Vanadate ions, reactions

It has been suggested that an increase in the coordination number of vanadium from 4 to 5 already takes place in the second protonation step, i.e. when [H2V04] is formed (21). For reactions (1) and (2), however, the protonation constants and thermodynamic parameters are comparable with those reported for P04 and As04 , providing firm evidence that reaction (2) is not accompanied by incorporation of water in the vanadate ion (15, 17). Further, the estimated thermodynamic quantities for reaction (6), AH° = -39 kJ/mol and AS0 = —51 J/(mol K), obtained by extrapolation from the experimental values for reactions (1) and (2) and those for the three protonation steps of P04 and As04 , are not typical of a simple protonation reaction (17). For such a reaction the entropy change is normally a positive quantity often amounting to 100 50 J/(mol K) and the enthalpy... 

This enzyme [EC 3.4.21.53], also known as endopepti-dase La, ATP-dependent serine proteinase, and ATP-dependent protease La, catalyzes the hydrolysis of peptide bonds in large proteins (for example, globin, casein, and denaturated serum albumin) in the presence of ATP (which is hydrolyzed to ADP and orthophosphate). Vanadate ion inhibits both reactions. A similar enzyme occurs in animal mitochondria. Protease La belongs to the peptidase family S16. 

A triple complex is formed between V-(a-pyridyl)-2-thioquinaldamide (120), vanadate ions and H2O2 that becomes fluorescent in acidic solution (Aex = 340 nm, An = 490 nm). This reaction, originally developed for determination of V(V) in a FIA system, can be adapted for determination of H2O2 and organic peroxides. LOD for H2O2 is 0.05 p,M, with RSD 2.4% (ri = 3) at 1 p,M, with linearity in the 0.2 to 50 p,M range330. 

A systematic study to identify solid oxide catalysts for the oxidation of methane to methanol resulted in the development of a Ga203—M0O3 mixed metal oxide catalyst showing an increased methanol yield compared with the homogeneous gas-phase reaction.1080,1081 Fe-ZSM-5 after proper activation (pretreatment under vacuum at 800-900°C and activation with N20 at 250°C) shows high activity in the formation of methanol at 20°C.1082 Density functional theory studies were conducted for the reaction pathway of the methane to methanol conversion by first-row transition-metal monoxide cations (MO+).1083 These are key to the mechanistic aspects in methane hydroxylation, and CuO+ was found to be a likely excellent mediator for the reaction. A mixture of vanadate ions and pyrazine-2-carboxylic acid efficiently catalyzes the oxidation of methane with 02 and H202 to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde.1084 1085... 

Reactions of vanadate ions To study these reactions use a 0.1m solution of ammonium metavanadate, NH4VO3, or sodium metavanadate, NaV03. The addition of some sulphuric acid keeps these solutions stable. 

Since vanadium chloroperoxidase from Curvularia inaequalis is structurally closely related to acid phosphatases and transition metal oxoanions are potent inhibitors of the related phytases, (Figure 10.8) [35,36], Sheldon and coworkers investigated the peroxidase activity of phytase from Aspergillus ficuum in the presence of sodium orthovanadate Na3V04 [37-39]. Oxidation of thioanisole with H2O2 proceeded to produce the sulfoxide in quantitative yield in the presence of [VO4]-phytase. The reaction rate showed saturation kinetics with respect to the vanadate concentration, indicating a maximum rate of 120pmol/h (TOF = 11 min ) and a dissociation constant for the vanadate ion of 15.4 pM. 

A vanadate ion can react with itself through the redox reaction... 

An example of an equilibrium potential of a solution when it contains an ampholyte is provided by the reaction of the hypovanadous ion V + with the vanadyl ion VO + to give the vanadate ion V + according to... 

Reduced oxide soldering activation is another method being evaluated [161,162]. Oxides of Sn, Pb, and Cu are reduced by placing the component in a solution of vanadous ion under a blanket of nitrogen. The half-cell reactions are ... 

In a slightly different method, hydrazinium metavanadate (a brown amorphous solid) is prepared by the addition of solid ammonium metavanadate (NH4VO3) to hydrazine hydrate at 0°C using an ice-salt mixture as this reaction is highly exothermic [8]. The compound formed decomposes at room temperature and so a lower temperature is always maintained during its preparation. In addition, the vanadate ion (VO3 ) is known to oxidize hydrazine, but this is considerably suppressed if the reaction is conducted at a low temperature. Furthermore, it is extremely difficult to recrystallize hydrazinium metavanadate as it is insoluble in water. 

