Strongly Lewis Acidic Metal–Organic Frameworks for Continuous Flow

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Cite This: J. Am. Chem. Soc. 2019, 141, 14878−14888 pubs.acs.org/JACS

Strongly Lewis Acidic Metal−Organic Frameworks for Continuous Flow Catalysis † § † § † † ‡ † ‡ Pengfei Ji, , Xuanyu Feng, , Pau Oliveres, Zhe Li, , Akiko Murakami, Cheng Wang, † and Wenbin Lin*, † Department of Chemistry, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States ‡ College of Chemistry and Chemical Engineering, iCHEM, State Key Laboratory of Physical Chemistry of Solid Surface, Xiamen University, Xiamen 361005, China

*S Supporting Information

ABSTRACT: The synthesis of highly acidic metal−organic frameworks (MOFs) has attracted significant research interest in recent years. We report here the design of a strongly Lewis acidic MOF, ZrOTf-BTC, through two-step transformation of MOF-808 (Zr-BTC) secondary building units (SBUs). Zr-BTC was first treated with 1 M hydrochloric acid solution to afford ZrOH-BTC by replacing each bridging formate group with a pair of hydroxide and water groups. The resultant ZrOH-BTC fl was further treated with trimethylsilyl tri ate (Me3SiOTf) to afford ZrOTf-BTC by taking advantage of the oxophilicity of the Me3Si group. Electron paramagnetic resonance spectra of Zr-bound superoxide and fluorescence spectra of Zr-bound N- methylacridone provided a quantitative measurement of Lewis acidity of ZrOTf-BTC with an energy splitting (ΔE) of 0.99 eV π * π * between the x and y orbitals, which is competitive to the homogeneous benchmark Sc(OTf)3. ZrOTf-BTC was shown to be a highly active solid Lewis acid catalyst for a broad range of important organic transformations under mild conditions, including Diels−Alder reaction, epoxide ring-opening reaction, Friedel−Crafts acylation, and alkene hydroalkoxylation reaction. The MOF catalyst outperformed Sc(OTf)3 in terms of both catalytic activity and catalyst lifetime. Moreover, we developed a ffi fl ZrOTf-BTC@SiO2 composite as an e cient solid Lewis acid catalyst for continuous ow catalysis. The Zr centers in ZrOTf- BTC@SiO2 feature identical coordination environment to ZrOTf-BTC based on spectroscopic evidence. ZrOTf-BTC@SiO2 displayed exceptionally high turnover numbers (TONs) of 1700 for Diels−Alder reaction, 2700 for epoxide ring-opening reaction, and 326 for Friedel−Crafts acylation under flow conditions. We have thus created strongly Lewis acidic sites in MOFs fl fl via tri ation and constructed the MOF@SiO2 composite for continuous ow catalysis of important organic transformations.

■ INTRODUCTION solubility in nonpolar organic solvents, are sensitive to Lewis acids efficiently catalyze many different types of organic moisture, and have short lifetimes during catalysis. In order See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. to overcome these drawbacks, researchers have devoted Downloaded via DALIAN INST OF CHEMICAL PHYSICS on November 21, 2019 at 12:05:37 (UTC). reactions by withdrawing electron density from functional − fi ff groups to make them susceptible to nucleophilic attacks.1 3 In signi cant e orts to developing heterogeneous Lewis acid Diels−Alder reactions, for example, coordination of a catalysts such as zeolites, metal oxides and resins over the past dienophile to a Lewis acid significantly decreases its LUMO few decades. Solid acid catalysts can be readily separated from 4 − reaction mixtures for reuse and are compatible with flow energy to facilitate its reaction with a diene. Catalytic Diels 9−11 Alder reactions allow for efficient synthesis of functionalized catalysis, but they have moderate Lewis acidity, low active site density, and nonuniform active sites.12,13 cyclic compounds that havefoundwidespreaduseas − pharmaceutical ingredients, artificial flavors, fragrances, and Metal organic frameworks (MOFs) have recently emerged agrochemicals.5 Lewis acids are also used to catalyze the as a novel porous material platform for designing homoge- neously inaccessible catalysts by taking advantage of their formation of epoxides and aromatic ketones and regioselective − regular framework structures and site isolation effects.14 19 In ring opening of epoxides to yield alcohols. ff Early Lewis acid catalysts were typically based on main particular, metal centers in MOF nodes have a orded group or early transition metal halides. Lewis acidity of these outstanding single-site solid catalysts with unique electronic metals increases when the halides are replaced with more properties and steric environments that are not accessible via electron-deficient and less coordinating anions, such as triflate − fl − 6−8 ( OTf) or bis(tri uoromethylsulfonyl)imide ( NTf2). Received: July 23, 2019 However, these Lewis acidic metal salts tend to have low Published: September 4, 2019

