Vanadium compounds solution structures

Conte, V., O. Bortolini, M. Carraro, and S. Moro. 2000. Models for the active site of vanadium-dependent haloperoxidases Insight into the solution structure of peroxo-vanadium compounds. J. Inorg. Biochem. 80 41 -9. 

The chemistry already described is reproduced by numerous ligands that have not specifically been addressed in the previous discussion. The V-salicylidenehydrazides (Scheme 4.18a) and related compounds provide a good example. The structure [76] of a typical complex, represented in Scheme 4.18b, is not very different from that proposed for the solution structures of dipeptide complexes (Scheme 4.17). Interestingly, other similar complexes, based on Schiff base-derived ligands, form dimeric [VO]2 core complexes (Scheme 4.1) via two long ( 2.4 A) VO bonds [2], The cyclic core is not necessary for dimer formation, and a dimer can form via a linear VOV bond [77], These complexes otherwise are not significantly different in their vanadium coordination from that depicted in Scheme 4.18b. 

Vanadium, a typical transition element, displays well-characterized valence states of 2—5 in solid compounds and in solutions. Valence states of — 1 and 0 may occur in solid compounds, eg, the carbonyl and certain complexes. In oxidation state 5, vanadium is diamagnetic and forms colorless, pale yellow, or red compounds. In lower oxidation states, the presence of one or more 3d electrons, usually unpaired, results in paramagnetic and colored compounds. All compounds of vanadium having unpaired electrons are colored, but because the absorption spectra maybe complex, a specific color does not necessarily correspond to a particular oxidation state. As an illustration, vanadium(IV) oxy salts are generally blue, whereas vanadium(TV) chloride is deep red. Differences over the valence range of 2—5 are shown in Table 2. The structure of vanadium compounds has been discussed (6,7). 

Hydroxyquinoline (oxine) reacts with numerous metal ions, in neutral, ammoniacal, or acetic acid solution, to produce inner complex salts of the coordination structure (II). Anions of metallo acids (molybdic, tungstic, vanadic) react in acetic acid solution to form insoluble compounds of a different structure probably the oxine esters of the particular metallo acid are formed. The vanadium compound is represented by (III). ... 

A brief introduetion to the spectroscopic properties of vanadium(IV) is provided, along with identification of key concepts crucial to the successful application of EPR methods to research questions in the field. Specific discussions of EPR spectroscopies, sueh as continuous wave and pulsed methods, have been recently reviewed [19,20]. The discussion of the use of VO " ions as spin probes by N.D. Chasteen remains a seminal review paper in the field for its complete overview of the speetroseopie properties of the ion and application to biological systems [21]. Numerous reviews on the biochemmical activity of vanadium compounds are available [22-26], and the rationales for development and synthetic routes to new vanadium compounds are also reported [27,28] A reeent study evaluated the in-vitro activity of 22 compounds currently studied in the literature [29]. In this review, EPR s role in the delineation of structure, chemistry, and in-vivo behavior of vanadium compounds will be discussed. The first seetion of die review foeuses on the use of EPR for the description of solution structures, ternary eomplex formation, and redox chemistry of vanadium(IV) and (V) compounds, wifli the general flieme of highlighting in-vitro studies. This seetion is followed by a diseussion of die application of EPR for in-vivo investigations of vanadium cellular uptake, pharmacokinetics, and in-vivo coordination structure. 

EPR methods have uncovered a number of properties of insulin-enhancing vanadium compounds that would otherwise be difficult, if not impossible, to obtain by other methods. The strengths of EPR spectroscopy, such as its natural selectivity, the high-resolving power of closely related structural isomers, and ease of in-vivo application have been exploited to reveal important details regarding the solution chemistry, absorption, metabolism, and bioaccumulation of antidiabetic vana-... 

Reactions of these ligands have not been studied in aqueous solution. However, their complexes are readily synthesized and are stable but reactive towards heteroligands [41,42], The reported structures all show the vanadium coordinated in monomeric units after the fashion depicted in Scheme 4.9. The multidentate thiolato complexes with tri- or tetradentate functionality are sufficient to satisfy the coordination requirements of the vanadium nucleus. Structurally, the compounds are not much different from analogous complexes formed with oxygen ligands (Section 4.4.2). 

Crans, D.C., A.D. Keramidas, M. Mahroof-Tahir, O.P. Anderson, and M.M. Miller. 1996. Factors affecting solution properties of vanadium(V) compounds x-ray structure of 3-cw-NH4[V02(EDDA)]. Inorg. Chem. 35 3599-3606. 

In addition to the linear oligomers, cyclic derivatives (V33, V44, V/ ) are also known (see Section 2.2). The latter two are readily formed and are ubiquitous components of aqueous solutions. These compounds are relatively inert to complex-ation reactions that preserve their cyclic structure rather, the equilibria shift towards products of other vanadium stoichiometry. Presumably, one way to generate complexes of these oligomers would be to protonate one or more oxygens so that the reactivity is increased. However, neither protonated V4 nor V5 has been identified in solution, nor has there yet been any indication from studies in alcoholic solution that an alkoxo group can replace an oxo ligand, as might be expected if protonation occurs. 

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