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Open AccessCCS ChemistryRESEARCH ARTICLE1 Jul 2021Copper (I)–Organic Frameworks for Catalysis: Networking Metal Clusters with Dynamic Covalent Chemistry Rong-Jia Wei†, Hou-Gan Zhou†, Zhi-Yin Zhang, Guo-Hong Ning and Dan Li Rong-Jia Wei† College of Chemistry and Materials Science, Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications, Jinan University, Guangzhou 510632 , Hou-Gan Zhou† College of Chemistry and Materials Science, Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications, Jinan University, Guangzhou 510632 , Zhi-Yin Zhang College of Chemistry and Materials Science, Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications, Jinan University, Guangzhou 510632 , Guo-Hong Ning *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Chemistry and Materials Science, Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications, Jinan University, Guangzhou 510632 and Dan Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Chemistry and Materials Science, Guangdong Provincial Key Laboratory of Functional Supramolecular Coordination Materials and Applications, Jinan University, Guangzhou 510632 https://doi.org/10.31635/ccschem.020.202000401 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Metal clusters exhibit diverse structures, emerging functions, and applications; thus, incorporating clusters into metal–organic frameworks (MOFs) brings tremendous merits. Although the construction of cluster-based MOFs is sophisticated, the reticular materials constructed from a combination of the chemistry of metal clusters and covalent organic frameworks (COFs) remain unexplored. Herein, we prepared two Cu(I) cluster-based MOFs with cyclic trinuclear units (CTUs), termed JNM-1 and JNM-2, either by a stepwise synthetic approach or by a one-pot reaction, for networking clusters with dynamic covalent chemistry, rarely utilized in MOF synthesis. The generated JNMs exhibited excellent stability and could be used as recyclable catalysts for palladium-free Sonogashira coupling reactions with high efficiency and tolerance (>90% yield for nine examples), without loss of performance for at least five cycle runs. In addition, conjugated single molecular wires with lengths ranging from 1.6 to 2.7 nm were synthesized feasibly using the JNM-1 catalyst. Download figure Download PowerPoint Introduction Reticular chemistry1 endows chemists to link molecular building blocks into extended and crystalline framework structures such as metal–organic frameworks (MOFs)2–6 and covalent organic frameworks (COFs)7–11 via strong coordinate and covalent bonds, respectively. Owing to relatively weaker strengths of coordinate bonds, compared with covalent bonds, MOFs are often suffering from stability issues, especially in harsh chemical environments such as strong bases and acids, boiling water, and reactions involving highly reactive substrates.12 In contrast, with the development of dynamic covalent chemistry (DCC), COFs could achieve high stabilities toward harsh conditions.13 However, the lack of metals restricts their functionalities and further applications. Therefore, it is envisioned that "cream-skimming" of coordination chemistry and DCC would address these shortcomings and might bring unprecedented structural complexity, along with functional diversity. Recently, incorporation of a single metal ion or mononuclear metal complexes into COFs, namely metal–covalent organic frameworks (MCOFs),14 was proposed and even shown to facilitate crystal growth of COFs with unusual topology,15,16 leading to emerging applications in catalysis, molecular adsorption and separation, optics, and sensing.14 Compared with single metal ion units, metal clusters or polynuclear metal complexes are much more attractive because of their intriguing aesthetic and diverse structures, as well as fascinating functions such as magnetism, catalytic activities, and luminescence properties.17–19 Although the preparation of cluster-based MOFs has been well established,20,21 the