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.