Organometallic catalysis in aqueous solution. Oxygen transfer to bromide

Article abstract of DOI:10.1021/ja00086a020

The reaction between hydrogen peroxide and bromide ions in aqueous acidic solutions, ordinarily very slow, is strongly catalyzed by CH₃ReO₃, a water-soluble organometallic oxide. The complex catalytic kinetics showed that the rate-controlling process consists of two steps: (1) reversible formation of the independently-known 1:1 and 2:1 adducts of hydrogen peroxide and methylrhenium trioxide (the formulas, including the water that had been shown to be coordinated, are CH₃Re(O)₂(η²-O₂)(H₂O) and CH₃Re(O)(η²-O₂)₂(H₂O)) and (2) their reactions with bromide ions that yield HOBr. The rate constants for these steps were evaluated by several steady-state kinetic techniques. The HOBr intermediate reacts with Br to yield Br₂. When hydrogen peroxide was in excess, the reaction yielded oxygen instead of bromine. This can be accounted for by the reaction of HOBr with H₂O₂. The 2:1 peroxide-rhenium adduct, formed only at the higher concentrations of hydrogen peroxide, also reacts with bromide ions, but more slowly. Kinetic modeling by numerical techniques was used to provide verification of the reaction scheme. The various steps of peroxide activation consist of nucleophilic attack of bromide ions on peroxide ions that have become electrophilically activated by binding to the rhenium compound. The rhenium catalyst bears some resemblance to the enzyme vanadium bromoperoxidase.