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

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.