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.