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.

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