© 2019 American Chemical Society 14878 DOI: 10.1021/jacs.9b07891 J. Am. Chem. Soc. 2019, 141, 14878−14888 Journal of the American Chemical Society Article conventional homogeneous chemistry or heterogenization M aqueous HCl at 100 °C for 18 h to replace the formate approaches. Although MOF nodes have been used as Lewis groups with six pairs of Zr-coordinated hydroxide and water acidic sites to catalyze organic transformations, such as groups to afford ZrOH-BTC with the inorganic node 20 − 21 22,23 μ μ cyclization reactions, C H iodinations, hydrolysis, composition of Zr6( 3-O)4( 3-OH)4(RCO2)6[(OH)(OH2)]6. and dehydrations,24 using activated substrates, Lewis acidity ZrOH-BTC was then treated with trimethylsilyl triflate fi ° ff of these metal sites is signi cantly lower than the (Me3SiOTf) in benzene at 80 C for 8 h to a ord ZrOTf- 25 homogeneous benchmark Sc(OTf)3. We have previously BTC. As the oxophilic Me3Si group forms stronger bond with − − developed two quantitative methods for measuring Lewis the OH moiety than with the OTf moiety, Me3SiOTf readily acidity of MOF active sites through electron paramagnetic removes OH− from ZrOH-BTC to generate ZrOTf-BTC with μ μ resonance (EPR) spectroscopy of MOF-bound superoxide the node composition of Zr6( 3-O)4( 3-OH)4(RCO2)6[OTf]6. •− fl (O2 )or uorescence spectroscopy of MOF-bound N- The Me3SiOH byproduct generated through this activation 21 methylacridone (NMA). Quantitative measurements of reacted with excess Me3SiOTf to form (Me3Si)2O. The fi Lewis acidity of MOFs can potentially guide the discovery of amount of (Me3Si)2O was quanti ed to be 1.92 equiv w.r.t. highly Lewis acidic MOF catalysts for efficient organic Zr by 1H NMR, agreeing well with the proposed activation synthesis through continuous flow catalysis.21 process (Figure S3, Supporting Information). Several examples of Zr MOFs have recently been shown to − Thermogravimetric analysis (TGA) of ZrOTf-BTC showed possess relatively strong acidity.20,22,23,26 29 Notably, Yaghi a weight loss of 63.8% in the 220−800 °C range (Figure S2, ̈ and co-workers developed MOF-808-SO4 as a strong Bronsted Supporting Information), consistent with the expected weight μ μ acid catalyst for alkene dimerization. The unsaturated Zr6 loss of 62.8% for the conversion of Zr6( 3-O)4( 3- fi ff nodes of MOF-808 were modi ed by sulfuric acid to display OH)4(BTC)2(OTf)6 to (ZrO2)12. Powder X-ray di raction strong Brönsted acidity (Figure 1).30 By utilizing a distinct (PXRD) studies showed that ZrOTf-BTC remains crystalline and maintains the structure of ZrOH-BTC (Figure 2b). Transmission electron microscope (TEM) images of ZrOTf- BTC displayed highly crystalline octahedral particles of ∼200 nm in dimensions. (Figure 2c) The porosity of ZrOTf-BTC fi − was con rmed by N2 sorption isotherms, with a Brunauer Emmett−Teller (BET) surface area of 779 m2/g. (Figure 2d) Pore size analysis by nonlinear density functional-theory (NL- DFT) showed a uniform pore at ∼16 Å that is attributable to the large hexagonal cages of the MOF. The pore size of ZrOTf- BTC is smaller than that of ZrOH-BTC (21 Å) due to the presence of six large triflate groups per SBU (Figure 2e). Figure 1. Chemical structures of reported acidic Zr MOFs and Computational and Spectroscopic Studies of Zr-OTf ZrOTf-BTC. Active Sites. We used a range of computational and spectroscopic techniques, including density functional theory (DFT), extended X-ray absorption fine-structure (EXAFS) and strategy, we aim to develop strongly Lewis acidic Zr MOF X-ray absorption near-edge spectroscopy (XANES), to catalysts that work through different and complementary elucidate the Zr coordination environment in ZrOTf-BTC. reaction mechanisms in this work.31,32 We synthesized Lewis The first question we attempted to address is whether triflate acidic ZrOTf-BTC MOF from Zr-BTC (MOF-808) through groups are incorporated into the MOF. We performed sulfur sequential postsynthetic formate removal and triflation of K-edge XANES analysis to prove the presence of triflate MOF nodes. ZrOTf-BTC is a highly active solid Lewis acid groups. The sulfur centers in ZrOTf-BTC displayed significant catalyst for a broad range of important organic transformations, edge-step, with the K-edge energy of at 2479.8 eV. This energy including Diels−Alder reaction, epoxide ring-opening reaction, is much higher than that of sodium thiosulfate (Na S O ) − 2 2 3 Friedel Crafts acylation, and alkene hydroalkoxylation reac- standard and matched well to the edge energy of Sc(OTf)3, tion. ZrOTf-BTC exhibits higher Lewis acidity than Sc(OTf)3 indicating the presence of triflate groups in ZrOTf-BTC and outperforms Sc(OTf)3 in these reactions with higher (Figure 3b). catalytic activity and longer catalyst lifetime. We further The next question we attempted to address is how triflate ff supported ZrOTf-BTC on SiO2 to a ord a composite material groups are coordinated to Zr centers. We envisioned two as an efficient solid Lewis acid catalyst for continuous flow possible modes of triflate coordination (Figure 3a): OTf μ catalysis. bridging two proximal Zr centers (the 2-model) or OTf chelation to a single Zr center (the η2-model). Both of these ■ RESULTS AND DISCUSSION two coordination modes are commonly observed in the crystal Synthesis and Characterization of ZrOTf-BTC. We have structures of molecular metal-OTf complexes. Both structures developed a novel strategy to install metal triflate active sites at were optimized using DFT at the B3LYP level of theory, and MOF nodes through postsynthetic triflation of metal their free energies were calculated to compare their relative μ hydroxide groups. Zr-BTC was synthesized following the stability. The optimized structure for the 2-model showed an literature procedure by a solvothermal reaction of trimesic acid average Zr−OOTf bond distance of 2.269 Å, which is similar to · − OTf and ZrOCl2 8H2O in a mixture of N,N-dimethylformamide reported Zr O bond distances in molecular Zr2(OTf) (DMF) and formic acid.33 The inorganic node of Zr-BTC has complexes.34,35 The optimized structure for the η2-model μ μ − OTf the composition of Zr6( 3-O)4( 3-OH)4(RCO2)6(HCO2)6, showed an average Zr O bond distance of 2.299 Å, which μ where six formate groups cap all the peripheral sites around is longer than that of the 2 model by 0.030 Å. Bond distance μ fl the Zr6 octahedron. Zr-BTC was subsequently activated with 1 analysis thus showed stronger bonding between 2-tri ates and

