Synthesis of Hollowed Polyoxometalate with a Flipped VO₅ Unit by the Elimination of a Centered Organic Molecule

Mechanistic understanding of the formation of clusters plays a role in designing the structure-dependent properties. Based on the fact that anions act as templates to form spherical polyoxovanadates, various structures were reported by changing anions in the synthetic solution. In this work, another factor in the formation of spherical polyoxometalates was demonstrated. By the reaction of [V₁₀O₂₆]^4- in acetonitrile with a reductant to increase the number of tetravalent V⁴⁺ and p-toluene sulfonic acid to convert tetrahedral VO₄ units to square-pyramidal VO₅, acetonitrile-containing polyoxovanadate [V₂₄O₆₀(CH₃CN)]^6- (I^CH3CN) was synthesized. The bulky and hydrophobic aromatic rings prevented the formed anions from acting as a template. By changing the synthetic solvent, encapsulated moieties were controlled. Nitromethane was also encapsulated to afford [V₂₄O₆₀(CH₃NO₂)]^6- (I^CH3NO2). When acetone was used as the solvent, the contaminated water was encapsulated to form [V₂₄O₆₀(H₂O)]^6- (I^H2O). The encapsulated acetonitrile molecule was eliminated by heating I^CH3CN up to 230 °C under N₂ flow conditions to give hollowed polyoxovanadate [V₂₄O₆₀]^6- (II), even though I^CH3CN possesses no pores for acetonitrile to pass. From the X-ray crystallographic analysis of II, one of the 24 VO₅ units was flipped. The electrochemical properties and catalytic performances between I^CH3CN and II were also investigated.