High-field pulsed EPR spectroscopy for the speciation of the reduced [PV(2)Mo(10)O(40)](6-) polyoxometalate catalyst used in electron-transfer oxidations

Abstract

An in-depth spectroscopic EPR investigation of a key intermediate, formally notated as [PV(IV)V(V)Mo(10)O(40)](6-) and formed in known electron-transfer and electron-transfer/oxygen-transfer reactions catalyzed by H(5)PV(2)Mo(10)O(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 (95)Mo and (17)O hyperfine interactions, and electron-nuclear double resonance (ENDOR), which gave the weak (51)V and (31)P interactions. In this way, two paramagnetic species related to [PV(IV)V(V)Mo(10)O(40)](6-) were identified. The first species (30-35 %) has a vanadyl (VO(2+))-like EPR spectrum and is not situated within the polyoxometalate cluster. Here the VO(2+) was suggested to be supported on the Keggin cluster and can be represented as an ion pair, [PV(V)Mo(10)O(39)](8-)[V(IV)O(2+)]. This species originates from the parent H(5)PV(2)Mo(10)O(40) in which the vanadium atoms are nearest neighbors and it is suggested that this isomer is more likely to be reactive in electron-transfer/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 H(5)PV(2)Mo(10)O(40) isomers to yield an intact cluster, [PV(IV)V(V)Mo(10)O(40)](6-).