Structural and EPR/ENDOR/ESEEM spectroscopic investigations of a vanadomolybdate Keggin-type polyoxometalate in organic solvent

An EPR spectrum of as synthesized [G.A. Tsigdinos, C.J. Hallada, Inorg. Chem. 7 (1968) 437-441], orange colored, H₅PV₂Mo₁₀O₄₀ polyoxometalate showed the presence of a reduced vanadium(IV) addenda atom. Surprisingly, further ³¹P ENDOR (electron-nuclear double resonance) measurements indicated the absence of a phosphorous heteroatom leading to the suggestion that H₅V^VV^IVMo₁₁O₄₀ exists as a previously unrecognized impurity in the typically synthesized H₅PV₂Mo₁₀O₄₀ compound. H_5/4PV^VO₄V^IV/VMo₁₁O₃₆ was then synthesized in low yield (0.8 mol%) by omitting the addition of phosphate in a typical H₅PV₂Mo₁₀O₄₀ preparation. The molecular formulation and structure was supported by X-ray crystallography, infrared and mass spectrometry. Further use of EPR/ENDOR/ESEEM (electron-spin echo envelope modulation) allowed the formulation of [V^VV^IVMo₁₁O₄₀]⁵⁻ as [V^VO₄V^IVMo₁₁O₃₆]⁵⁻. Accordingly, the polyoxometalate has a heteroatom core with 11 molybdenum addenda and one VO²⁺ moiety at the polyoxometalate surface. The redox potential and the catalytic activity of the new vanadomolybdate polyoxometalate compound were essentially identical to the often-studied H₅PV₂Mo₁₀O₄₀ polyoxometalate isomeric mixture.