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-).

The H(5)PV(2)Mo(10)O(40) polyoxometalate and Pd/Al(2)O(3) were used as co-catalysts under anaerobic conditions for the activation and oxidation of CO to CO(2) by an electron transfer-oxygen transfer mechanism. Upon anaerobic reduction of H(5)PV(2)Mo(10)O(40) with CO in the presence of Pd(0) two paramagnetic species were observed and characterized by continuous wave electron paramagnetic resonance (CW-EPR) and hyperfine sublevel correlation (HYSCORE) spectroscopic measurements. Major species I (65-70%) is assigned to a species resembling a vanadyl cation that is supported on the polyoxometalate and showed a bonding interaction with (13)CO. Minor species II (30-35%) is attributed to a reduced species where the vanadium(IV) atom is incorporated in the polyoxometalate framework but slightly distanced from the phosphate core. Under aerobic conditions, CO/O(2), a nucleophilic oxidant was formed as elucidated by oxidation of thianthrene oxide as a probe substrate. Oxidation reactions performed on terminal alkenes such as 1-octene yielded a complicated mixture of products that was, however, clearly a result of alkene epoxidation followed by subsequent reactions of the intermediate epoxide. The significant competing reaction was a hydrocarbonylation reaction that yielded a approximately 1:1 mixture of linear/branched carboxylic acids.

No abstract

Aerobic Catalysis. -Two paramagnetic species are observed in the anaerobic oxidation of CO using H5PV2Mo10O40 and Pd/Al2O3 as co-catalysts. One of these species resembles a vanadyl cation and shows a bonding interaction with CO. Oxygen transfer from the polyoxometalate results in the formation of CO2. Under aerobic conditions in the presence of terminal alkenes such as 1-octene, epoxidation and hydrocarbonylation is observed. -(GOLDBERG, H.; KAMINKER, I.; GOLDFARB, D.; NEUMANN*, R.; Inorg. Chem. 48 (2009) 16, 7947-7952; Dep. Org. Chem., Weizmann Inst. Sci., Rehovot 76100, Israel; Eng.) -Schramke 43-014