Tungsten complexes oxidation

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

Epoxidation systems based on molybdenum and tungsten catalysts have been extensively studied for more than 40 years. The typical catalysts - MoVI-oxo or WVI-oxo species - do, however, behave rather differently, depending on whether anionic or neutral complexes are employed. Whereas the anionic catalysts, especially the use of tungstates under phase-transfer conditions, are able to activate aqueous hydrogen peroxide efficiently for the formation of epoxides, neutral molybdenum or tungsten complexes do react with hydrogen peroxide, but better selectivities are often achieved with organic hydroperoxides (e.g., TBHP) as terminal oxidants [44, 45],... 

The reaction of alkynyllithium compounds with alkoxycarbene tungsten complexes leads to anionic propargyl tungsten complexes (Figure 2.33 see also Figure 3.9). These intermediates are stable at low temperatures and react upon Lewis acid catalysis with aldehydes or A -sulfonyl imines to yield five-membered heterocycles [280]. Oxidative methoxycarbonylation [375] of the intermediate vinyl tungsten complex, followed by elimination of methanol leads to pyrroles or furanes (Figure 2.33 Entry 6, Table 2.22). 

Tungsten is a weak reducing agent ( ow/wo3 = —0.09 V), which can be oxidized into WOj, W2O5, or WO3. Passivation is achieved in aqueous solutions by WO3 which is stable at pH lower than 4 except in the presence of HF, which forms fluorinated tungsten complexes. Never-therless corrosion is very low in diluted ammonia. 

Catalytic activities of [Ru PW (H30)(0)3, ] -/03/CH3CN/80°C and of [Ru PMOjj(H30)(0)3, ] "/03/CH3CN were compared. The tungsten complex did not catalyse the aerobic oxidation of cumene and 1-octene to cumyl alcohol and 1-octene oxide while the Mo analogue did so the tungsten complex underwent structural change with to an inert form, while its molybdenum analogue did not... 

Group 4, surface chemistry on oxides, 12, 515 Group 5, surface chemistry on oxides, 12, 524 molybdenum, surface chemistry on oxides, 12, 529 into nickel complexes, 8, 59-60 with palladium, GO insertion, 8, 200 tungsten, surface chemistry on oxides, 12, 531 on zinc compounds, 2, 366 Alkoxido-imido tungsten complexes structure and properties, 5, 754—755 synthesis, 5, 750/... 

The chemical transport of molybdenum and tungsten as oxides or sulphides in the presence of iodine has been shown to proceed by the formation of the corresponding M02I2 complex.510... 

Only one nitrido Ww complex has been documented. This was prepared by the action of Me3SiN3 followed by oxidation on dinitrogen W° complex 83 (Eq. (31)) [36]. It has been proposed that the reaction of 83 with trimethylsilylazide affords a Wn complex bearing two N3-ligands a subsequent two-electron oxidation of the metal center then affords nitrido tungsten complex 84. 

---