14879 DOI: 10.1021/jacs.9b07891 J. Am. Chem. Soc. 2019, 141, 14878−14888 Journal of the American Chemical Society Article

Figure 2. (a) Synthesis of ZrOTf-BTC through stepwise activation of Zr-BTC with 1 M HCl and Me3SiOTf. (b) Similarity between PXRD patterns of ZrOTf-BTC (magenta), Zr-BTC (red), and ZrOH-BTC (blue) indicates the maintenance of MOF crystallinity throughout postsynthetic treatments. (c) TEM image of ZrOTf-BTC showing octahedral morphology with an average diameter of 200 nm. (d) N2 sorption isotherms of ZrOTf-BTC, with a calculated BET surface area of 779 m2/g. (e) Pore size distribution of ZrOTf-BTC (blue) showing a uniform pore size of 16 Å that is smaller than that of ZrOH-BTC (21 Å, gray plot) due to the presence of six triﬂate groups per hexagonal pore in ZrOTf-BTC.

μ •− 36 Zr centers. Free energy calculations also indicated the 2 active Zr(O2 ) species (Figure 4a). Coordination to Lewis · fi •− model is thermodynamically more stable, with 24.85 kcal acids signi cantly shifts the EPR signature of O2 , especially −1 η2 Δ mol lower free energy than the -model per six Zr centers. the gzz-tensor that is determined by the energy splitting ( E) μ fi π * π * 37 •− The DFT optimized 2 model tted well to the Zr k-edge between the x and y orbitals. ZrOTf-BTC bound O2 Δ EXAFS spectrum of ZrOTf-BTC, with an R-factor of 0.007 exhibited a gzz of 2.0310, which corresponds to a E of 0.99 (Figure 3c, Table S2, Supporting Information). The fitted Zr- eV (Figure 4b), comparable with the benchmark homogeneous Δ OTf bond distances have an average value of 2.20 Å, similar to Lewis acid catalyst Sc(OTf)3, which displays a E of 1.00 eV that calculated by DFT (2.269 Å). The ZrOTf-BTC spectrum as reported.38 The nontriflated ZrOH-BTC has much lower was also fitted with the DFT optimized η2 model, but the Lewis acidity than ZrOTf-BTC, with a measured ΔE of 0.90 fitting afforded significant shortening of the Zr-η2-OTf bond eV. The Lewis acidity of ZrOTf-BTC is also much higher than Δ distance by 0.090 Å to 2.209 Å when compared to the DFT our previously reported Zr6-fBDC, which has the measured E optimized Zr−OOTf distance (Figure S5, Table S3, Supporting of 0.93 eV. The 0.06 eV increase in Lewis acidity makes μ Information). EXAFS studies thus favor the 2 coordination ZrOTf-BTC a much more active catalyst than Zr6-fBDC in mode as well. Diels−Alder reactions. Furthermore, ZrOTf-BTC has a much fi Quanti cation of ZrOTf-BTC Lewis Acidity by Spec- higher Lewis acid site density than Zr6-fBDC. troscopic Methods. We then quantified the Lewis acidity of We also used NMA fluorescence to probe the Lewis acidity λ ZrOTf-BTC using two spectroscopic methods that were of ZrOTf-BTC. Free NMA has an emission maximum ( max)at recently developed, including electron paramagnetic resonance 433.0 nm. Upon coordination to ZrOTf-BTC, the Zr(NMA)- •− λ (EPR) spectroscopy of MOF-bound superoxides (O2 ) and BTC complex displayed a max at 471.0 nm (Figure 4c). The fluorescence spectroscopy of MOF-bound N-methylacridone energy shift of NMA emission is established to be linearly (NMA).21 ZrOTf-BTC was shown to be more Lewis acidic correlated to the Lewis acidity of metal centers.21,38 Following than ZrOH-BTC and the previously reported Zr6-fBDC based the empirical equation that was described previously, we on the Lewis acidity measurement using both techniques. calculated the ΔE of ZrOTf-BTC to be 0.98 eV, which is •− − O2 probe was generated in situ by the 1 e photoreduction almost identical to the value measured from superoxide EPR. of O2, which readily binds to Lewis acidic Zr-centers by In comparison, ZrOH-BTC only shifted the NMA emission fl λ Δ displacing the weakly coordinating tri ates to form the EPR- max to 467.0 nm, with a calculated E of 0.89 eV. Identical

