Vanadium complexes bromides

Vanadium IlI) bromide, VBr3. Dark green or black (V plus Br2) gives green solution in water and green crystalline VBr3,6H20. Forms many complexes. 

The oxidation of bromide can be conveniently and quantitatively measured by the bromination of TMB in DMF solution. Bromination is stoichiometric with the vanadium complex concentration in the absence of added acid (35). The stoichiometric addition of acid results in an... 

Vanadium complexes of the first class of ligands catalyze the oxidation of bromide by hydrogen peroxide. These ligands (Figure 4) in-... 

Another system is relevant to this discussion of the oxidation of bromide by hydrogen peroxide. It does not catalyze precisely the same chemistry instead, the vanadium complex of tetraethylene glycol (H2teg) catalyzes the aerobic oxidation of bromide (from HBr) in 1,2-dichloroethane (30). 

Similarly, the reaction of 1,2,3-triphenylcyclopropenylium bromide (7) with sodium hexa-carbonylvanadate gave the vanadium complex 21. ... 

Strongly acidic vanadium(V) oxidises bromide in a sulphate ion medium . The reaction is first-order in both oxidant and sulphuric acid. The dependence of the rate on bromide ion concentration is complex and a maximum is exhibited at certain acidities. A more satisfactory examination is that of Julian and Waters who employed a perchlorate ion medium and controlled the ionic strength. They used several organic substrates which acted as captors for bromine radical species. The rate of reduction of V(V) is independent of the substrate employed and almost independent of substrate concentration. At a given acidity the kinetics are... 

A similar preparative procedure was used to isolate tetrahedral complexes of bidentate donors (L = bipy or phen) [VL2](SCN)3. However, complexes of vanadium(lll) chloride or bromide with these same bidentate donors have stoichiometries depending on conditions.199 With excess of bipy or phen, complexes [VX JX were obtained. In EtOH or in MeCN two series of complexes were isolated. VC13LL (L = bipy or phen L = EtOH or MeCN) and [VC12L2]+ + [VCUL]-. In the reaction of vanadium(III) with phenanthroline202 203 or bipyridine204 there is evidence for extensive hydrolysis. Spectrophotometry was used to estimate the equilibrium constant (Kcq = 0.43) for the reaction in equation (7). 03... 

Alkyl hydroperoxides, including ethyl hydroperoxide, cuminyl hydroperoxide, and tert-butyl hydroperoxide, are not used by V-BrPO to catalyze bromination reactions [29], These alkyl hydroperoxides have the thermodynamic driving force to oxidize bromide however, they are kinetically slow. Several examples of vanadium(V) alkyl peroxide complexes have been well characterized [63], including [V(v)0(OOR)(oxo-2-oxidophenyl) salicylidenaminato] (R = i-Bu, CMe2Ph), which has been used in the selective oxidation of olefins to epoxides. The synthesis of these compounds seems to require elevated temperatures, and their oxidation under catalytic conditions has not been reported. We have found that alkyl hydroperoxides do not coordinate to vanadate in aqueous solution at neutral pH, conditions under which dihydrogen peroxide readily coordinates to vanadate and vanadium( V) complexes (de la Rosa and Butler, unpublished observations). Thus, the lack of bromoperoxidase reactivity with the alkyl hydroperoxides may arise from slow binding of the alkyl hydroperoxides to V-BrPO. 

Neither V0(02)+ nor V0(02)2 oxidizes bromide rather they associate to form the dinuclear (V0)2(02)3, which oxidizes bromide (Scheme 3) [76], The association constant for the dinuclear compound is rather small (K3 = 9 M at pH 0-2) this explains why it is not observed by 51V NMR at low concentrations of vanadium [73,76], The dinuclear compound has now been observed by 51V NMR at very high concentrations of vanadium [76], A complex of V202(02)3(HGly)2(H20)2 has been isolated in which a p-peroxo coordination of one peroxide is proposed [76],... 

Clague, M.J. and A. Butler. 1995. On the mechanism of cw-dioxovanadium(V)-catalyzed oxidation of bromide by hydrogen peroxide Evidence for a reactive, binu-clear vanadium(V) peroxo complex. J. Am. Chem. Soc. 117 3475-3484. 

Figure 4. Ligands whose vanadium(V) complexes have been tested for catalysis of bromide oxidation.

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. 

Complexes of the first two ligands are prepared by reaction of vanadyl sulfate with the ligand, preformed in situ in aqueous ethanol (38). Isolated as the vanadium(IV) complexes, they can be oxidized in aerobic methanol and reisolated as the methoxymethanol vanadium(V) derivatives (39). Alternatively, the reduced complexes can be dissolved in DMF and allowed to oxidize aerobically before use. Their reactivity parallels that of (HPS)VO(OEt)(EtOH). They have reaction rates for catalyzing bromide oxidation comparable to (HPS)VO(OH) (Table I). Interestingly, the rates of these reactions are quite sensitive to the amount of water. The increase from 0.5% to 1.0% added water causes a 10-20% decrease in rate. 

The V-BrPO-catalyzed oxidation of bromide by hydrogen peroxide is the first step in a series of reactions that results in brominated organic products or the halide-assisted disproportionation of hydrogen peroxide. As studies continue on both the enzymatic and model systems, we can make several comments. The ability to catalyze this reaction seems to be a fairly general property of vanadium(V) complexes with an available coordination site in the equatorial plane in which to bind peroxide. The oxidation rates of the model complexes, with respect to the enzyme, raise the question of what microscopic differences give rise to the high... 

Interestingly, a study of the bromide-dependent chloroperoxidase bromination of tyrosine reveals that the active brominating agent is free bromine and not a bromine-enzyme complex [122]. The situation is very different for bromoperoxidase- and vanadium peroxidase-catalyzed brominations [106-108]. 

Binuclear vanadium(V) peroxo complexes such as (NH4)5[V202(02)4P04]-H20 are also known.30 They have been implicated as the reactive species in the bromide oxidation by hydrogen peroxide,31 a process which is important in some biological systems (Section 17-B-8). Some stable peroxo compounds display insulin-mimetic activity both in vivo and in vitro.32... 

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