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

摘要

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 P-31 ENDOR (electron-nuclear double resonance) measurements indicated the absence of a phosphorous heteroatom leading to the suggestion that (H5VVMo11O40)-V-V-Mo-IV exists as a previously unrecognized impurity in the typically synthesized H5PV2Mo10O40 compound. (H5/4PVO4VMo11O36)-O-V-Mo-IV/V 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 [(VVMo11O40)-V-V-Mo-IV](5-) as [(VO4VMo11O36)-O-V-Mo-IV](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. (c) 2006 Elsevier B.V. All rights reserved.