14880 DOI: 10.1021/jacs.9b07891 J. Am. Chem. Soc. 2019, 141, 14878−14888 Journal of the American Chemical Society Article

μ η2 μ Figure 3. (a) DFT optimized structures of ZrOTf-BTC with Zr2( 2-OTf) and Zr( -OTf) coordination modes. The 2-model is more stable than η2 · −1 the -model by 24.85 kcal mol per Zr6O4(OH)4(OTf)6 node. (b) The sulfur XANES spectra of ZrOTf-BTC, sodium thiosulfate, and Sc(OTf)3 fl μ fi indicate the presence of tri ate groups in ZrOTf-BTC. (c) Fitting of ZrOTf-BTC EXAFS data using the 2-model; the R-factor for the tting is 0.007. fl fi NMA uorescence test with homogeneous Sc(OTf)3 also gave simply mix the MOF with inert ller can reduce the column a ΔE of 0.98 eV (Figure S8, Supporting Information). pressure, but organic substrates cannot easily access the Triflation of metal-hydroxides thus provides an effective interiors of such MOF particles due to the increased diffusion strategy to enhance MOF Lewis acidity and to generate barrier. To overcome this limitation, we have directly grown ff porous solid acids with Lewis acidity comparable to Sc(OTf)3. ZrOTf-BTC on silica to a ord the ZrOTf-BTC@SiO2 47−49 fi Synthesis and Characterization of ZrOTf-BTC@SiO2. composite material. Zr-BTC@SiO2 was rst prepared Although Lewis acids have broad applications in organic analogously to Zr-BTC except with the addition of 30 weight synthesis, little progress has been made in immobilizing Lewis equiv of SiO2. Surface silanol groups on SiO2 served as the fl 39,40 acids for ow synthesis. BF3 and Sc(OTf)3 are commonly nucleation sites for the formation of octahedron-shaped Zr- used to catalyze organic transformations, but they are difficult BTC particles of an average dimension of ∼50 nm (Figure S9, to incorporate into heterogeneous supports without sacrificing Supporting Information). Due to enhanced nucleation on silica acidity.41,42 As a result, most flow processes used Lewis acids in surface, the MOF particles in the composite material are stoichiometry, causing significant waste of catalysts and smaller than pure MOF particles and are densely packed on quenching reagents and corrosion of reaction vessels. Owing silica. to their tunable structures and high porosity, MOFs and Zr-BTC@SiO2 was treated with 1 M HCl to remove formate − fl related organic inorganic hybrid materials represent an capping groups and then tri ated with Me3SiOTf to form fl 43−46 ff attractive class of solid catalysts for ow catalysis. We ZrOTf-BTC@SiO2 as an o -white powder. TEM imaging ff fi − sought to utilize ZrOTf-BTC as an e ective xed-bed Lewis showed that ZrOTf-BTC@SiO2 possessed core shell struc- acid catalyst to achieve flow catalysis of a broad scope of tures with the MOF particles displaying high crystallinity and reactions. octahedral morphology (Figure 5c, 5d). SEM imaging also Due to the tight packing of MOF particles and the slow showed a dense coating of ZrOTf-BTC particles on the silica substrate diffusion rate through the nanosized MOF channels, surface (Figure S13, Supporting Information). ZrOTf-BTC@ packing pure MOFs into a column can lead to very high SiO2 exhibited the same PXRD pattern as Zr-BTC, indicating column pressure. Growing MOFs into much larger sizes or that the MOF in the composite maintained the identical

