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

The autocatalytic oxidation of the bis­(terpyridine)­iron­(II) complex, Fe­(tpy)2 2+ by peroxomonosulfate ion (PMS) proceeds via the formation of the corresponding iron­(III) complex (Fe­(tpy)2 3+) as the primary oxidation product. The proton-assisted dissociation of Fe­(tpy)2 2+ 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)2 3+ was also confirmed in the overall process. This effect is interpreted by considering the formation of an additional adduct between PMS and Fe­(tpy)2 3+. 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)2 2+ by PMS. The equilibrium constants for the formation of the adducts between PMS and complexes Fe­(tpy)2 2+ and Fe­(tpy)2 3+ were estimated as 129 ± 18 M–1 and 87 ± 10 M–1, respectively. In contrast to the closely related Fe­(phen)3 2+-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.