BIOMIMICS OF VANADIUM BROMOPEROXIDASE - VANADIUM(V)-SCHIFF BASE-CATALYZED OXIDATION OF BROMIDE BY HYDROGEN-PEROXIDE

A six-coordinate vanadium(V) complex, LVO(OEt)(EtOH), where H2L = N-(2-hydroxyphenyl)salicylideneamine, is shown to be a catalyst precursor in the oxidation of bromide by hydrogen peroxide in DMF solution. The oxidized bromine species is trapped by organic substrates and, by product analysis, is shown to be a two-electron-oxidized species, rather than a bromine radical. The bromination reaction is quantitative with respect to hydrogen peroxide. The acid dependence of the stoichiometry of the bromination reaction is established. Using UV/vis spectrophotometry, V-51 NMR spectroscopy, and kinetic data, a mechanism is proposed: the active catalyst is LVO(OH), which, upon binding peroxide and releasing H3O+, oxidizes bromide and binds another equivalent of peroxide, generating the observed LVO(O2)-. L*VO(OEt)(EtOH), where H2L* is N-(2-carboxyphenyl)salicylideneamine, also supports catalytic bromination reactions. V-51 NMR data indicate that L*2- dissociates upon the complexation of peroxide. The structures of LVO(OEt)(EtOH) and L*VO(OEt)(EtOH) have been determined by X-ray crystallography. LVO(OEt)(EtOH), VC17H2ONO5, crystallizes in the orthorhombic system, space group Pbca, with a = 20.218(5) angstrom, b = 20.955(4) angstrom, c = 16.196(3) angstrom, and Z = 16. The refinement converged to R = 0.066 and R(w) = 0.073. L*VO(OEt)(EtOH), VC18H20NO6,crystallizes in the orthorhombic system, space group Fdd2, with a = 17.7951(7) angstrom, b = 33.118(3) angstrom, c = 12.8834(5) angstrom, and Z = 16. The refinement converged to R = 0.040 and R(w) = 0.048.