14881 DOI: 10.1021/jacs.9b07891 J. Am. Chem. Soc. 2019, 141, 14878−14888 Journal of the American Chemical Society Article

obvious change was observed in the MOF crystallinity by PXRD (Figure S17, Supporting Information), whereas only minimum Zr (0.13%) leached into the supernatant after each reaction run by ICP-MS analysis. When the catalyst loading was reduced to 0.1 mol %, the cyclization product was obtained in 76% yield in 10h, affording a TON of 760. The Diels−Alder reaction is significantly more challenging with electron-rich dienophiles such as α,β-unsaturated ketones and aldehydes due to their much higher LUMO energy. Impressively, at 5.0 mol % catalyst loading, ZrOTf-BTC catalyzed the cyclization of a series of challenging dienophiles, including 3-buten-2-one, methacrolein and crotonaldehyde, with both cyclohexa-1,3-diene and 2,3-dimethylbuta-1,3-diene in high yields (Table 1). The heterogeneity of ZrOTf-BTC in the Diels−Alder reaction was confirmed by the “hot filtration” test. After a reaction run, the MOF and supernatant were separated and separately used as catalyst for another reaction run. While the catalytic activity was maintained in the solid MOF, no activity was observed for the supernatant. This “hot filtration” test rules out the possibility of leached Zr salts or soluble Brönsted acids contributing to the catalytic reactivity (Figure S18, Supporting Figure 4. (a) Proposed structures for superoxide-coordinated ZrOH- Information). BTC (left) and ZrOTf-BTC (right). (b) EPR spectra of ZrOTf-BTC- − •− •− ff We then tested the Diels Alder reaction using the ZrOTf- (O2 ) (blue) and ZrOH-BDC-(O2 ) (red) show di erent gzz values fl ff BTC@SiO2 packed column as a continuous ow catalyst. A owing to di erent Lewis acidity of ZrOTf-BTC and ZrOH-BTC. The CH Cl solution of benzoquinone (limiting reagent) and EPR spectrum for ZrOTf-BTC is vertically shifted for clarity. (c) 2 2 Fluorescence spectra of ZrOTf-BTC-(NMA) (blue), ZrOH-BDC- cyclohexa-1,3-diene (0.5 M) in a molar ratio of 1:1.6 was fl · −1 (NMA) (red), and free NMA. owed through the column at a rate of 10 mL h to achieve complete conversion of benzoquinone and to produce the cyclization product in quantitative yield. The turnover fl structure to pristine MOF (Figure 5b). N2 sorption isotherms frequency (TOF) of the ow process was calculated to be 2 −1 fl of ZrOTf-BTC@SiO2 gave a BET surface area of 321 m /g. 100 h . The ow reaction was run for a TON of 1700 in 17 h The composite material showed a sharp N2 sorption at P/P0 < without a drop in the reaction yield (Figure 6). 0.2, which can be attributed to micropore filling of ZrOTf- ZrOTf-BTC Catalyzed Epoxide Ring-Opening Amina- − BTC, and a broad adsorption with hysteresis at P/P0 = 0.2 tion. Epoxides are an important class of industrial chemicals 0.9, which is attributable to mesopore sorption by silica for conversion into a broad range of commodity and fine particles. Pore size analysis by NL-DFT indicated the presence chemicals.51,52 For instance, the nucleophilic ring-opening with of a uniform micropore at 1.6 nm and a broad range of larger amines affords β-amino alcohols, which are useful intermedi- pores at 2−10 nm (Figure 5e, 5f). The Zr coordination ates for organic synthesis.40,53 Several homogeneous catalysts, μ fl fl environment was studied by EXAFS. By using the 2-tri ate including metal tri ates and metal halides have been used for μ μ 54−57 and Zr coordination sphere of Zr(OTf)2( 3-O)2( 3- this reaction. However, very few heterogeneous catalysts OH)2(RCO2)2, the Zr K-edge EXAFS data of ZrOTf-BTC- have been tested for this reaction to achieve catalyst reuse and fi fl SiO2 was well tted with an average Zr-OTf bond distance of ow catalysis. ZrOTf-BTC is a very active catalyst for epoxide 2.217 Å (Figure 5g, Table S4, Supporting Information). The ring-opening with anilines. At 5.0 mol % ZrOTf-BTC loading, ff Zr active sites in ZrOTf-BTC@SiO2 thus adopts identical many di erent epoxides, including styrene oxide, cyclohexene coordination environment as those in ZrOTf-BTC. The oxide and cyclopentene oxide, reacted with aniline to form ZrOTf-BTC@SiO2 powder was slurry-packed with CH2Cl2 corresponding amino alcohols (Table 2) without heating. A into a stainless-steel column for flow catalysis studies. The broad range of aniline derivatives, including electron-deficient amount of ZrOTf-BTC was quantified to be 40 μmol Zr per aniline (4-chloroaniline) and electron-rich aniline (2,4,6- gram by 1H NMR analysis of the digested material. trimethylaniline), and sterically hindered secondary anilines ZrOTf-BTC Catalyzed Diels−Alder Reactions. The (N-methylaniline) can all be used for the ring-opening of Diels−Alder reaction is a very efficient strategy for epoxides to afford amino alcohols in high yields (Table 2). constructing six-membered ring structures with regioselectivity The epoxide ring-opening reaction also worked well under and stereoselectivity.5,50 This reaction generally requires the flow conditions. Styrene oxide (limiting reagent) and aniline fl addition of an acid catalyst to reduce reaction temperature and solution in CH2Cl2 (0.5 M) were owed through ZrOTf- · −1 time. Unlike homogeneous Lewis acidic metal complexes, BTC@SiO2 column at a rate of 60 mL h to form the amino MOF-based Lewis acids offer significant advantages including alcohol product in 93% yield. High yields (83−93%) of easy separation from reaction mixtures and catalyst recovery/ product were consistently obtained in 30 runs to afford a total reuse. Moreover, uniform active sites of MOF catalysts TON of >2700 in 30 h (Figure 7). generally afford higher reaction selectivity. For the reaction ZrOTf-BTC Catalyzed Friedel−Crafts Acylation Reac- of 1,4-benzoquinone with cyclohexa-1,3-diene, the cyclization tions. As one of the most convenient and useful strategies for product was obtained in quantitative yield within 1 h at only the construction of aryl ketone moieties in a wide range of 1.0 mol % ZrOTf-BTC loading under room temperature. No pharmaceuticals and agricultural chemicals, Friedel−Crafts

