High-field pulsed EPR spectroscopy for the speciation of the reduced [PV₂MO₁₀O₄₀]^6- polyoxometalate catalyst used in electron-transfer oxidations

An in-depth spectroscopic EPR investigation of a key intermediate, formally notated as [PV^IVV^VMo₁₀O₄₀] ^6- and formed in known electron-transfer and electrontransfer/oxygen- transfer reactions catalyzed by H₅PV₂Mo₁₀O ₄₀, has been carried out. Pulsed EPR spectroscopy have been utilized: specifically, W-band electron-electron double resonance (ELDOR)-detected NMR and two-dimensional (2D) hyperfine sub-level correlation (HYSCORE) measurements, which resolved ⁹⁵Mo and ¹⁷O hyperfine interactions, and electron-nuclear double resonance (ENDOR), which gave the weak ⁵¹V and ³¹P interactions. In this way, two paramagnetic species related to [PV^IVV^VMo₁₀O₄₀]^6- were identified. The first species (30-35%) has a vanadyl (VO²⁺)-like EPR spectrum and is not situated within the polyoxometalate cluster. Here the VO²⁺ was suggested to be supported on the Keggin cluster and can be represented as an ion pair, [PV^VMo₁₀O₃₉] ^8--[V^IVO²⁺]. This species originates from the parent H₅PV₂Mo₁₀O₄₀ in which the vanadium atoms are nearest neighbors and it is suggested that this isomer is more likely to be reactive in electrontransfer/oxygen-transfer reaction oxidation reactions. In the second (70-65 %) species, the V^IV remains embedded within the polyoxometalate framework and originates from reduction of distal H₅PV₂Mo₁₀O₄₀ isomers to yield an intact cluster, [PV^IVV^VMo₁₀O ₄₀]^6-.