High-Nuclearity Polyoxometalate-Based Metal-Organic Frameworks for Photocatalytic Oxidative Cleavage of C-C Bond

High-nuclearity polyoxometalate (POM) clusters are attractive building blocks (BBs) for the synthesis of metal-organic frameworks (MOFs) due to their high connectivity and inherently multiple metal centers as functional sites. This work demonstrates a strategy of step-wise growth on ring-shaped [P₈W₄₈O₁₈₄]^40- precursor, which produced two new high-nuclearity polyoxotungstates, a half-closed [H₁₆P₈W₅₈O₂₁₈]^32- {W₅₈} and a fully-closed [H₁₆P₈W₆₄O₂₃₆]^32- {W₆₄}. By in situ synthesis, unique MOFs of copper triazole-benzoic acid (HL) complexes incorporating the negatively-charged {W₅₈} and {W₆₄} as nodes, {Cu₁₁(HL)₉W₅₈} HNPOMOF-1 and {Cu₉(HL)₉W₆₄} HNPOMOF-2, were constructed by delicately tuning the reaction conditions, mainly solution pH, which controls the formation of {W₅₈} and {W₆₄}, and at the same time the protonation of triazole-benzoic acid ligand thus its coordination mode to copper ion that creates the highest nuclearity POM-derived MOFs reported to date. HNPOMOF-1 features 3D framework possessing cage-like cavities filled with exposed carboxyl groups, while the inherent 2D layer-like HNPOMOF-2 allows for facile exfoliation into ultrathin nanosheets, and the resulted HNPOMOF-2_NS exhibits superior activity towards photocatalytic oxidative cleavage of C-C bond for a series of lignin models. This work not only provides a strategy to build high-nuclearity POM cluster-based frameworks, but also demonstrates their great potential as functional materials for green catalysis.