Strongly Lewis Acidic Metal–Organic Frameworks for Continuous Flow Catalysis

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 was further treated with trimethylsilyl triflate (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 <i>N</i>-methylacridone provided a quantitative measurement of Lewis acidity of ZrOTf-BTC with an energy splitting (Δ<i>E</i>) of 0.99 eV between the π_<i>x</i>* and π_<i>y</i>* 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@SiO₂ composite as an efficient solid Lewis acid catalyst for continuous flow 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 via triflation and constructed the MOF@SiO₂ composite for continuous flow catalysis of important organic transformations.