construction of cluster-based, crystalline reticular materials via linkage of covalent bonds is highly challenging and remain scarcely explored.22–24 This is due to the incompatibility of the condition for cluster formation with those of DCC, and the stability along with solubility issues of clusters during their synthesis and crystallization. Cyclic trinuclear units (CTUs) with d10 metals are a class of metal clusters exhibiting unique properties such as unsaturated metal centers with a medium oxidation state, metallophilic attraction, π-acidity/basicity, and luminescence properties. Therefore, they are potentially useful for a wide range of applications, including chemical sensing, full-color display, gas absorption, and catalysis.25–28 In 2006, our group29 first introduced the solvothermal synthesis of a MOF with Cu3Pz3 CTUs (pyrazolate ligand [Pz]), with reaction conditions similar to those used in COF synthesis. Therefore, we reasoned that the Cu(I) cluster-based organic frameworks could be constructed in a hierarchical assembly fashion via a combination of metal clusters chemistry and COF, which is rarely adopted in MOFs' fabrication (Scheme 1).22–24 Unlike the one-pot synthesis, assembly of metal clusters-based COFs in a stepwise fashion could exclude the disturbance from other metal ions or ligands; thus, the extended structure could be predictable and designed precisely employing reticular chemistry. Scheme 1 | Stepwise and one-pot syntheses and structural illustration of the JNMs. Download figure Download PowerPoint In this study, we demonstrated the preparation of two-dimensional (2D) Cu(I) CTU-based organic frameworks, namely JNM-1 and JNM-2 (JNM represents Jinan material), from either imine condensation reaction between Cu3L3 [1H-pyrazole-4-carbaldehyde (HL)] as cluster units and organic linkers [i.e., 1,3,5-tris(4-aminophenyl)benzene ( 1) for JNM-1 or 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline ( 2) for JNM-2, respectively], or in a one-pot reaction (Scheme 1). Interestingly, these two MOFs featured much higher stability and porosity than those of Cu3L3 CTUs, making them promising platforms for catalytic study. Indeed JNM-1 exhibited excellent catalytic activities and broad substrate scope with various functional groups for the palladium (Pd)-free Sonogashira cross-coupling reaction. Besides, JNM-1 showed much better catalytic activities than those of Cu3L3, and it could be applicable in the synthesis of conjugated single molecular wires. Overall, the strategy of combining reticular chemistry of metal clusters and COF in a stepwise manner allowed us to merge their advantages for constructing new types of function-led reticular materials with rational design. Experimental Methods Synthesis of the complex Cu3L3 A mixture of the ligand 1H-pyrazole-4-carbaldehyde (HL) (24.0 mg, 0.25 mmol), Cu2O (14.3 mg, 0.1 mmol), 4 mL ethanol, and 0.1 mL pyridine was sealed in an 8 mL Pyrex tube, heated in an oven at 120 °C for 72 h, and then slowly cooled to room temperature at a rate of −5 °C·h−1. The light-yellow needle crystals of Cu3L3 formed were filtered and collected under a microscope manually. The yield of Cu3L3: 23.7 mg (75.8%, based on Cu2O). Chemical formula, C12H9Cu3N6O3: C, 30.29; H, 1.91; N, 17.66. Found: C, 30.45; H, 2.13; N, 17.42. IR (KBr, cm−1): 3481 w, 3109 w, 2782 w, 1667 s, 1537 s, 1416 m, 1337 w, 1203 s, 1044 m, 872 w, 767 m, 625 w. Solid-state 13C cross-polarization/magic-angle spinning nuclear magnetic resonance (CP/MAS NMR) (400 MHz), δ (ppm) 124, 142, 184. Stepwise synthesis of JNM-1 and JNM-2 A 10 mL Schlenk tube was charged with Cu3L3 (23.7 mg, 0.05 mmol), 1,3,5-tris(4-aminophenyl)benzene ( 1) (26.3 mg, 0.075 mmol) or 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline ( 2) (26.5 mg, 0.075 mmol), 0.5 mL of mesitylene, 0.5 mL of dioxane, and 0.1 mL of 6 M aqueous acetic acid. Each tube containing 1 or 2 was flash-frozen at 77 K in a liquid nitrogen bath and degassed with three freeze-pump-thaw cycles. Upon warming to room temperature, each tube