Ferrate(vi) Formation by Hydrogen Peroxide in Presence of Ethylenediaminetetraacetate

Detailed Spectroscopic and Theoretical Studies on [Fe(EDTA)(O2)]3-: Electronic Structure of the Side-on Ferric−Peroxide Bond and Its Relevance to Reactivity

“…Usually for high-spin ferric complexes, g ≈ g 0 1 , where 1 is the 3 × 3 unit matrix and g 0 is close to the free electron g-value ( g e = 2.0023...). 41a, For D ≫ h ν (the energy of the microwave, ∼0.3 cm -1 at X-band), the three Kramers doublets of a S = 5 / 2 system act approximately like independent “ S = 1 / 2 ” systems with effective g-values that are dependent only on E/D . From plots of the effective g-values versus E/D 24,41 we conclude that E/D ≈ 0.21 for the Fe(III)−EDTA−peroxide complex at pH 11.5, consistent with an earlier study 18c. According to this analysis, the dominant signal in the g = 3−6 region arises from the “ M S = ± 3 / 2 ”, the feature at g = 9.2 from the “ M S = ± 1 / 2 ”, and the resonance at g = 9.9 from the “ M S = ± 5 / 2 ” Kramers doublet (Figure ).…”

“…Upon addition of H 2 O 2 to Fe(III)−EDTA at elevated pH, an absorption band at 520 nm (19 200 cm -1 , the “purple band”) develops (ε 520 = 520 M -1 cm -1 ), which has been ascribed to a peroxide-to-iron ligand-to-metal charge transfer (LMCT) transition. 16a,18a,b Under the conditions chosen, this band titrates with an equilibrium binding constant of ∼1700 M -1 , somewhat lower than previously reported. , ABS data were obtained at room temperature (not shown) and at low temperature (14 K) on frozen-solution glass samples (Figure , upper panel). Solutions of the peroxo complex change their color from purple to orange on freezing, which has been attributed to a narrowing of the charge transfer (CT) band 18b. However, the spectra in Figure show only a limited narrowing; they shift by ∼3000 cm -1 to higher energy on freezing .…”

“…This was reported to shift to 815 cm -1 in the frozen state 18b and was observed at 816 cm -1 in the present study. Based on its frequency, 16 O 18 O isotope shift (Δ(H 2 16 O 2 /H 2 16,18 O 2 ) = −21 cm -1 ),18b and profile behavior, this feature was assigned to the dominantly O−O stretching vibration. 18a,b Data were also obtained for the 18 O 18 O derivative and an isotope shift of −40 cm -1 was observed (530-nm trace in Figure , right; Table ). In previous studies 18a,b no metal−ligand vibration could be unambiguously identified, in part because of the weak enhancement of the features with visible laser irradiation and in part because of strong background signals from the quartz tubes and non-resonance enhanced metal−ligand vibrations in the 250−500 cm -1 region .…”

“…Fe(III)−EDTA also catalyzes superoxide dismutation, hydrogen peroxide decomposition , and organic substrate oxidation in the presence of H 2 O 2 . It has therefore been attractive as a mimic of the superoxide dismutase, catalase, and peroxidase enzymes and, consequently, detailed kinetic information is available. − The Fe(III)−EDTA−peroxide complex has been studied by a variety of spectroscopic methods, ,, which have demonstrated that it is a 1:1 adduct and that it exists in an S = 5 / 2 ground state, the latter indicating that a ferric−peroxide rather than a ferrous-superoxide description is appropriate.…”

“…The binding mode of the peroxo ligand has previously been addressed by resonance Raman (rR) spectroscopy with the use of mixed isotope (H 16 O 18 OH) hydrogen peroxide 18b. The absence of an isotope splitting on the ν OO stretching mode observed at 815 cm -1 has been taken as definitive evidence for a side-on (η 2 ) peroxide coordination geometry.…”