101 MHz, CDCl3, δ): 157.3, 154.8, 152.0, 151.7, 150.9, 147.4, 142.4, 135.7, 135.2, 124.7, 121.2, 112.6, 59.5. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS): 733 [M + H]+. Results and Discussion The solid-state 13C crosspolarization magic angle spinning (CP-MAS) NMR spectra spectra of SP-3D-COF-BPY and C showed very similar characteristic peaks indicating similar chemical structures. The resonance signals noted at 156 ppm were attributed to the carbon of the C=N bond ( Supporting Information Figure S1). Moreover, the Fourier transform infrared (FT-IR) spectrum of SP-3D-COF-BPY revealed strong, apparent C=N stretch modes characteristic of imines at 1625 cm−1 ( Supporting Information Figure S2). Thermogravimetric analysis (TGA) showed good thermal stability up to 500 °C ( Supporting Information Figure S3). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the SP-3D-COF-BPY is a bulk crystal with regular continuous edges (Figure 4a and Supporting Information Figure S4). To confirm the proposed rigid channel configuration and the existence of coordination sites within the structure of SP-3D-COF-BPY, we performed PXRD analysis with the help of structural simulation (Figure 1). A tetragonal-disphenoid diamond ( dia) topology lattice with interpenetration was proposed.28 After a detailed comparison between the calculated PXRD profiles from different interpenetrated structures and the experimental profiles ( Supporting Information Figure S5), we finally identified a sevenfold-interpenetrated ( dia-c7) net, highly similar to a previously reported SP-3D-COF-BPH in which a biphenyl ( BPH) unit was used instead of bipyridine ( BPY) (Scheme 1a).28 As shown in Figure 1a, the PXRD pattern of SP-3D-COF-BPY exhibited very intense peaks for the {200} Bragg planes (actual planes {200} at 4.5°, secondary {400} diffraction at 9.0°, and tertiary {600} diffraction at 13.5°). In our simulated unit cell (Figure 1b as stacking of seven unit cells along the interpenetration axis), {200} planes corresponded to the smooth channels' coplanar walls. Undoubtedly, the atom density of these {200} planes was several orders of magnitude higher than any other planes, and hence, exhibited the strongest peak signals. Indeed, this type of pattern (strong peaks solely from one plane group) is very different from those of other 3D COFs and could only be observed in SP-3D-COFs, evidently proving the proposed structure with unobstructed channel configuration. The structural simulation was performed by the Forcite package with geometry optimization at the molecular-mechanics level (Figure 1b). As seen in Figure 1a and inset, the experimental profile (black) correlates well with the predicted profile from the simulated model (green). Pawley refinement revealed minimal differences (Figure 1a, red; Rp = 4.24% and Rwp = 5.02%). Unit cell parameters of SP-3D-COF-BPY were determined from Pawley refinement to be a = 37.9 nm, b = 39.5 nm, c = 6.9 nm, and α = β = γ = 90°, in excellent agreement with the simulated values of a = b = 38.0 nm, c = 7.1 nm, and α = β = γ = 90°. Structures with degrees of interpenetration higher than seven were found to cause structural distortion, and thus, excluded ( Supporting Information Table S1). The degrees of interpenetration —one to seven were compared in detail, exclusively revealing sevenfold as the actual interpenetration mode ( Supporting Information Table S1 and Figure S5). Hence, the structural analysis of the experimental PXRD pattern proved our proposed structure for SP-3D-COF-BPY successfully bearing numerous periodic metal chelation sites and channels. Figure 1 | (a) Indexed experimental (black), Pawley refined (red) PXRD patterns with their difference (blue), and the calculated pattern (green) from dia-c7 net of SP-3D-COF-BPY. Inset: zoomed view of detailed PXRD profile without the primary peak; (b) structural representation of SP-3D-COF-BPY (dia-c7). Top left: ball-and-stick images; top right: the representation of interpenetration; bottom: space-filling model views perpendicular (left) and parallel to 1D channels (right). Download figure Download PowerPoint The sevenfold interpenetration of SP-3D-COF-BPY yielded numerous coordination sites that could chelate metal ions firmly (Scheme 1b). The remarkable advantage of the SP-3D-COF family is that the uniform square pore channel was constructed from rigid orthogonal planar SP units. The channel walls with a high density of discrete coordination sites could efficiently bind metal ions and fully expose the