Vanadyl cation

Vanadium occurs in soils predominantly as the +5 vanadate species (VO(OH)3°, V02(0H)2 and V03(0H) ) and under reducing conditions as the +4 vanadyl cation (VO +). Less commonly V + may also form and substitute for Fe in minerals. Interchange between these oxidation states with redox conditions greatly alters the solubility of V in soils. 

The use of a special microtechnique has permitted the precipitation, weighing, and determination of about 2 /xg of technetium with a standard deviation of 0.08 /xg . The precipitate is filtered, washed with ice-cold water, dried at 110 °C and weighed as (CgH5) AsTcO. Permanganate, perchlorate, periodate, iodide, fluoride, bromide, thiocyanate anions and mercury, bismuth, lead, silver, tin and vanadyl cations as well as nitrate concentrations above 0.5 M interfere with the determination. 

The complex is normally described as the oxochromium(IV) [(H20)5Cr=0]2+ or chromyl cation, by analogy with the vanadyl cation [(H20)5Viv0]2+. It can also be regarded as a chromium(III) substituted hydroxyl radical (Fig. 1) and its H-atom abstracting properties are parallel to those of -OH (15). 

Analysis of vanadium-loaded model materials (such as EuY, amorphous aluminosilicate gels and EuY-gel mixtures) by electron paramagnetic resonance (EPR) has provided information concerning metal oxidation state and stereochemistry (67). EPR data has indicated that when vanadyl cations are introduced in the form of vanadyl naphthenate, they were stabilized in a zeolite with the faujasite structure as pseudo-octahedral V02+ even after calcination at 540°C. Upon steaming, these V02+ cations were then converted almost entirely to V+5 species (67). The formation of EuV04 was verified but the concentration of this vanadate was never proportional to the total rare-earth content of the zeolite. In EuY-gel mixtures the gel preferentially sorbed vanadium where it was stabilized mainly in the form of V205. 

Vanadate transport in the erythrocyte was shown to occur via facilitated diffusion in erythrocyte membranes and was inhibited by 4,4 -diisothiocyanostilbene-2,2 -disulfonic acid (DIDS), a specific inhibitor of the band 3 anion transport protein [23], Vanadium is also believed to enter cells as the vanadyl ion, presumably through cationic facilitated diffusion systems. The divalent metal transporter 1 protein (called DMT1, and also known as Nramp2), which carries iron into cells in the gastrointestinal system and out of endosomes in the transferrin cycle [24], has been proposed to also transport the vanadyl cation. In animal systems, specific transport protein systems facilitate the transport of vanadium across membranes into the cell and between cellular compartments, whereas the transport of vanadium through fluids in the organism occurs via binding to proteins that may not be specific to vanadium. 

Moreover, no adsorption band near 600 nm typical of VO ions is observed in the spectra of the two as-made samples, although vanadyl cations in a square pyramidal geometry have been detected by ESR in V-MCM-41. This is probably due to the fact that d-d transitions in VO are generally 10-30 times weaker than those of CT transitions and therefore are undetectable by diffuse reflectance UV-vis [34]. However, a small band at ca 605 nm has been detected by Chao et al. [24] in the as-synthesized V-MCM-41 samples having Si/V= 30, This band is responsible for the pale violet colour of the as-made sample. Nevertheless, our as-made samples are white in colour, suggesting that VO species should occur, if any, in very small concentration... 

Several Vlv materials form via complexation with different protonation states of phosphate481 84 and pyrophosphate485,486 and have been characterized in the solid state. Corresponding studies with Viv hypophosphites487 and arsenates488 490 have also been reported. Imidophosphinato complexes with the vanadyl cation are characterized in the solid state and in solution.491 The... 

Bidentate ligands containing OS donors, particularly in conjunction with carboxylate moieties, form strong complexes at pH values near the p/y, value for the thiol.584,602,687-689 The reduction of Viv to V111 by cysteine methyl ester in water has been found to be enhanced in the presence of amino polycarboxylates.690 The ligand 2-mercapto-4-methylphenol forms a square-pyramidal five-coordinate complex with the vanadyl cation (150) the reported crystal structure is of the cis isomer.691 The bidentate ligand 2-aminocyclopent-l-ene-l-carbodithioic acid forms non-oxo-Viv complexes based on spectroscopic studies and chemical precedence, an eight-coordinate vanadium atom is inferred.692... 

Vanabins, storage proteins for the vanadyl cation found in ascidians, and containing up to 20 VO + attached to amine terminals of lysines (section 4.1.2 and Figure 4.4). A vanadium(IV)-binding protein was also isolated from the bacterium Pseudomonas isachenkovii (Section 4.5) which utilises vanadate as terminal electron acceptor in anaerobic respiration, and appears as well to be present in the hydrogen bacterium Xanthobacter autotrophicus. ... 

Fig. 12. ESR spectra of isolated vanadyl cations (IV)) introduced into H-ZSM-5 (Si/Al 35) by solid-state reaction with VoOk at 1025 K Ufter Ref. [31], wiui permission).

Normal-type spectra are shown by metals of the d- or f-block where the metal d- or f-electrons are of such low energy that they do not interact significantly with the porphyrin ir-electrons for example, the vanadyl cation, (d ) and the europium cation, Eu(III) (f ). 

Henry and Van Lierde developed a three-compartment cell equipped with an reticulated vitreous carbon cathode mixed with ion-exchange resin and two Ti—Pt anodes separated by a Daramic diaphragm to selectively separate the vanadium from molybdenum (Henry Van Lierde, 1998). In this designed membrane reactor, the pentavalent vanadyl anion was selectively desorbed with respect to molybdenum after its reduction to a tetravalent vanadyl cation. Consequently, 93% vanadium recovery yields were obtained. 

The formation of complexes between the vanadyl cation and malonate, oxalate, L-tartrate, and racemic tartrate has been studied by the pressure-jump technique. Three relaxation times were found for the first two systems in the case of malonate they were attributed to the formation of the complexes [(VOOHlattial], [VO(mal)], and [VOfmaljg] -, whereas in the case of oxalate they were attributed to the formation of [VO(ox)] and the two isomeric complexes [VO(ox)2] j and [VO(ox)2]. The two relaxation times for the tartrate systems were ascribed to the formation of [VO(tar)] or [VO(tarH)]+ and [VO(tar)]2. It was also found that the formation of the dimer takes place stereospecifically such that no mixed complex (VO-L-tar-VO-D-tar) is formed. 

Fig. 53. Schematic representation of the close proximity of a vanadyl cation to framework Al in a zeolite structure giving rise to the super-hyperfme splitting of the ESR signal shown in Fig. 52 (after [191], with permission)...

An attempt was made to co-introduce Cu into the above-characterized VO(OH),H-ZSM-5 via SSIE by calcining a mixture of Cu compounds and the V-containing zeolite [93]. This, however, led to a decrease in the VO(OH)+ concentration as was indicated by a decrease in the intensity of the signals related to vanadyl species (cf. Fig. 54) and to the appearance of the ESR signal of isolated Cu+ in ZSM-5 (vide supra. Sect. 5.3.2.1). Obviously, Cu + is more strongly held in the zeolite structure than VO(OH)+ and is able to easily replace the vanadyl cation. 

Finally, it is worth mentioning that, in a zeolite in which Al had been isomor-phously substituted by Ga, i.e., in H-[Ga[ZSM-5, vanadyl cations could not be... 

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