was heated at 120 °C for 72 h. The pale green solid from each tube was isolated by filtration, washed, and solvent exchanged with tetrahydrofuran (THF) and fresh dimethylformamide (DMF). The resultant solids were dried under vacuum at 100 °C for 8 h to give JNM-1 and JNM-2 both as pale green powders. For JNM-1: Elemental analysis calcd (%) for C36H24Cu3N9: C, 55.92; H, 3.11; N, 16.31. Found: C, 53.58; H, 2.40; N, 15.71. Yield: 28.1 mg (73%, based on Cu3L3). IR (KBr pellets, cm−1): 3355 w, 3112 w, 2872 w, 1667 m, 1617 s, 1539 m, 1490 m, 1375 w, 1199 m, 1050 w, 863 m, 748 w, 641 m. For JNM-2: Elemental analysis calcd (%) for C33H21Cu3N12: C, 51.06; H, 2.70; N, 21.66. Found: C, 49.52; H, 2.01; N, 21.20. Yield: 29.5 mg (76%, based on Cu3L3). IR (KBr pellets, cm−1): 3369 w, 1667 m,1591 m, 1507 s, 1417 w, 1369 m, 1309 w, 1246 w, 1203 m, 1144 w, 1071 w, 1013 w, 877 w, 814 w. One-pot synthesis of JNM-1 and JNM-2 A 10 mL Schlenk tube was charged with Cu2O (10.7 mg, 0.075 mmol), HL (14.4 mg, 0.15 mmol), 1 (26.3 mg, 0.075 mmol) or 2 (26.5 mg, 0.075 mmol), 0.5 mL of mesitylene, 0.5 mL of dioxane, and 0.1 mL of 6 M aqueous acetic acid. The tube was flash-frozen at 77 K in a liquid nitrogen bath and degassed with three freeze-pump-thaw cycles. Upon warming to room temperature, the tube was heated at 120 °C for 72 h. The pale green solid was isolated by filtration, washed, and solvent exchanged with THF and DMF. The resultants were dried under vacuum at 100 °C for 8 h to give JNM-1 and JNM-2 as pale green powders for catalytic performance experiments. General procedure for the Sonogashira cross-coupling reaction Before the catalytic experiment, the catalysts were dried in a vacuum at 120 °C for 8 h. About 4 mol % of the dried catalysts and 5 mL of DMF were added into a 10 mL Pyrex tube. Then phenylacetylene (0.5 mmol, 51.5 mg), iodobenzene (0.6 mmol, 122.4 mg), and K2CO3 (1 mmol, 138.2 mg) were added into the tube, orderly. The mixture was stirred at 140 °C under N2 atmosphere for 8 h. After that, 50 µL of the reaction solution was taken and diluted with CH2Cl2 to 1 mL, followed by centrifugation at 10,000 rpm·min−1 for 5 min. Then the supernatant was analyzed by gas chromatography–mass spectrometry (GC–MS). The reaction conversion was calculated based on the phenylacetylene reference substrate. Also, after the 8 h completion of the reaction, the mixture was quenched with water. The aqueous layer was extracted with ethyl acetate (3 × 150 mL), and the combined organic layers were washed with water, dried with anhydrous MgSO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography to give a solid white product. Different catalysts, solvents, temperatures, catalyst loading, and others were investigated in a similar procedure. Results and Discussion The initial synthetic attempts were carried out in a stepwise manner, in which single crystals of discrete, planar Cu3L3 were obtained under solvothermal conditions. A single crystallographic analysis revealed that Cu3L3 complexes formed a column packing with intermolecular Cu⋯Cu distances of 3.74 Å, indicating weak metal–metal interactions (Figures 1a–c). The solvothermolysis of a suspension of triangular Cu3L3 and organic linkers 1 or 2 in a 5∶5∶1 (v/v) mixture of mesitylene, 1,4-dioxane, and 6 M aqueous acetic acid led to crystalline products with hexagonal symmetry of hxl lamellar structures of JNM-1 and JNM-2 (Scheme 1). Although the high crystalline JNMs could be prepared feasibly from a one-pot reaction of the Cu2O, HL, and 1 or 2, where Cu3L3 was formed in situ, it was hard to remove unreacted Cu2O and remained as impurities, as confirmed by powder X-ray diffraction (PXRD) patterns (see Supporting Information Figures S2 and S3). A similar observation of metal oxide impurities was also reported previously in the one-pot synthetic approaches.22 Figure 1 | X-ray structure of Cu3L3. (a) ORTEP diagram at 50% level; (b) top view and (c) side view showing the crystal packing of Cu3L3 with intermolecular Cu⋯Cu distance of 3.74 Å. (C, N, O, H, and Cu atoms