14882 DOI: 10.1021/jacs.9b07891 J. Am. Chem. Soc. 2019, 141, 14878−14888 Journal of the American Chemical Society Article

Figure 5. (a) Schematic depiction of the synthesis of ZrOTf-BTC@SiO2 and its packing into a column reactor. (b) PXRD patterns of ZrOTf- BTC@SiO2 (magenta), ZrOH-BTC@SiO2 (blue), and ZrFA-BTC@SiO2 (red) compared to the simulated one for Zr-BTC, indicating the crystallinity of composite materials. (c) TEM image of a ZrOTf-BTC@SiO2 particle. (d) Zoomed-in view of the surface region of coated silica and ∼ the octahedral morphology of MOF particles of 50 nm in dimensions. (e) N2 sorption isotherms of ZrOTf-BTC@SiO2 display two steps of 2 sorption in the micropore and mesopore regions. The calculated BET surface area is 321 m /g for the ZrOTf-BTC@SiO2 composite. (f) Pore size − distributions of ZrOTf-BTC@SiO2 show a uniform micropore with a diameter of 16 Å and a series of mesopores in the range of 20 100 Å. (g) μ fi Fitting of ZrOTf-BTC@SiO2 EXAFS data using the 2-model structure of ZrOTf-BTC; the R-factor for the tting is 0.009. acylation has drawn continuous research interests.58 Conven- acetyl-2-methoxynaphthalene in 98% isolated yield. This level tional Lewis acids for Friedel−Crafts acylation include of activity is much higher than those of most Lewis acid fl traditional metal halides (e.g., ZnCl2,AlCl3, TiCl4) and catalysts including homogeneous metal tri ates and solid acid fl 59−61 metal tri ates (e.g., Sc(OTf)3, Hf(OTf)4). However, catalysts. Several substituted arenes, including anisole, due to the coordination between the Lewis acid and the dimethoxybenzene, mesitylene, and benzofuran, underwent produced aromatic ketone, stoichiometric amount of metal acylation in the presence of 1−5 mol % of ZrOTf-BTC at halides are usually required in these reactions. Furthermore, room temperature to afford desired products in 63−83% yields since the workup procedure typically requires aqueous (Table 3). Furthermore, 5.0 mol % ZrOTf-BTC catalyzed − treatment, the recovery of these homogeneous Lewis acidic Friedel Crafts acylation with benzoic anhydride in CH2Cl2 to metal salts and the generation of large amounts of wastes are afford 1-benzoyl-2-methoxynaphthalene in 81% yield. long-standing challenges. As a result, significant efforts have The catalytic performance of Friedel−Crafts acylation was been devoted to the development of heterogeneous Lewis acid also tested in the continuous flow mode. When a solution of 2- − fi 62,63 catalyst for Friedel Crafts acylation. Modi ed zeolites, methoxynaphthalene (0.05 M) in Ac2O/CH2Cl2 (1:4, v/v) 64 65 66,67 fl metal oxides, heteropoly acids, and hybrid materials was owed through ZrOTf-BTC@SiO2 column at a rate of 30 − have displayed moderate to good activity in Friedel−Crafts mL·h 1, 1-acetyl-2-methoxynaphthalene was formed as the acylation. desired product in excellent yields (85−99%) in the first 5 runs At 1.0 mol % of loading, ZrOTf-BTC catalyzed Friedel− (5 mL solution per run). The yield dropped to 65% in the six Crafts acylation between 2-methoxynaphthalene and neat run but the catalytic performance was restored by simply ff acetic anhydride in 2 h at room temperature to a ord 1- washing the column with a CH3CN/CH2Cl2 (1:9, v/v)

