耐水性超強塩基ニオブ酸化物クラスターの研究がAngew. Chem. Int. Ed.に掲載されました．

Water-Tolerant Superbase Polyoxometalate [H₂(Nb₆O₁₉)]⁶⁻ for Homogeneous Catalysis

S. Kikkawa, Y. Fujiki, V. Chudatemiya, H. Nagakari, K. Shibusawa, J. Hirayama, N. Nakatani, S. Yamazoe

Angew. Chem. Int. Ed., e202401526 (2024).

Here, doubly protonated Lindqvist-type niobium oxide cluster [H₂(Nb₆O₁₉)]^6–, fabricated by microwave-assisted hydrothermal synthesis, exhibited superbase catalysis for Knoevenagel and crossed aldol condensation reactions accompanied by activating C−H bond with pK_a >26 and proton abstraction from a base indicator with pK_a=26.5. Surprisingly, [H₂(Nb₆O₁₉)]⁶⁻ exhibited water-tolerant superbase properties for Knoevenagel and crossed aldol condensation reactions in the presence of water, although it is well known that the strong basicity of metal oxides and organic superbase is typically lost by the adsorption of water. Density functional theory calculation revealed that the basic surface oxygens that share the corner of NbO₆ units in [H₂(Nb₆O₁₉)]⁸⁻ maintained the negative charges even after proton adsorption. This proton capacity and the presence of un-protonated basic sites led to the water tolerance of the superbase catalysis.