are shown as gray, light blue, light red, white, and orange, respectively.) Structural modeling of JNM-1 exhibiting (d) AA and (e) AB packing modes shown as space-filling models. (f) PXRD structural analysis of JNM-1. Experimental (black) and refined (red) PXRD patterns of JNM-1 with difference curve (blue), and calculated profiles of JNM-1 displaying AA (purple) and AB (green) packing modes. N2 adsorption (filled) and desorption (open) isotherm profiles of (g) JNM-1 and (h) JNM-2 at 77 K. Inset, showing pore size distribution profiles of JNM-1 and JNM-2 calculated by nonlocal DFT modeling based on N2 adsorption data, showing a uniform pore size of 1.89 nm. ORTEP, Oak Ridge thermal ellipsoid plot; PXRD¸ powder X-ray diffraction; DFT, density functional theory. Download figure Download PowerPoint The Fourier-transform infrared (FT-IR) spectra of the JNMs confirm the formation of imine linkages, supported by the disappearance of the N−H stretching signals located at 3462–3208 cm−1 and exhibition of the C=N stretching bands located at 1623–1617 cm−1 ( Supporting Information Figures S4 and S5). Also, the solid-state 13C CP/MAS NMR spectra of the JNMs revealed the vanish of aldehyde carbon signals located at 184 ppm and the appearance of characteristic resonance peaks of imine carbons at 157 and 155 ppm for JNM-1 and JNM-2, respectively, which evidenced the existence of imine linkages ( Supporting Information Figures S7 and S8). Furthermore, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) displayed rod-shaped morphologies of the products and consisted of highly crystalline nanolayered structures ( Supporting Information Figures S9–S12). Energy-dispersive X-ray spectroscopy (EDS) elemental mapping of the JNM powder particles displayed a uniform distribution of C, N, and Cu in JNMs ( Supporting Information Figures S13 and S14). The PXRD experiments and theoretical simulations were performed to analyze the crystal structure of the microcrystals of JNMs obtained. The structural calculations were carried out using BIOVIA Materials Studio (Accelrys, San Diego, CA, USA; see Supporting Information Figures S15–S19) during which the eclipsed stacking (AA) and staggered stacking (AB) structures were simulated (Figures 1d and 1e). The PXRD patterns of JNM-1 show an intense peak at 4.27° accompanied by four small peaks at 7.42°, 8.55°, 11.43°, and 25.98°, which can be attributed to (100), (110), (200), (120), and (001) diffractions. The experimental PXRD patterns were in good agreement with the calculated PXRD profiles of the AA stacking model (Figure 1f), suggesting that JNM-1 featured a uniform pore distribution with an eclipsed structure. In particular, Pawley refinements gave a hexagonal space group P 6 ¯ with unit cell parameters of a = b = 24.4344 Å, c = 4.2298 Å, with refinement parameters of Rp = 3.84% and Rwp = 8.61%. The refined PXRD patterns match the experimental PXRD data, as confirmed by the negligible difference plot in Figure 1f. JNM-2 featured a similar AA stacking structure with JNM-1 (see Supporting Information for details). The nitrogen adsorption isotherms, measurements at 77 K of JNM-1 and JNM-2 both illustrate the Type IV adsorption curves featuring the mesoporous nature (Figures 1g and 1h). The Brunauer−Emmett−Teller (BET) surface areas of JNM-1 and JNM-2 were calculated to be 534.61 and 505.32 m2·g−1 and the total pore volumes were 0.28 and 0.39 cm3·g−1 (P/P0 = 0.99), respectively. The calculated eclipsed-stacked structures of JNM-1 and JNM-2 using the nonlocal density functional theory (DFT) suggested a narrow pore size distribution with an average pore width both of ∼1.89 nm (Figures 1g and 1h), identical to their theoretical values of 1.89 nm predicted from the eclipsed AA stacking structures, thereby further supporting the eclipsed structures of JNM-1 and JNM-2. Interestingly, the JNMs exhibited high stability toward heat, air, and water, in spite of the common observation of fast oxidation and decomposition of Cu(I) CTU-based compounds when exposed to air and