Exploring the Role of Hexanuclear Clusters as Lewis Acidic Sites in Isostructural Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have been extensively investigated as Lewis acidic catalysts for a variety of reactions. However, the identity and nature of the MOF nodes in the catalysis remain ill-defined. Herein, a series of isostructural MOFs (M-NU-1008), with M being hexanuclear clusters of transition metals (Zr and Hf), a lanthanide (Ce), or an actinide (Th), were successfully synthesized and evaluated as Lewis acid catalysts, and CO₂ cycloaddition of styrene oxide was used as a test reaction. Superior catalytic activity was observed with Ce-NU-1008 over the other M₆-based MOFs. In addition to the different Lewis acidities among the metals in the four M-NU-1008, the dissociation of terminal water molecules from the M₆ clusters before exposing the Lewis acidic metal sites was found to vary between the MOFs, as supported by variable-temperature diffuse reflectance infrared Fourier transform spectroscopy studies. In the MOFs where water dissociation from the metal nodes readily occurs, the catalytic activity is higher than the MOFs where water is bound more strongly. Upon further investigation, Hf-NU-1008-dehy with terminal water partially removed was found to exhibit enhanced activity compared to the as-prepared catalyst.