Multistep Oxidation Kinetics of [FeII(cdta)] [cdta =N,N′,N′′,N′′′-(1,2-Cyclohexanediamine)tetraacetate] with Molecular Oxygen

Abstract

The complicated oxidation kinetics of the reaction of [FeII(cdta)] [cdta = 1,2‐(N,N′‐ cyclohexanediamine)tetraacetate] with molecular oxygen was investigated as a function of [FeII], [O2], pH, temperature and pressure. In the presence of an excess of [FeII(cdta)] three steps could be observed, for which the following rate constants were found at 25°C; k1 = 1080 ± 16 M−1 s−1, k2 = 103 ± 4 M−1 s−1 and k3 = 59 ± 5 M−1 s−1. These reaction steps can be accounted for in terms of the following mechanism: (1) [FeII(cdta)H2O]2– reacts with O2 by a substitution process to form [FeII(cdta)O2]2–; (2) electron‐transfer to form an FeIII‐superoxo species; (3) subsequent bridge formation followed by electron‐transfer to give [(cdta)FeIII‐O22–‐FeIII(cdta)]4–; and (4) a fast decomposition of the peroxide intermediate yielding the monomeric [FeIII(cdta)] and H2O2. Rate and activation parameters for these steps are reported and discussed in terms of the postulated mechanism and in reference to available literature data.

An abiotic analogue of the diiron(iv)oxo “diamond core” of soluble methane monooxygenase generated by direct activation of O2 in aqueous Fe(ii)/EDTA solutions: thermodynamics and electronic structure

“…The van Eldik mechanism [100][101][102] of Fe(II)/Fe(III) autoxidation in water in the presence of the EDTA ligand assumes the intermediate generation of a dinuclear Fe(III) complex, in which O 2 is partially reduced to a (formally) peroxo ion:…”

“…[100][101][102] The O 2 reduction, in water solution, has been shown to involve the generation of a reactive dinuclear Fe(IV)oxo species, [EDTAHÁFeOÁOFeÁ EDTAH(H 2 O)] 2À , whose structure and electronic properties bear important analogies with the active centre of intermediate Q of sMMO. We have demonstrated the crucial role of solvation in promoting the cleavage of O 2 by stabilising the more reactive reactant spin state (S = 4) and in driving the reaction toward the formation of two high-spin (quintet) Fe(IV)oxo species in [EDTAHÁFeOÁOFeÁEDTAH(H 2 O)] 2À .…”