Vanadium(IV) Oxide. Vanadium(IV) oxide (vanadium dioxide, VO2) is a blue-black solid, having a distorted mtile (Ti02) stmcture. It can be prepared from the reaction of V20 at the melting point with sulfur or carbonaceous reductants such as sugar or oxaUc acid. The dioxide slowly oxidizes in air. Vanadium dioxide dissolves in acids to give the stable (VO) " ions and in hot alkaUes to yield vanadate(IV) species, eg, (HV20 ) . 

H. 8-Hydroxyquinaldine (XI). The reactions of 8-hydroxyquinaldine are, in general, similar to 8-hydroxyquinoline described under (C) above, but unlike the latter it does not produce an insoluble complex with aluminium. In acetic acid-acetate solution precipitates are formed with bismuth, cadmium, copper, iron(II) and iron(III), chromium, manganese, nickel, silver, zinc, titanium (Ti02 + ), molybdate, tungstate, and vanadate. The same ions are precipitated in ammoniacal solution with the exception of molybdate, tungstate, and vanadate, but with the addition of lead, calcium, strontium, and magnesium aluminium is not precipitated, but tartrate must be added to prevent the separation of aluminium hydroxide. 

The lower oxidation states are stabilized by soft ligands e.g. CO (Prob. 3). The aquated vanadium ions represent an interesting series of oxidation states. They are all stable with respect to disproportionation and labile towards substitution. They undergo a number of redox reactions with one another, all of which have been studied kinetically. Many of the reactions are [H ]-dependent. There has been recent interest in the biological aspects of vanadium since the discovery that vanadate can mimic phosphate and act as a potent inhibitor (Prob. 4). 

Extrapolating from well-characterized enzymatic inhibition in test tubes, numerous mechanistic ideas concerning the in vivo effects of vanadium compounds have been advanced. The effects of vanadium compounds as transition-state analogs of certain enzymes with a phosphoprotein intermediate in their reaction scheme is proposed to account for the action of vanadium [11] in many biological systems. Unfortunately, it is often difficult to determine if the inhibition observed in the test tube occurs in vivo. For example, although vanadate is a potent inhibitor of plasma membrane ion pumps (such as the sodium potassium ATPase) in the test tube, it is difficult to determine if these pumps are actually inhibited in animals exposed to vanadium compounds. Currently, the role of vanadium compounds as protein phosphatase (PTP) inhibitors is believed to be related to the metabolic effects of this... 

The active sites of these enzymes can have a nitrogen ligand, usually as histidine (acid phosphatases and some protein phosphatases), a nucleophilic serine residue (alkaline phosphatases), a cysteine residue in which the thiol group can form a covalent species with the phosphate ester (protein phosphatases), or an aspartate-linked phosphate (plasma membrane ion pumps). The inhibitory form of vanadium is usually anionic vanadate V(V), but cationic vanadyl V(IV) has also shown strong inhibition of some types of phosphorylase reactions. Above neutral pH, speciation of vanadyl ions produces anionic V(IV) species capable of inhibition of enzymes in the traditional transition-state analogue manner [5],... 

Operating below -900 K ensures the presence of Reactions 8.1 to 8.4 s active ions in the molten catalyst. Above -900 K, these active ions transform to inactive vanadates (V043 ) causing S02 oxidation to cease (Rasmussen, 2001). Also, above 900 K, the molten catalyst and solid substrate tend to irreversibly form a viscous inactive liquid. 

Chromates, vanadates, and cerium salts give colour reactions with the reagent and should therefore be absent. Iron salts give a yellow colour with hydrogen peroxide, but this is eliminated by the addition of syrupy phosphoric acid. Fluorides bleach the colour (stable [TiF6]2 ions are formed), and large amounts of nitrates, chlorides, bromides, and acetates as well as coloured ions... 

The reactions discussed entail coordination of simple aquo ions such as Cu2+, Mg2+, etc. Metal-containing oxo-anions such as VO, MoO2 and WO2 also form stable complexes, especially with cis-diols13. These reactions are quite rapid vanadate, for example, has bimolecular complex formation rate constants on the order of 104 M 1 s"114). 

Cr(II) may be used to carry out all the reactions of Ti(III), but usually under milder conditions. Applications of Cr(II) as a reductant have been reviewed. The applications include Sn(IV) chloride in the presence of catalysts such as Sb(V) or Bi(III), Sb(V) in 20% HCl at elevated temperatures, Cu(II), silver, gold, mercury, bismuth, iron, cobalt, molybdenum, tungsten, uranium, dichromate, vanadate, titanium, thallium, hydrogen peroxide, oxygen in water and gases, as well as organic compounds such as azo, nitro, and nitroso compounds and quinones. Excess Cr(II) in sulfuric acid solution reduces nitrate to ammonium ion. The reduction is catalyzed by Ti(IV), which is rapidly reduced to Ti(III). 

---