Kinetics and Mechanism of the Autocatalytic Oxidation of Bis(terpyridine)iron(II) by Peroxomonosulfate Ion (Oxone) in Acidic Medium.

Abstract

The autocatalytic oxidation of the bis(terpyridine)iron(II) complex, Fe(tpy)22+ by peroxomonosulfate ion (PMS) proceeds via the formation of the corresponding iron(III) complex (Fe(tpy)23+) as the primary oxidation product. The proton-assisted dissociation of Fe(tpy)22+ and subsequent oxidation of Fe2+ are side reactions in this system. In the initial stage of the reaction, a 1:1 adduct is formed between PMS and bis(terpyridine)iron(II), which decomposes in an intramolecular electron transfer reaction step. The autocatalytic role of Fe(tpy)23+ was also confirmed in the overall process. This effect is interpreted by considering the formation of an additional adduct between PMS and Fe(tpy)23+. The decomposition of the adduct yields two strong oxidizing intermediates, an Fe(IV) species and SO4-•, which consume the iron(II) complex in rapid reaction steps. A detailed kinetic model was postulated for the overall oxidation of Fe(tpy)22+ by PMS. The equilibrium constants for the formation of the adducts between PMS and complexes Fe(tpy)22+ and Fe(tpy)23+ were estimated as 129 ± 18 M-1 and 87 ± 10 M-1, respectively. In contrast to the closely related Fe(phen)32+-PMS reaction, the N-oxide derivative of the ligand (tpyO) does not have any kinetic role in the overall process because of the very slow formation of the N-oxide in the reaction.