Photochemistry of the Fe(III)–EDTA complexes: A mechanistic study

Abstract

The Fe(III) photoreduction in the EDTA complexes studied by pulse photolysis and continuous irradiation revealed that LMCT excitation is followed by reaction of the primary photoproduct with the parent complex, generating intermediate species formulated as [(H 2O)(EDTA +)Fe II(μ-OH x )Fe IIIEDTA] x − 4 . The presence of molecular oxygen is of crucial relevance for the intermediate life time and pathway of decay. In deoxygenated media, the intermediate is relatively long-lived ( k obs ∼1 × 10 −3 s −1) and its degradation proceeds by the back electron transfer regenerating the parent complex and by the second inner-sphere electron transfer producing Fe(II) species and EDTA oxidation products. In this case the product ratio of the Fe(II) to (EDTA) ox should be 2:1, consistent with the previous reports. In the presence of molecular oxygen, the intermediate dimer decomposes much faster ( k obs ∼1.4 × 10 2 s −1), presumably via outer-sphere two-electron oxidation of the dangling CH 2COO group and the Fe(II) centre, yielding [Fe IIIEDTA] −, [Fe IIIED3A] and EDTA oxidation products. Formation of the [Fe IIIED3A] complex was recorded only when Fe(III)–EDTA was irradiated in the presence of O 2. Under prolonged irradiation, it undergoes photoredox reaction resulting in oxidation of successive EDTA fragments and Fe aq 3+ production. Thus, in aerated media EDTA is oxidized by O 2 in the photocatalytic process, in which Fe(III) species plays a role of photocatalyst. Quantum yields of the Fe(III)–EDTA decay depend on the irradiation wavelengths and solution pH; in deoxygenated media, the post-irradiation substrate regeneration makes the measured quantum yields apparently dependent on measurement time ( ϕ 0 ≈ 4–5 ϕ ∞).