Vanadium 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). 

Benzene-Based Catalyst Technology. The catalyst used for the conversion of ben2ene to maleic anhydride consists of supported vanadium oxide [11099-11-9]. The support is an inert oxide such as kieselguhr, alumina [1344-28-17, or sUica, and is of low surface area (142). Supports with higher surface area adversely affect conversion of benzene to maleic anhydride. The conversion of benzene to maleic anhydride is a less complex oxidation than the conversion of butane, so higher catalyst selectivities are obtained. The vanadium oxide on the surface of the support is often modified with molybdenum oxides. There is approximately 70% vanadium oxide and 30% molybdenum oxide [11098-99-0] in the active phase for these fixed-bed catalysts (143). The molybdenum oxide is thought to form either a soUd solution or compound oxide with the vanadium oxide and result in a more active catalyst (142). 

Photooxidafions are also iudustriaHy significant. A widely used treatment for removal of thiols from petroleum distillates is air iu the presence of sulfonated phthalocyanines (cobalt or vanadium complexes). Studies of this photoreaction (53) with the analogous ziuc phthalocyanine show a facile photooxidation of thiols, and the rate is enhanced further by cationic surfactants. For the perfume iudustry, rose oxide is produced iu low toimage quantifies by singlet oxygen oxidation of citroneUol (54). Rose bengal is the photosensitizer. 

We refrain here from giving an extensive overview of studies on the surface structure of vanadium oxide nanolayers, as this has already been done for up to year 2003 in our recent review [97]. Instead, we would like to focus on prototypical examples, selected from the V-oxide-Rh(l 1 1) phase diagram, which demonstrate the power of STM measurements, when combined with state-of-the-art DFT calculations, to resolve complex oxide nanostructures. Other examples will highlight the usefulness of combining STM and STS data on a local scale, as well as data from STM measurements, and sample area-averaging spectroscopic techniques, such as XPS and NEXAFS, to derive as complete a picture as possible of the investigated system. 

These dimeric complexes involve, in their neutral state, two metal atoms in the (III) oxidation state. In the vanadium complexes such as [CpV(bdt)]2 and [CpV(tft)]2, the V—V bond length, 2.54 A in [CpV(bdt)]2, are shorter than observed in model complexes with a single V—V bond, indicating a partial double-bond character, also confirmed by a measured magnetic moment of 0.6 fiB in [CpV(tfd)]2, lower than expected if the two remaining unpaired electrons contribute to the magnetic susceptibility [20, 49]. This class of complexes most probably deserves deeper attention in order to understand their exact electronic structure. 

It was discovered nearly 20 years ago that V(V) as vanadate and V(IV) as vanadyl can mimic some of the effects of insulin (stimulate glucose uptake and oxidation and glycogen synthesis) (512, 513). Vanadate is an effective insulin mimetic in the diabetic rat (514), but has proved to be too toxic for human use. Vanadyl, as VOS04, is also unsuitable because high doses are needed on account of its poor oral absorption. Vanadium complexes with organic ligands have proved to be less toxic and can have improved aqueous solubility and lipophil-icity. 

Halides (chloride, in particular) also react promptly with surface OH groups, as has been shown for [W(=CCMe3)Cl3(dme)] and several inorganic oxides (silica, alumina, silica-alumina, niobia) [5-7]. The same was observed for the reaction of [V(=0)Cl3] with silica in this case, using a large excess of the vanadium complex, one mole of HCl is released per mole of grafted vanadium [8]. 

Newer types of the dinuclear vanadium(IV) complex catalysts 84 have been developed. The abovementioned dinuclear vanadium complexes possess a VO V linkage whereas the ESR study on the catalyst 84 revealed no V—O—V linkage. The sense of enantioselection by the catalyst 84 of the (R,5,5)-structure is opposite to that of the binuclear complex 78a of the same (R,5,5)-structure. These results suggested two active sites attached to the binaphthyl skeleton in the catalyst 84 performed the dual activation of 2-naphthols in the oxidative couphng to achieve high enantioselectivity ... 

Reports have appeared claiming that triperoxo vanadates behave as nucleophilic oxidants. In particular, triperoxo vanadium complexes, A[V(02)3]3H20 (A=Na or K), are proposed as efficient oxidants of a,-unsaturated ketones to the corresponding epoxide, benzonitrile to benzamide and benzil to benzoic acid, reactions which are usually carried out with alkaline hydrogen peroxide. Subsequent studies concerning the oxidation of cyclobutanone to 4-hydroxybutanoic acid, carried out with the above-cited triperoxo vanadium compound, in alcohol/water mixtures, clearly indicated that such a complex does not act as nucleophilic oxidant, but only as a source of HOO anion. 

Alkoxo monoperoxo vanadium complexes are also efficient radical oxidants of the alcoholic function again with a radical chain mechanism whose details are indicated in Scheme 16 in the case of 2-propanol. Similar radical peroxo species have been indi- ... 

Shul pin and coworkers have demonstrated, in several papers, that other peroxo vanadium complexes closely related to 36, containing in the coordination sphere amino acids, nitrogen-containing bases or weak carboxylic acids, are effective oxidants of satnrated and aromatic hydrocarbons. An accnrate account containing this work, together with results related to the use of other transition metals, has appeared and aU the relevant literature can be found there. 

This chapter primarily covers studies of the electrochemistry of vanadium, which have appeared in the literature during the period 1985-2005. The material is organized on the basis of the oxidation state of the starting complex and is not meant to be an exhaustive review. Cyclic voltammetry (CV) studies outnumber other methods of electrochemistry for the study of vanadium complexes, and redox potentials will be reported as referenced to the Cp2Pe /+ couple in the appropriate solvent except as noted [1]. 

Complex vanadium-phosphorus-oxide catalysts are the most successful industrial catalysts for the selective oxidation of /i-butane to maleic anhydride (MA) with uses in tetrahydrofurans (THE) and polyurethane intermediates. A schematic diagram of the reaction is shown in figure 3.21(a). These catalysts have been studied extensively (e.g. Centi et al 1993, Bordes 1987). In the selective catalysation of a-butane to MA, the best active phase in the V-P-0 system is identified as the vanadyl pyrophosphate, (VO)2P207 (hereafter... 

ENANTIOSELECTIVE OXIDATIVE COUPLING OF 2-NAPHTHOLS CATALYZED BY A NOVEL CHIRAL VANADIUM COMPLEX... 

CATALYTIC OXIDATIVE COUPLING OF 7-ALKOXY-l-NAPHTHOLS BY CHIRAL VANADIUM COMPLEXES... 

An interesting series of vanadium complexes whose formal oxidation number is 4+ has been reported by Davison et al. (75). They have investigated the solution spectra of the ions of general formula V(S2C2R2)3, where R = —C6H5, —CF3, —CN. They find g 1.98 and /f 60xl0-4 cm-1. They do not, however, consider these complexes to have one unpaired spin on the vanadium but rather consider the complex to be V(III) plus a radical anion with the two electrons in vanadium orbitals being paired up. 

Table 2 Oxidation States and Stereochemistries of Vanadium Complexes...

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