Peroxovanadium complexes

Casny, M. and D. Rehder. 2004. Molecular and supramolecular features of oxo-peroxovanadium complexes containing OsN, 02N2 and ON3 donor sets. J. Chem. Soc., Dalton Trans. 839-846. 

Other Complexes. Before we turn to a discussion of other complexes, it is worth making a few general comments about the biomimetic systems studied to date. The model systems are much slower ( 105-fold) than the enzyme (33). All peroxovanadium complexes, whether competent to catalyze bromide oxidation reactions or not, contain tj2-coordinated peroxide (4). Little is known about the binding of peroxide in the enzyme (see above), but one wonders whether the enhanced reactivity is derived from an alternative binding mode, such as end-on peroxide or hydroperoxide. The rapid enzymatic rate could also arise from the nature or configuration of the ligands to the vanadium ion. 

In the vanadate-dependent haloperoxidases, an 0x0 group of the vanadate centre is exchanged for peroxide prior to 0x0 transfer, referred to as an activation of peroxide, and 0x0 transfer to a substrate such as halide or R2S is afforded by nucleophilic attack of the substrate to the activated (hydro)peroxide as shown in Figure 4.18. Generally, peroxovanadium complexes can thus be considered to model the active catalytic site in the haloperoxidases. A mechanism for the peroxovanadium complex formation as depicted in Equation (4.14), proposed by Pecoraro and co-workersl on the basis of... 

Although peroxovanadium complexes are abundant, none actually structurally models the active centre of the peroxo form of the structurally characterised chloroperoxidase from the... 

Despite the fact that synthetic peroxovanadium complexes do not model satisfactorily the active centre of the VHPOs in the sense outlined above, most of them are effective oxidants under mild conditions, or even catalysts for, or active intermediates in the catalysis of, oxygenation reactions using hydrogen peroxide as the oxo transfer agent. Examples of 0X0 transfer reactions to organic substrates carried out via peroxovanadium complexes -the hydroxylation of alkanes, alkenes and aromatic compounds by the picolinato complex 22 (Figure 4.21) - are depicted in Scheme 4.8. 

Meanwhile, the mechanism proposed for the hydroxylation of aromatics catalyzed by cationic complexes of platinum(ll) involves an electrophilic meta-lation of the aromatic ring to yield platinum-aryl intermediates followed by oxygen transfer from a platinum-hydroperoxy species (Scheme X.3) [23b]. Finally, the oxidation [26a] of aromatic compounds by hydrogen peroxide catalyzed by the peroxovanadium complex VO(02)(Picolinate)(H20)2 is proposed to occur via oxygenation of the arene by this complex, which is restored under the action of H2O2. 

Reports on the ring hydroxylation of electron-poor aromatic componnds are relatively scarce. Chlorobenzene, nitrobenzene, benzonitrile, benzaldehyde, and benzoic acid are inert toward hydroxylation with over TS-1 [81]. The hydroxylation of benzene derivatives XC j (X=F, Cl, Br, NOj) by and in the presenee of eatalytie amonnts of [V0(02)(pie)(H20)2] prodnced corresponding phenols in moderate yields [94]. The reaction was proved to be a radical chain process where initiation prodnces a radical anion of the peroxovanadium complex as the actnal oxidizing species [34]. 

Sarmah, S., D. Kalita, P. Hazarika, R. Borah, and N.S. Islam. 2004. Synthesis of new dinuclear and mononuclear peroxovanadium(V) complexes containing biogenic coligands A comparative study of some of their properties. Polyhedron 23 1097-1107. 

Vanadium Compounds Used for Treatment of Diabetes Salts, Chelate Complexes, and Peroxovanadium Compounds... 

Perox- Species. Addition of aqueous H2O2 to aqueous results in the formation of various peroxo-complexes. In neutral or alkaline solutions, the yellow diperoxoorthovana-date ion, [V02(02)2], is formed, whereas in acid solutions the red-brown peroxovanadium ion, [V(02)] +, is the main species present. Acidic salts such as KH2[V02(02)2] H20 and (NH4)2H[V02(02)2]-nH20 have been prepared and their IR spectra are characterized by the presence of a strong band at about 850 cm, probably associated with the 0-0 stretching vibration of the peroxo-group, which is coordinated... 

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