14883 DOI: 10.1021/jacs.9b07891 J. Am. Chem. Soc. 2019, 141, 14878−14888 Journal of the American Chemical Society Article

Table 1. Catalyst Evaluation and Substrate Scope of ZrOTf- Table 2. Catalyst Evaluation and Substrate Scope of ZrOTf- a a BTC Catalyzed Diels−Alder Reactions BTC Catalyzed Epoxide Ring-Opening Reactions

aReaction conditions: epoxides (1 equiv, 1.0 mmol), anilines (1.2 − equiv, 1.2 mmol), ZrOTf-BTC (1.0 5.0 mol %), CH2Cl2 (4.0 mL), 25 °C. Reaction yields were determined by 1H NMR using mesitylene aReaction conditions: dienophile (1 equiv, 1 mmol), diene (1.2 as internal standard. − ° equiv), ZrOTf-BTC (0.1 5.0 mol %), CH2Cl2 (4.0 mL), 25 C. Reaction yields were determined by 1H NMR using mesitylene as internal standard.

Figure 7. ZrOTf-BTC@SiO2 catalyzed epoxide ring-opening reaction with aniline in a continuous flow mode. − Figure 6. ZrOTf-BTC@SiO2 catalyzed Diels Alder reaction in a continuous flow mode. For the cyclization of 4-penten-1-ol, 2.0 mol % of ZrOTf- BTC afforded 2-methyltetrahydrofuran in quantitative yield after heating at 135 °C under inert atmosphere for 18 h (Table 4). 5-Hexen-1-ol was also hydroalkoxylated in 91% yield at 4.0 solution. Fifteen consecutive runs afforded a total TON of 326 mol % ZrOTf-BTC to afford 2-methyltetrahydropyran and 2- and a TOF of 130 h−1 (the column was washed after the 6th ethyltetrahydrofuran in a 3:1 ratio. Besides aliphatic alcohols, and 11th run). The catalyst performance in the flow mode alkene substrates containing phenol groups such as 2- significantly outperformed that of the batch mode (Figure 8). allylphenol and 2-allyl-6-methylphenol were also catalytically ZrOTf-BTC Catalyzed Alkene Hydroalkoxylation Re- cyclized with ZrOTf-BTC at a lower temperature of 100 °C. actions. ZrOTf-BTC is also a highly active catalyst for alkene Alkene-containing carboxylic acids such as pent-4-enoic acid hydroalkoxylation reactions. Oxygen-containing cyclic com- and hex-5-enoic acid were also hydroalkoxylated to form pounds are abundant in polyether antibiotics and other corresponding lactones in excellent yields (91−95%) at 1.0 biologically active natural products as well as in chemical mol % catalyst loading. feedstocks.68,69 The addition of alcohols across CC bonds is To further demonstrate the advantage of ZrOTf-BTC over 70,71 the most straightforward synthetic route to cyclic ethers. conventional Sc(OTf)3 in catalyzing alkene hydroalkoxylation This reaction pathway is widely adopted by biological systems reactions, we conducted catalytic hydroalkoxylation of pent-4- 72,73 to synthesize cyclic ether-containing natural products. enoic acid at 0.2 mol % loading of ZrOTf-BTC or Sc(OTf)3. Some homogeneous catalysts are effective for this reaction, but As shown in Figure 9, ZrOTf-BTC catalyzed the hydro- many of them require activated alkenes (e.g., alkenes, dienes, alkoxylation of pent-4-enoic acid in octane to afford 90% of γ- − and Michael acceptors).74 77 We report here the first example valerolactone in 72 h, while very little product was detected in of MOF-catalyzed alkene hydroalkoxylation using highly acidic the presence of Sc(OTf)3 under the same condition. The lack ZrOTf-BTC. of catalytic activity of Sc(OTf)3 is likely due to its poor

14884 DOI: 10.1021/jacs.9b07891 J. Am. Chem. Soc. 2019, 141, 14878−14888 Journal of the American Chemical Society Article

Table 3. ZrOTf-BTC Catalyzed Friedel−Crafts Acylation Table 4. ZrOTf-BTC Catalyzed Intramolecular Alkene a a Reactions Hydroalkoxylation

aReaction conditions: substrate (1.0 mmol), ZrOTf-BTC (1.0−4.0 aReaction conditions: arene (1 equiv, 1.0 mmol), acetic anhydride mol %), octane (2.0 mL), 100−135 °C, 18 h, inert atmosphere. (1.0 mL, excess) or benzoic anhydride (3 equiv, 3.0 mmol, in 2.0 mL Reaction yields were determined by 1H NMR using mesitylene as − ° CH2Cl2), ZrOTf-BTC (1.0 5.0 mol %), 25 C. Isolated yields are internal standard. listed.

