A Tetranuclear Thorium-Containing Hexameric Antimonotungstate: Synthesis, Structure, and Catalysis of the Knoevenagel Condensation

A novel hexameric thorium-antimony-tungstate compound, [NH₂(CH₃)₂]₁₆Na₁₀-{[(SbW₉O₃₂)(OH)(SbW₉O₃₁)(OH)₂(SbW₉O₃₃)(Th₂WSb₉O₁₃)(OH)₄(H₂O)]₂}·57H₂O (Th4Sb24), has been synthesized. It represents the first tetranuclear thorium-containing polyoxometalates (POMs) cluster, consisting of a [{(Th₂WSb₉O₂₉)(OH)₅(H₂O)}₂]²²⁻ ({Th₄Sb₁₈W₂}) heterometallic cluster and six {SbW₉} units, and it exhibits the highest Th and Sb nuclearity in POM chemistry. Th4Sb24 serves as an efficient heterogeneous catalyst for the Knoevenagel condensation under mild conditions, achieving 93% yield, a turnover number (TON) of 1860, and a turnover frequency (TOF) of 3720 h⁻¹, while showing good recyclability and scalability. Mechanistic studies indicate a synergistic effect where the Lewis-acidic Th centers activate benzaldehyde, while the {SbW₉} units facilitate proton abstraction. This work offers a viable strategy for designing high-nuclearity actinide POMs with practical catalytic utility.