Tuning Lewis Acidity of Metal–Organic Frameworks via Perfluorination of Bridging Ligands: Spectroscopic, Theoretical, and Catalytic Studies

Abstract

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 (O₂^•–) and fluorescence spectroscopy of MOF-bound <i>N</i>-methylacridone (NMA)and a simple strategy that significantly enhances MOF Lewis acidity through ligand perfluorination. Two new perfluorinated MOFs, Zr₆-fBDC and Zr₆-fBPDC, where H₂fBDC is 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid and H₂fBPDC 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 Zr₆-BDC-NO₂ and Zr₆-BPDC-(NO₂)₂. Zr₆-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