Vanadium complexes electron-transfer

The experimental observations were interpreted by assuming that the redox cycle starts with the formation of a complex between the catalyst and the substrate. This species undergoes intramolecular two-electron transfer and produces vanadium(II) and the quinone form of adrenaline. The organic intermediate rearranges into leucoadrenochrome which is oxidized to the final product also in a two-electron redox step. The +2 oxidation state of vanadium is stabilized by complex formation with the substrate. Subsequent reactions include the autoxidation of the V(II) complex to the product as well as the formation of aVOV4+ intermediate which is reoxidized to V02+ by dioxygen. These reactions also produce H2O2. The model also takes into account the rapidly established equilibria between different vanadium-substrate complexes which react with 02 at different rates. The concentration and pH dependencies of the reaction rate provided evidence for the formation of a V(C-RH)3 complex in which the formal oxidation state of vanadium is +4. 

Vanadium(iv) Complexes.—Aqueous electron-transfer reactions involving V as a reductant have been reviewed. 

Mixed-valence species containing V11 and Viv bridged by an oxygen atom were intermediate in electron transfer between [V(Hedta)]- and [VO(Hedta)], and [V(edta)]2 and [VO(edta)]2-.815 Similarly bridged species were assumed in the reaction of [V(H20)6]2+ with [V0(H20)5]2+. Vanadium(III) dimeric complexes are the reaction products (see Section 33.4.9.3). 

Krylov (62) studied the adsorption of oxygen and propylene on vanadium oxide/MgO and molybdenum oxide/MgO catalysts by ESR and IR at 25°C. He observed the formation of Qr radicals and ir-allyl complexes during the simultaneous adsorption of 02 and C3H . The data indicated that an electron transfer took place from the olefin to the oxygen through the transition metal ion forming the following complex ... 

A molecular conversion system based on a four-electron transfer to 02 was accomplished in the 02-oxidative polymerization of diphenyl disulfide (Figure 14) [116]. This is the first example of a multielectron mediator that is applied to molecular conversion systems. The multielectron transfer process from the reduced vanadium(III) complex (VOV+) to 02 not only revealed the 02 oxidation mechanism but also provided additional insight into the unique chemistry of vanadium with possible relevance to metal mono-oxygenases. 

Tables 1 and 2 gives the numerical data for a series of vanadium (II), chromium (III), manganese (IV), molybdenum (III), rhenium (IV), iridium (VI), cobalt (II), and nickel (II) complexes. The first spin-allowed absorption band, caused by an internal transition in the partly filled shell, has the wavenumber equal to A. If spin-forbidden transitions are superposed on this band, a certain distortion from the usual shape of Gaussian error curve can be observed, and one takes the centre of gravity of intensity as the corrected wavenumber ai. One has to be careful not to confuse electron transfer or other strong bands with the internal transitions discussed here. Obviously, one has also to watch for absorption due to other coloured species, produced e. g. by oxidation or hydrolysis of the solutions. In the case of certain octahedral nickel (II), and nearly all tetrahedral cobalt (II) complexes, the first band has not actually been...

The complex [V(terpy)2]l2 may be isolated as a green air-sensitive solid from aqueous ethanolic solution (2S8). Taube and co-workers have investigated the electron-transfer reactions of [V(terpy)2], prepared by the direct interaction of terpy with aqueous vanadium(ll) solutions (49). 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

From comparison of antitumor activity and toxicity of hetero-ligand vanadium(V) complexes, Djordjevic and Wampler (79) arrived at the conclusion that the hetero-ligand is able to affect the redox potential of the V(V)/V(IV) couple in such a way that intramolecular electron transfer can occur within the V(V)-peroxoadduct. As a consequence, vanadium(V) is reduced to the IV state, and the peroxo group is oxidized to a superoxide radical. It is conceivable that such a species is present also during the reaction of vanadium bromoperoxidase with H2O2. However, there is no evidence for a radical type of reaction with bro-moperoxidases. 

Vanadium(V) complexes of I -oxa-4,7-dia/acyclononane-/V,/V -diacetate were prepared and undergo one-electron transfer to form the corresponding vanadium(IV) complexes.434... 

In both cases, the cobalt containing product is the aqua complex because H2O is present in abundance, and high-spin d complexes of Co(II) are substitution labile. However, something that distinguishes the two pathways is the composition of the vanadium-containing product. If [V(N3)(OH2)s] is the product, then the reaction has proceeded via an inner-sphere pathway. If [V(OH2)6] " is the product, then the electron-transfer reaction is outer-sphere. The complex [V(N3)(OH2)5] is inert enough to be experimentally observed before the water molecule displaces the azide anion to give [V(OH2)6]. ... 

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