active metal ions to the channel space to collide with substrates (Scheme 1b). Thus, SP-3D-COF-BPY is considered superior in serving as a heterogeneous catalyst to its counterparts due to the unique topological structure. The surface area and porosity of SP-3D-COF-BPY were determined by nitrogen adsorption–desorption analysis at 77 K. As shown in Figure 2a, under low-pressure stage (P/P0 < 0.05), SP-3D-COF-BPY shows type I isotherm with a sharp uptake. The Brunauer–Emmett–Teller (BET) surface area was obtained as SBET = 1945 m2 g−1, which is relatively large and beneficial for the introduction of Pd(II). The total pore volume is Vp = 1.10 cm3 g−1 (calculated at P/P0 = 0.99). The pore size distribution curve clearly shows that the pore-limiting diameter is 1.36 nm (Figure 2b), which agrees with the simulated model (1.56 nm, Figure 1b). Figure 2 | (a) N2 adsorption–desorption isotherms (77 K) of SP-3D-COF-BPY. (b) Pore-size distribution from the quenched solid density functional theory of SP-3D-COF-BPY. Download figure Download PowerPoint The Pd(II) was then introduced into the SP-3D-COF-BPY through a simple wet-chemistry approach (see Supporting Information). Pd(II)@SP-3D-COF-BPY was obtained as an ionic framework, with Pd(II) chelated to the bipyridine moieties and the acetate counter anions located nearby. The metal content adsorbed by the framework was determined to be 14.8 wt % by inductively coupled plasma (ICP) analysis, which is slightly lower than the theoretical value of 17.9%. A comparison of the PXRD patterns of SP-3D-COF-BPY and Pd(II)@SP-3D-COF-BPY indicated that the crystallinity and the periodic structure of Pd(II)@SP-3D-COF-BPY were well kept after introducing Pd(II) ions ( Supporting Information Figure S6). The BET surface area of Pd(II)@SP-3D-COF-BPY decreased to SBET = 640 m2 g−1 due to the introduction of Pd(OAc)2 ( Supporting Information Figure S7), clearly supporting the efficient deposition of Pd(II) in the channels of SP-3D-COF-BPY. The X-ray photoelectron spectroscopy (XPS) was then performed to investigate the oxidation state of the Pd and incorporation with the COF. As shown in Figure 3, the binding energy of Pd 3d3/2 in Pd(II)@SP-3D-COF-BPY (338.4 eV) was very similar to the reference compound Pd(OAc)2 (338.5 eV). The XPS results confirmed the palladium's divalent nature in the COF and efficient coordination with the bipyridine units in the COF. Figure 3 | XPS spectra of the Pd 3d orbital in Pd(II)@SP-3D-COF-BPY (red), Pd(OAc)2 (blue), and [email protected] (green). Download figure Download PowerPoint Notably, no characteristic peak of Pd(0) was observed from the PXRD patterns of Pd(II)@SP-3D-COF-BPY ( Supporting Information Figure S6), which further confirmed the Pd(II) state in Pd(II)@SP-3D-COF-BPY. As for TEM images, it has been reported that the electron beams in TEM would reduce Pd(II) to Pd(0) effectively during analysis,40,41 which also led to the invention of an electron-beam-induced synthetic approach of Pd(0) NPs.42–44 In periodic structures, this phenomenon has been often observed in MOFs.45,46 Recently, Wang's group14 also noticed this in their Pd(II)-containing two-dimensional (2D)-COF system. Indeed, we observed some ultrasmall PdNPs from the TEM of Pd(II)@SP-3D-COF-BPY (Figure 4b). The statistic histogram for randomly selecting more than 100 particles indicated that the Pd NPs had an average size diameter of 1.28 nm with narrow size distributions ( Supporting Information Figure S8). The interplanar spacing of these PdNPs was calculated to be 0.17 nm, consistent with the plane spacing of the Pd structure (Figure The corresponding area electron diffraction pattern of diffraction revealed the nature of PdNPs and typical diffraction for and planes, as well as their secondary diffraction with to the crystal structure of Pd Figure | (a) TEM of SP-3D-COF-BPY. (b) of Pd(II)@SP-3D-COF-BPY. pattern of the Pd(II)@SP-3D-COF-BPY. of the Pd(II)@SP-3D-COF-BPY. of Pd(II)@SP-3D-COF-BPY after Download figure Download PowerPoint Recently, COF-supported PdNPs have been reported as heterogeneous catalysts for coupling For comparison we also prepared [email protected] the by Pd(II) ions after their deposition in the COF experimental Supporting Information). The XPS results of the [email protected] showed that the binding of Pd 3d3/2 and Pd are located at and which is than the Pd(II)@SP-3D-COF-BPY (Figure The metal content adsorbed by the framework was determined to be wt % by analysis, slightly lower than that of Pd(II)@SP-3D-COF-BPY. To