− Figure 8. ZrOTf-BTC@SiO2 catalyzed Friedel Crafts acylation between 2-methoxynaphthalene and acetic anhydride in the continuous flow mode. Figure 9. Catalytic performance of ZrOTf-BTC and Sc(OTf)3 for alkene hydroalkoxylation. The catalyst loadings are both 0.2 mol %. solubility in nonpolar octane. Changing the solvent to polar MeNO afforded 2-methyltetrahydrofuran in 20% yield at 0.2 of organic transformations, including Diels−Alder reaction, 2 − mol % Sc(OTf)3 in 5 h. However, no further substrate epoxide ring-opening reaction, Friedel Crafts acylation, and conversion was observed beyond 5 h, suggesting the alkene hydroalkoxylation reactions. The catalytic performance deactivation of Sc(OTf)3 in polar solvent at elevated of ZrOTf-BTC is superior over Sc(OTf)3 in terms of catalyst temperature. ZrOTf-BTC thus shows much longer lifetime activity, lifetime, and reusability. We have further developed fl than Sc(OTf)3 in alkene hydroalkoxylation. Additionally, as a ZrOTf-BTC@SiO2 composite for continuous ow catalysis. heterogeneous catalyst, ZrOTf-BTC was easily recovered from ZrOTf-BTC@SiO2 displayed exceptionally high turnover the reaction mixture via centrifugation and reused at least 5 numbers (TONs) of 1600 for Diels−Alder reaction, 2700 for times without significant activity decrease (Figure S28, epoxide ring-opening reaction, and 326 for Friedel−Crafts Supporting Information). ZrOTf-BTC thus shows significant acylation under flow conditions. We have thus demonstrated advantage over traditional homogeneous Lewis acid catalysts in the creation of strongly Lewis acidic sites in MOFs via fl catalytic activity, lifetime, and catalyst recovery and reuse. tri ation and utility of MOF@SiO2 composite in continuous flow catalysis of important organic transformations. ■ CONCLUSION ■ ASSOCIATED CONTENT We have designed a strongly Lewis acidic MOF, ZrOTf-BTC, * through two-step SBU transformations of MOF-808. The S Supporting Information Lewis acidity of the Zr-triflate active site was quantified The Supporting Information is available free of charge on the through spectroscopic methods to be comparable to the ACS Publications website at DOI: 10.1021/jacs.9b07891. homogeneous benchmark Sc(OTf)3. ZrOTf-BTC was shown Synthesis and characterization of ZrOTf-BTC and to be a highly active solid Lewis acid catalyst for a broad range ZrOTf-BTC-SiO2procedures for DFT calculations;

14885 DOI: 10.1021/jacs.9b07891 J. Am. Chem. Soc. 2019, 141, 14878−14888 Journal of the American Chemical Society Article

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Single-Site Heterogeneous Catalysts for Olefin Pengfei Ji: 0000-0002-8109-7929 Polymerization Enabled by Cation Exchange in a Metal-Organic Xuanyu Feng: 0000-0003-1355-1445 Framework. J. Am. Chem. Soc. 2016, 138 (32), 10232−10237. Cheng Wang: 0000-0002-7906-8061 (16) Ji, P.; Feng, X.; Veroneau, S. S.; Song, Y.; Lin, W. Trivalent − Wenbin Lin: 0000-0001-7035-7759 Zirconium and Hafnium Metal Organic Frameworks for Catalytic 1,4-Dearomative Additions of Pyridines and Quinolines. J. Am. Chem. Author Contributions § Soc. 2017, 139 (44), 15600−15603. P. Ji and X. Feng contributed equally to this work. (17) Drake, T.; Ji, P.; Lin, W. Site Isolation in Metal−Organic Notes Frameworks Enables Novel Transition Metal Catalysis. Acc. Chem. − The authors declare no competing financial interest. Res. 2018, 51 (9), 2129 2138. (18) Feng, X.; Song, Y.; Li, Z.; Kaufmann, M.; Pi, Y.; Chen, J. S.; Xu, − ■ ACKNOWLEDGMENTS Z.; Li, Z.; Wang, C.; Lin, W. 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14888 DOI: 10.1021/jacs.9b07891 J. Am. Chem. Soc. 2019, 141, 14878−14888