water.30–32 Thermal gravimetric analyses (TGA) and various temperature PXRD spectra under N2 atmosphere proved that the JNMs had high thermal stability and remained the high crystallinity up to 320 °C ( Supporting Information Figures S21–S24). It is known that the Cu(I) ions in CTU-based MOFs underwent fast oxidation to give Cu(II) ions.31,32 In contrast, the JNMs exhibited superior stability even when exposed to air over 1 month. X-ray photoelectron spectroscopy (XPS) measurements evidenced only intense sharp and symmetrical Cu(I) 2p3/2 signals at 933.4 and 933.5 eV for JNM-1 and JNM-2, respectively, without satellite peaks, implying that the Cu(I) ions remained intact within the frameworks (Figure 2a and Supporting Information Figure S26). Furthermore, the crystallinities of the JNMs were sustained upon suspension in various organic solvents, water, and even NaOH solutions for 24 h, documented by PXRD analyses ( Supporting Information Figures S27 and S28). Figure 2 | XPS profiles of (a) JNM-1 exposed to air for over 1 month and before oxidation with H2O2; (b) JNM-1 after oxidation with H2O2 (the asterisk represents the Cu(II) satellite peaks); (c) oxidized JNM-1 was reduced in NMP at 160 °C. XPS, X-ray photoelectron spectroscopy; NMP, N-methyl-2-pyrrolidone. Download figure Download PowerPoint We tested the reversible redox reactivities of Cu(I) CTUs in JNMs by initially treating JNM-1 with a solution of H2O2 in THF at room temperature for 24 h (see Supporting Information Figure S29). PXRD analysis of the resultant dark green powder suggested a slight decrease in the crystallinity of JNM-1, which might be due to vigorous stirring. In addition, the XPS experiments revealed an intense asymmetrical Cu 2p3/2 peak along with satellite peaks that could be deconvoluted into two contributions located at 932.9 and 934.8 eV, corresponding to Cu(I) 2p3/2 and Cu(II) 2p3/2 with an integrated Cu(I):Cu(II) ratio of ∼4∶5, respectively (Figure 2b).32 These results confirmed that the Cu(I) ions in JNM-1 were able to oxidize to Cu(II) ions in the presence of an oxidant. When the samples of oxidized JNM-1 were heated in N-methyl-2-pyrrolidone (NMP) at 160 °C, the Cu(II) ions reduced entirely to Cu(I) ion, as shown by the XPS analysis (Figure 2c). The excellent stability and reversible redox reactivities of JNMs promoted us to investigate their catalytic performance further. Since the of the Sonogashira cross-coupling it has been used in synthetic chemistry as an for Recently, with a more and Cu catalyst has of the Sonogashira cross-coupling reaction by the JNMs was tested via an initial of a model reaction of phenylacetylene and We the reaction including reaction temperature, and catalyst shown in 1 and Supporting Information Figure at 140 °C, and in the presence of K2CO3 as the the mixture of and JNM-1 mol based on Cu formed the coupling in 8 h with the temperature to 120 °C or the reaction to 2 h the conversion to and respectively. Besides, the reaction in the of the catalyst. the a was indicating the high of the using the JNMs catalyst. It is that phenylacetylene underwent a coupling reaction in the presence of air and JNMs catalyst to give with a high yield of (see Supporting Information for details). 1 | JNM Sonogashira (%) 1 2 mol % JNM-1 DMF 140 2 mol % JNM-1 DMF 140 4 mol % JNM-1 DMF 140 4 5 mol % JNM-1 DMF 140 5 4 mol % JNM-1 THF 6 4 mol % JNM-1 4 mol % JNM-1 8 4 mol % JNM-1 DMF 100 4 mol % JNM-1 DMF 120 10 4 mol % JNM-2 DMF 140 6 mol % Cu2O DMF 140 4 mol % Cu3L3 DMF 140 phenylacetylene 0.5 mmol, iodobenzene mL), N2 and reaction is 8 h. The reported conversion is based on chromatography–mass spectrometry Although Cu3L3 exhibited similar catalytic activities with JNM-1 it after the reaction, confirmed by In addition, the green mixture further oxidation of Cu(I) to In contrast, the JNM-1 catalyst excellent stability and after five catalytic the crystallinity and structural of JNM-1 remained supported by the PXRD analysis ( Supporting Information Figure the catalyst could feasibly from the reaction