confirm the of the bipyridine as a chelated metal SP-3D-COF-BPH reported in our was used as a reference (Scheme 1a).28 The structure of SP-3D-COF-BPH is to SP-3D-COF-BPY except that the was used in SP-3D-COF-BPH instead of The was under the same As Pd(II) could also be into SP-3D-COF-BPH, given its nanoporous structures. the analysis showed that the Pd content was only wt which is lower than that of SP-3D-COF-BPY wt the bipyridine had a on the coordination of Pd(II). The XPS also confirmed the divalent nature of the palladium in SP-3D-COF-BPH ( Supporting Information Figure we the catalytic of Pd(II)@SP-3D-COF-BPY and compared it with and [email The Suzuki–Miyaura reaction was as an The reaction was performed with a catalytic of Pd(II)@SP-3D-COF-BPY under typical one-pot Suzuki–Miyaura coupling using a small molecule catalyst acid mg, 0.75 mmol), aryl halide (0.5 mmol), potassium carbonate (138 mg, 1.0 mmol), and Pd(II)@SP-3D-COF-BPY (2 mg, 0.5 mol % were dispersed in 1 mL of p-xylene. The suspension was heated to 70 °C for 2 h. The mixture was centrifuged and Pd(II)@SP-3D-COF-BPY was washed with dichloromethane for the The product was purified by chromatography over and calculated the yield Pd(II)@SP-3D-COF-BPY showed excellent catalytic with only 0.5 mol % of Pd loading under 2 substrates with different such as or groups with were were obtained for the For the substrates, a reaction was to a 95% less active substrates also up to A catalyst mol % was then in the reaction to the catalytic performance We found that the yield could still when the reaction was to h. these results clearly indicated that Pd(II)@SP-3D-COF-BPY is an excellent catalyst for the Suzuki–Miyaura coupling The catalytic performance of the reference catalyst was then The of the catalyst used in the reaction was on the analysis to that the same of Pd was As the was much lower than Pd(II)@SP-3D-COF-BPY. the same (10 with 0.5 mol % the yield only compared with Pd(II)@SP-3D-COF-BPY 1). In contrast, [email protected] as a catalyst also exhibited lower catalytic than that of Pd(II)@SP-3D-COF-BPY. The yield 1). This superiority of Pd(II)@SP-3D-COF-BPY could be due to the of bipyridine sites to ligands) in the of the which could the of the in Indeed, the catalytic is well to be superior to given the same of active due to the of sites. To the of the crystallinity of the SP-3D-COF-BPY, the of the SP-3D-COF-BPY was used as The catalyst prepared using this under the same showed decreased Pd content 14.8 wt The product in Suzuki–Miyaura coupling reactions decreased by as much as indicating the lower catalytic performance of the catalyst under the same Pd loading (0.5 mol % of ( Supporting Information Table S2). results indicated that the crystallinity of is for Pd loading and catalytic performance. Table 1 | of and [email protected] in the Suzuki–Miyaura 1 2 2 2 3 2 2 I 2 6 I 2 I 2 12 6 a Pd(II)@SP-3D-COF-BPY mg, 0.5 mol % as b mg, 0.5 mol % as c [email protected] mg, 0.5 mol % as yield after 12 h. yield 95% after 12 h. mol % Pd was The and stability of Pd(II)@SP-3D-COF-BPY were then The catalyst be easily through and washed with The was performed using 0.5 mol % Pd of the catalyst in the reaction of phenylboronic acid and The results demonstrated that the high catalytic of Pd(II)@SP-3D-COF-BPY after ( Supporting Information Table S3). dispersed PdNPs were observed from the TEM (Figure The PXRD patterns of the catalyst showed a slightly solid in the of the diffraction peaks were that the of the crystalline structure was ( Supporting Information Figure S6). The XPS spectra of the Pd(II)@SP-3D-COF-BPY catalyst shows a (0.5 eV) of the Pd(II) peaks compared with the Pd(II)@SP-3D-COF-BPY ( Supporting Information Figure This could be attributed to the of Pd(II) to the active Pd(0) species. The analysis of the Pd(II)@SP-3D-COF-BPY after showed that the Pd content wt The in Pd loading after multiple wt % is due to the leaching of the Pd those or adsorbed on the surface and in the pores during the reaction and the which stirring in the reaction for several centrifugation, and was to the of the active from the Pd(II)@SP-3D-COF-BPY or Pd(II) from the Pd(II)@SP-3D-COF-BPY in the the same reaction except for the of the substrates, Pd(II)@SP-3D-COF-BPY was heated up to 70 °C for 2 h. The from the mixture was used to the reaction of phenylboronic acid and was these results further that the Pd(II) was in the SP-3D-COF-BPY. under of Suzuki–Miyaura coupling reaction,

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