Vanadium redox chemistry

The vanadium redox chemistry involves protons, since we have the equilibria... 

Dominant forms of V(V) are H2VOT and F1V04 at pH 7, analogous to those of P(V). However, V(V) species are more labile and relatively easily interconvertable. Vanadium also has a rich redox chemistry which is missing with phosphorus. 

The standard aqueous redox chemistry of vanadium and the other group 5 elements is summarized in the Latimer diagrams shown in Fig. 1 [2]. Under standard acidic aqueous conditions, the stability of the -1-5 oxidation state increases for the heavier group 5 elements at the expense of the +4 and -L3 states. 

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... 

Redox chemistry of vanadium-catechol systems is complicated References 256, 497 and 499-508 discuss this subject in detail. In complexes, the metal centre may be in the +5, +4, +3 (and +2) formal oxidation state and quinones complex in three localized electronic forms ... 

A wide range of oxidation states are known for all of the elements of this group, but only vanadium has an extensive redox chemistry. 

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-... 

A ubiquitous characteristic of vanadium chemistry is the fact that vanadium and many of its complexes readily enter into redox reactions. Adjustment of pH, concentration, and even temperature have often been employed in order to extend or maintain system integrity of a specific oxidation state. On the other hand, deliberate attempts to use redox properties, particularly in catalytic reactions, have been highly successful. Vanadium redox has also been successfully utilized in development of a redox battery. This battery employs the V(V)/V(IV) and V(III)AT(II) redox couples in 2.5 M sulfuric acid as the positive and negative half-cell electrolytes, respectively. Scheme 12.2 gives a representation of the battery. The vanadium components in both redox cells are prepared from vanadium pentoxide. There are two charge-discharge reactions occurring in the vanadium redox cells, as indicated in Equation 12.1 and Equation 12.2. The thermodynamics of the redox reactions involved have been extensively studied [8],... 

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. 

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... 

The chemistry of vanadium is characterized by multiple oxidation states (Fig. 1). The redox chemistry of this metal undoubtedly plays a role in its biochemistry. Of the four common oxidation states, only V(III), V(IV), and V(V) are important biologically, V(II) being too reducing to exist in any known organism. The best known example of the occurrence of V(III) is in the vanadocytes of the blood of tunicates22 otherwise, vanadium is largely found in the +4 and +5 oxidation states, both of which are readily accessible under physiological conditions. 

Figure 3.166 shows the reactivity and potential of these early transition metal pincer carbene complexes taking vanadium as an example. The broad range of compounds becomes possible owing to the rich redox chemistry of vanadium that makes different oxidation states easily accessible. Here, examples for vanadium(ll), vanadium(III) and vanadium(IV) are shown. Abstraction of halogen is facile making the introduction of different TT-donor ligands possible, even weak ones like acetonitrile. 

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... 

It appears to be well established by now that the reductant responsible for the conversion of vanadate(V) to oxovanadium(IV) is nicotine adenine dinucleotide phosphate (NADPH), produced in the pentose phosphate pathway.PI Further reduction to in the vacuoles of the vanadocytes of Phlebobranchia ascidians remains elusive. Several reductants have been proposed and shown to be effective in the in vitro and/or in vivo reduction of to V and/or to including tunichromes. Selected investigations of the related redox chemistry of vanadium can be briefly summarised as follows ... 

Catechols (and pyrogallols) readily reduce vanadium(V) to vanadium(IV) and, in some instances, further to vanadium(III). In the context of tunichromes as the presumed reducing agents in ascidians, the redox chemistry of catecholatovanadium complexes has been investigated to some extent. Results on reduction potentials for the and... 

VIV. As with the titanium porphyrins, these complexes occur as vanadyl complexes with an axial oxo group and produce a "normal electronic spectrum which changes to a hematin spectrum when trifluoroacetic acid is added [Bonnett (9)]. The vanadyl porphyrins contain a single unpaired electron (/i=l,4B.M.) and their esr spectra have been studied in some detail [Kivelson (113a), Roberts (152)]. The rich redox chemistry of the inorganic vanadium salts is not reflected in the vanadyl porphyrins, where the +4 state seems to be exceptionally stable. 

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