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 was further treated with trimethylsilyl triflate (Me 3 SiOTf) to afford ZrOTf-BTC by taking advantage of the oxophilicity of the Me 3 Si group. Electron paramagnetic resonance spectra of Zr-bound superoxide and fluorescence spectra of Zr-bound Nmethylacridone provided a quantitative measurement of Lewis acidity of ZrOTf-BTC with an energy splitting (ΔE) of 0.99 eV between the π x * and π 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 ZrOTf-BTC@SiO 2 composite as an efficient solid Lewis acid catalyst for continuous flow catalysis. The Zr centers in ZrOTf-BTC@SiO 2 feature identical coordination environment to ZrOTf-BTC based on spectroscopic evidence. ZrOTf-BTC@SiO 2 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 via triflation and constructed the MOF@SiO 2 composite for continuous flow catalysis of important organic transformations.

The Lewis acidity of metal-organic frameworks (MOFs) has attracted much research interest in recent years. We report here the development of two quantitative methods for determining the Lewis acidity of MOFs-based on electron paramagnetic resonance (EPR) spectroscopy of MOF-bound superoxide (O) and fluorescence spectroscopy of MOF-bound N-methylacridone (NMA)-and a simple strategy that significantly enhances MOF Lewis acidity through ligand perfluorination. Two new perfluorinated MOFs, Zr-fBDC and Zr-fBPDC, where HfBDC is 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid and HfBPDC is 2,2',3,3',5,5',6,6'-octafluoro-4,4'-biphenyldicarboxylic acid, were shown to be significantly more Lewis acidic than nonsubstituted UiO-66 and UiO-67 as well as the nitrated MOFs Zr-BDC-NO and Zr-BPDC-(NO). Zr-fBDC was shown to be a highly active single-site solid Lewis acid catalyst for Diels-Alder and arene C-H iodination reactions. Thus, this work establishes the important role of ligand perfluorination in enhancing MOF Lewis acidity and the potential of designing highly Lewis acidic MOFs for fine chemical synthesis.

The intrinsic heterogeneity of alumina (Al 2 O 3 ) surface presents a challenge for the development of aluminasupported single-site heterogeneous catalysts and hinders the characterization of catalytic species at the molecular level as well as the elucidation of mechanistic details of the catalytic reactions. Here we report the use of aluminum hydroxide secondary building units (SBUs) in the MIL-53(Al) metal− organic framework (MOF) with the formula Al(μ 2 −OH)-(BDC) (BDC = 1,4-benzenedicarboxylate) as a uniform and structurally defined functional mimic of Al 2 O 3 surface for supporting Earth-abundant metal (EAM) catalysts. The μ 2 − OH groups in MIL-53(Al) SBUs were readily deprotonated and metalated with CoCl 2 and FeCl 2 to afford MIL-53(Al)-CoCl and MIL-53(Al)-FeCl precatalysts which were characterized by powder X-ray diffraction, nitrogen sorption, elemental analysis, density functional theory, and extended X-ray fine structure spectroscopy. Activation with NaBEt 3 H converted MIL-53(Al)-CoCl to MIL-53(Al)-CoH which effectively catalyzed hydroboration of alkynes and nitriles and hydrosilylation of esters. X-ray photoelectron spectroscopy and X-ray absorption nearedge spectroscopy (XANES) indicated the presence of Al III and Co II centers in MIL-53(Al)-CoH while deuterium labeling studies suggested σ-bond metathesis as a key step for the MIL-53(Al)-CoH-catalyzed addition reactions. MIL-53(Al)-FeCl competently catalyzed oxidative Csp 3 −H amination and Wacker-type alkene oxidation. XANES analysis revealed the oxidation of Fe II to Fe III centers in the activated MIL-53(Al)-FeCl catalyst and suggested that oxidative Csp 3 −H amination occurs via the formation of Fe III −O t Bu species by single electron transfer between Fe II centers in MIL-53(Al)-FeCl and ( t BuO) 2 with concomitant generation of 1 equiv of t BuO• radical, C−H activation through hydrogen atom abstraction to generate alkyl radicals, protonation of Fe III −O t Bu by aniline to generate MIL-53(Al)-Fe III -anilide, and finally C−N coupling between the Fe IIIanilide and alkyl radical to form the Csp 3 −H amination product and regenerate the Fe II catalyst. These highly active single-site MOF-based solid catalysts were readily recovered and reused up to five times without significant decrease in catalytic activity. This work thus demonstrates the great potential of using the aluminum hydroxide SBUs in MOFs to support EAM catalysts for important organic transformations.