Vanadium redox with complexes

This book does not follow a chronological sequence but rather builds up in a hierarchy of complexity. Some basic principles of 51V NMR spectroscopy are discussed this is followed by a description of the self-condensation reactions of vanadate itself. The reactions with simple monodentate ligands are then described, and this proceeds to more complicated systems such as diols, -hydroxy acids, amino acids, peptides, and so on. Aspects of this sequence are later revisited but with interest now directed toward the influence of ligand electronic properties on coordination and reactivity. The influences of ligands, particularly those of hydrogen peroxide and hydroxyl amine, on heteroligand reactivity are compared and contrasted. There is a brief discussion of the vanadium-dependent haloperoxidases and model systems. There is also some discussion of vanadium in the environment and of some technological applications. Because vanadium pollution is inextricably linked to vanadium(V) chemistry, some discussion of vanadium as a pollutant is provided. This book provides only a very brief discussion of vanadium oxidation states other than V(V) and also does not discuss vanadium redox activity, except in a peripheral manner where required. It does, however, briefly cover the catalytic reactions of peroxovanadates and haloperoxidases model compounds. 

Hiatt et a/.34a-d studied the decomposition of solutions of tert-butyl hydroperoxide in chlorobenzene at 25°C in the presence of catalytic amounts of cobalt, iron, cerium, vanadium, and lead complexes. The time required for complete decomposition of the hydroperoxide varied from a few minutes for cobalt carboxylates to several days for lead naphthenate. The products consisted of approximately 86% tert-butyl alcohol, 12% di-fe/T-butyl peroxide, and 93% oxygen, and were independent of the catalysts. A radical-induced chain decomposition of the usual type,135 initiated by a redox decomposition of the hydroperoxide, was postulated to explain these results. When reactions were carried out in alkane solvents (RH), shorter kinetic chain lengths and lower yields of oxygen and di-te/T-butyl peroxide were observed due to competing hydrogen transfer of rm-butoxy radicals with the solvent. 

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. 

The Vra complex with L-cysteine was prepared and characterized the crystal structure shows a 1 2 stoichiometry and that the two S-donors are trans to one another while the O- and N-donors are cis to one another (194).810 With three different donor functionalities, this 1 2 complex illustrates key aspects of Vm coordination chemistry. To identify the mode of action of cellular vanadium species and the roles of cysteine and glutathione in cellular redox chemistry, the redox and complexation chemistry of vanadium with sulfur-containing ligands must be better... 

The action of redox metal promoters with MEKP appears to be highly specific. Cobalt salts appear to be a unique component of commercial redox systems, although vanadium appears to provide similar activity with MEKP. Cobalt activity can be supplemented by potassium and 2inc naphthenates in systems requiring low cured resin color lithium and lead naphthenates also act in a similar role. Quaternary ammonium salts (14) and tertiary amines accelerate the reaction rate of redox catalyst systems. The tertiary amines form beneficial complexes with the cobalt promoters, faciUtating the transition to the lower oxidation state. Copper naphthenate exerts a unique influence over cure rate in redox systems and is used widely to delay cure and reduce exotherm development during the cross-linking reaction. 

The Lo-Cat process, Hcensed by US Filter Company, and Dow/Shell s SulFerox process are additional Hquid redox processes. These processes have replaced the vanadium oxidizing agents used in the Stretford process with iron. Organic chelating compounds are used to provide water-soluble organometaHic complexes in the solution. As in the case of Stretford units, the solution is regenerated by contact with air. 

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. 

The great recent development in electrochemical techniques will certainly be helpful for the study of redox processes of a metal which can occur in so many oxidation states. Multinuclear NMR spectrometers will allow increased use of 51V resonance as a routine method for the characterization of complexes in solution. Other recent developments are the study of polynuclear complexes, metal clusters (homo and hetero-nuclear) and mixed valence complexes, and it can be anticipated that these topics will soon become important areas of vanadium coordination chemistry, although the isolation of compounds with such complex... 

The charge at the vanadium in the usual dithiolene complexes may be estimated49 as less than 2, and these complexes are therefore included in the section on low valence states. Various tris(dithiolene) complexes have been isolated and one-electron oxireductions studied by polarographic and voltammetric techniques. Table 3 summarizes methods of preparation and electrochemical behaviour. Schrauzer and co-workers50 correlated electrochemical data with Taft s constants the observed linear correlation reflected the ligand n orbital origin of the orbitals involved in the redox process. 

The insulin-enhancing activity of vanadium compounds is likely to be related to their interactions with cellular redox chemistry and ROS formation, in addition to direct inhibition of PTP-1B and other protein phosphatases as a transition-state analogue [100], Differences in the effects of V (III, IV or V)-dipicolinic acid complexes on blood glucose and absorption of V into serum after chronic oral admin-... 

The peroxo complexes of vanadium have not, by comparison with the other three elements (Ti, W, Mo) cited, been extensively employed for oxygen transfer reactions. The ease of the redox step vanadium(V) to vanadium(IV) introduces a mixture of two-electron and one-electron character into vanadium peroxo chemistry, which in the case of alkene epoxidation leads to side reactions of the substrate and products.81... 

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