New polyoxomolybdate compounds synthesized in situ using ionic liquid1-butyl-3-methyl-imidazolium tetrafluoroborate as green solvent

Abstract

Herein we report the facile, green syntheses of three new polyoxomolybdate-based inorganic–organic hybrid materials using room temperature ionic liquid (RTIL), 1-butyl-3-methyl-imidazolium tetrafluoroborate (bmim)[BF₄], as a green and reactive solvent. The organic imidazolium component of the RTIL was incorporated into all three structures, the μ₅-oxo octamolybdate cluster compound (bmim)₃NH₄[Mo₈O₂₆] (1) and two Keggin-type cluster compounds, one being a charge transfer salt (bmim)₄[PMo^VMo₁₁O₄₀] (2) and the other having the unreduced anion (bmim)₃[PMo₁₂O₄₀] (3). Phase pure and highly crystalline samples were obtained. In 1 the Mo₈O₂₆ moiety is a tetranionic cluster in its β phase. Compound 1 contains three bmim cations and an NH₄⁺ molecule to complement the −4 charge on the octamolybdate anion. The α-phase Keggin-type anion in 2, [PMo^VMo₁₁O₄₀]⁴⁻, contains one Mo atom in the +5 oxidation state, indicating that 2 is a charge transfer complex. The α-phase Keggin-type anion in 3, [PMo₁₂O₄₀]³⁻, has 12 fully-oxidized Mo atoms. Compound 3 has a band gap ∼3.5 eV. The catalytic nature of compound 3 in the oxidation of styrene to benzaldehyde was investigated. The maximum styrene conversion was 83%, while the maximum selectivity to benzaldehyde was 96.5%. The catalyst was successfully used for five cycles without significant loss in activity or selectivity. The structure of the catalyst remains unchanged after repeated use. Our work points to the feasibility of generating a wide variety of new and useful POM-based compounds through a ‘green’ synthesis route.