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

Article abstract of DOI:10.1021/jacs.8b05765

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 N-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.