Self-Assembly of a U(VI)-Containing Polytungstate Tetramer with Lewis Acid-Base Catalytic Activity for a Dehydration Condensation Reaction.

A novel U(VI)-containing polytungstate (U-POW) tetramer, {K 1.37 Na 26.63 [K 2 (UO 2 ) 4 Cl 0.5 (OH) 5.5 (γ-SiW 10 O 36 ) 4 ]}·ca66H 2 O ( U 4 ), was synthesized using the Keggin-type precursor [γ-SiW 10 O 36 ] 8- and UO 2 (NO 3 ) 2 . U 4 was further characterized by single-crystal X-ray diffraction, FT-IR, Raman spectroscopy, solid-state diffuse reflection spectroscopy, ICP-OES, ESI-MS, TGA, and PXRD. The central {K 2 (UO 2 ) 4 Cl 0.5 (OH) 5.5 } chromophore was constructed dexterously from four uranyl, four halves of K ions, 5.5 bridging μ 2 -OH, and disordered Cl ions, and was further stabilized by four {γ-SiW 10 } moieties to construct the tetramer [K 2 (UO 2 ) 4 Cl 0.5 (OH) 5.5 (γ-SiW 10 O 36 ) 4 ] 28- . Notably, U 4 could work as an effective bifunctional Lewis acid-base catalyst for the synthesis of pyrazoles via the condensation of hydrazines with 1,3-diketones under mild conditions, which is attributed to the synergetic effect of the Lewis acidity of U(VI) and the Lewis basicity of {γ-SiW 10 }.