mixture by and at least for five reaction without loss of catalytic performance ( Supporting Information Figure Furthermore, we investigated the of Cu ions in JNM-1 by XPS The XPS experiments of JNM-1 revealed an intense Cu(I) 2p3/2 at 933.4 eV without satellite peaks, that the Cu(I) ions are during the catalytic cycle and are and ( Supporting Information Figure the conditions in we further the scope of the JNM-1 coupling reaction with various We utilized with groups ( and and with ( and with both of which with excellent In addition, we investigated the tolerance of with reactive functional which could the imine or coordinate with metal the with and ( and also gave excellent ranging from to These results demonstrated that the and reactive functional groups were well in the Sonogashira cross-coupling reaction using the JNM-1 catalyst. 2 | of JNM-1 for Sonogashira 0.5 of 4 of the JNM-1 and 2 of DMF mL), 140 °C, N2 8 h. The reported are isolated The Sonogashira cross-coupling reaction is a approach for constructing a single molecular with a conjugated structure such as which are only for the electron also for We to such conjugated molecular by the JNM-1 catalyst JNM-1 showed superior catalytic compared with that of Cu3L3 The reaction of ( of and JNM-1 gave the ( with a good yield of only yield was obtained using Cu3L3 as a catalyst. yield of the might be to the stability of Cu3L3 in the presence of with which in Besides, with extended conjugated was synthesized with over yield using either JNM-1 or Cu3L3. JNM-1 was for with a conjugated up to 2.7 nm These results demonstrated that JNM-1 is a highly and promising catalyst for constructing molecular wires. | Synthesis of for 0.25 of for 0.5 of for 0.25 of and 4 of the JNM-1 or Cu3L3 2 of DMF mL), 140 °C, N2 h. The isolated yield using b JNM-1 or catalyst. Inset, the X-ray structure of and displaying the ORTEP diagram at a 50% and the calculated structure of showing the (C, O, and atoms are shown as red, and white, ORTEP, Oak Ridge thermal ellipsoid We a stepwise synthetic strategy for Cu(I) CTUs into extended frameworks by combining the chemistry of metal clusters and COFs, which is rarely demonstrated in These Cu(I) cluster-based JNM-1 and JNM-2, exhibited superior compared with that of their Cu(I) they reversible redox by their Cu(I) an of metal clusters and we illustrate that JNMs are promising platforms for the Sonogashira cross-coupling reactions with excellent performance and Besides, JNM-1 is a useful catalyst for constructing conjugated molecular wires and much higher catalytic activities than that of Cu3L3. the JNMs catalyst could be and for at least five reaction without loss of This a synthetic strategy for constructing function-led cluster-based reticular materials by networking metal clusters via linkage of dynamic covalent Supporting Information Supporting Information is of The of Information This was supported by the of and and the Guangdong of and is for the from Guangdong and Guangdong Scheme and the for the The and for the structural modeling and refinement and solid-state 13C measurements and their to Reticular Frameworks and Covalent Synthesis and the of Coordination and of of Frameworks to MOF and K. Chemistry and of Covalent Frameworks to of Covalent Frameworks on and Frameworks and for and of Li Covalent Li Frameworks A Frameworks and Covalent Zhang of into a Covalent Li Li K. of a Clusters of and Metal to the Metal and and Zhang Functional Coordination Li and K. as an of the Chemistry of and of and as a Li of from Different A for of of Supramolecular of and Li of Cyclic a to Cyclic d10 via and to the via and Ning Li Cyclic with to and to Li and Synthesis of Coordination and Zhang Coordination by Metal Zhang Metal and of Li Li in Frameworks from Sonogashira Synthesis of of with and and in Sonogashira A in and A with Chemical to Functional Information Chemical organic trinuclear and for the structural modeling and refinement and solid-state 13C measurements and their

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