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- (ICH3CN) 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- (ICH3NO2). When acetone was used as the solvent, the contaminated water was encapsulated to form [V24O60(H2O)]6- (IH2O). The encapsulated acetonitrile molecule was eliminated by heating ICH3CN up to 230 °C under N2 flow conditions to give hollowed polyoxovanadate [V24O60]6- (II), even though ICH3CN 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 ICH3CN and II were also investigated.

Reactivity control of nitrate-incorporating octadecavanadates by changing the oxidation state and metal substitution

Clarification and control of the active sites at the atomic/molecular level are important to develop nanocatalysts. The catalytic performance of two oxidation states of nitrate-incorporating octadecavanadates, [V18O46(NO3)]5- (V18) and [V18O46(NO3)]4- (V18ox), and a copper-substituted one, [Cu2V16O44(NO3)]5- (Cu2V16), in selective oxidation was investigated. Both V18 and V18ox possessed the same double-helical structures and one of two tetravalent vanadium sites of V18 was oxidized in V18ox. The comparison of the mobility of the incorporated nitrate reveals that tetravalent vanadium centres show stronger interaction with the incorporated anions than pentavalent ones. The oxidation reaction with V18ox proceeded more smoothly with tert-BuOOH as an oxidant than that with V18. The reactivity and selectivity of the oxidation of 2-cyclohexen-1-ol were different among the derivatives. V18ox showed the higher reactivity with 72% selectivity to epoxide. With V18, reactivity was lower but higher selectivity to epoxide was achieved. In the presence of Cu2V16, 2-cyclohexen-1-one was selectively obtained with 81% selectivity. The order of the reactivity for cyclooctene was V18ox, V18 and Cu2V16. These results shows that the cap part of the double-helix acts as the active site. Even though the vanadium-oxygen species exhibit the same structures, the catalytic properties can be controlled by changing the valence of vanadium and metal substitution.

Recent advances in oxidative catalytic applications of polyoxovanadate-based inorganic-organic hybrids

Polyoxovanadates (POVs) have received widespread attention in catalytic applications due to their various structures and remarkable redox properties. By introducing a second transition metal, POV-based inorganic-organic hybrid (POVH) catalysts show increasing stability and more catalytic active sites compared with pure POVs. In this perspective article, POVH materials as oxidative catalysts have been classified into two main categories according to the interactions between transition metal-complex units and POV clusters: (i) hybrids with metal-organic units act as isolated cations and (ii) hybrids with an organic ligand coordinate to the second transition metal, which is further linked to a POV cluster via oxygen bridges directly or indirectly to give zero-, one-, two- or three-dimensional supramolecular structures. The oxidative conversion of organic compounds, including thiophene derivatives, thioethers, alkanes, alcohols, and alkenes, and oxidative detoxification of a sulfur mustard simulant or degradation of lignin, along with the oxidative photo/electrocatalytic transformation of organic compounds catalyzed by POVH materials, are discussed in detail. Furthermore, the challenges and prospects toward the development of POVH catalysts are explored briefly from our perspectives.