| Abstract
| - The dioxovanadium(V) complexesVO2(bpg) (1),[VO2(pmida)]- (2), and[VO2(ada)]- (3) have beensynthesizedand characterized as models for the vanadium haloperoxidases.These compounds react with hydrogen peroxidein acetonitrile to form the corresponding peroxovanadium(V)complexes that have been previously studied bystopped-flow spectrophotometry. 1H and 51VNMR spectra of the VO2+ complexes in aqueoussolution providea clear picture of the solution structure of each complex. Theresults of these kinetic studies suggest an associativemechanism in which peroxide binds to a protonated form of the vanadiumcomplex, followed by loss of a boundhydroxide or water molecule in the rate-determining step of thereaction and rapid rearrangement to the finalproduct. The addition of acid to the reaction mixture results inrapid increases in the rate of peroxide binding byvanadium as a result of increased protonation of the complex. Asin previous studies of similar reactions inaqueous solution, the reaction is first order in [H+]for substoichiometric amounts of acid, but when acid ispresent in excess, the dependence on [H+] becomes morecomplex, implicating the presence of hydroxide- andwater-ligated intermediates. Under conditions in which no acid isadded to the reaction mixture, the rate constantsfor formation of the peroxovanadium complex from thevanadium−peroxide adduct are 0.12 ± 0.04s-1 for 1,0.33 ± 0.03 s-1 for 2, and 0.29± 0.06 s-1 for 3. Theimplications of this study with respect to catalysis bythevanadium-dependent haloperoxidase enzymes are discussed.
- Dioxovanadium(V) complexes oftetradentate amino carboxylate ligands have been synthesized andcharacterized as models for vanadium haloperoxidases. Thereactions of these compounds with hydrogen peroxide in acetonitrile toform the corresponding peroxovanadium complexes have been examined bystopped-flow spectrophotometry. Kinetic analyses of thesereactions implicate an associative mechanism in which peroxide binds tovanadium after protonation of an oxo ligand, followed by loss ofligated water or hydroxide in the rate-determiningstep.
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