Kinetics of oxidation of ascorbic acid by the [Mn-3(IV)(mu-O)(4)(bipy)(4)(H2O2](4+) ion (bipy=2,2 '-bipyridine)

In the pH range 1.91-5.24, an aqueous solution of the complex [Mn-3(IV)(mu- O)(4)(bipy)(4)( H2O)(2)](4+) (1) oxidises ascorbic acid quantitatively to d ehydroascorbic acid and is itself reduced to Mn-II. In the presence of exce ss ascorbic acid, the reaction follows first-order kinetics. The first-orde r rate constant k(t) measured in the presence of excess ascorbic acid incre ases linearly with increasing concentration of the reductant but decreases as more and more 2,2'-bipyridine is added. k(0) also increases with [H+] bu t tends to saturate at higher [H+]. The proposed rate-determining step invo lves simultaneous transfer of one electron and one proton from the ascorbat e ion [Mn-3(IV)( mu-O)4(bipy)(3)(H2O)](4+) an aquated form of 1. Subsequent rapid reactions via a dinuclear {Mn2O3}(3+) intermediate lead to the final products. (C) 2000 Elsevier Science Ltd All rights reserved.