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 H5VVVIVMo11O40 exists as a previously unrecognized impurity in the typically synthesized H5PV2Mo10O40 compound. H5/4PVVO4VIV/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 [VVVIVMo11O40]5− as [VVO4VIVMo11O36]5−. Accordingly, the polyoxometalate has a VO43- 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.

A previously unknown impurity formed in the synthesis of the known α-[PV2Mo10O40]5− polyoxometalate has been identified as α-[VVO4VIVMo11O36]5− with a VO43- heteroatom core and with 11 molybdenum addenda and one VO2+ moiety at the polyoxometalate surface by a combination of X-ray crystallography, infrared and mass spectrometry and EPR/ENDOR/ESEEM techniques. The catalytic activity of H5VVO4VIVMo11O36 was identical to that of H5PV2Mo10O40.Figure optionsDownload as PowerPoint slide