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, {K1.37Na26.63[K₂(UO₂)₄Cl0.5(OH)5.5(γ-SiW₁₀O₃₆)₄]}·ca66H₂O (U4), was synthesized using the Keggin-type precursor [γ-SiW₁₀O₃₆]⁸⁻ and UO₂(NO₃)₂. U4 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 {K₂(UO₂)₄Cl0.5(OH)5.5} chromophore was constructed dexterously from four uranyl, four halves of K ions, 5.5 bridging μ₂-OH, and disordered Cl ions, and was further stabilized by four {γ-SiW₁₀} moieties to construct the tetramer [K₂(UO₂)₄Cl0.5(OH)5.5(γ-SiW₁₀O₃₆)₄]²⁸⁻. Notably, U4 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-SiW₁₀}.