Formation of iron(VI) in ozonalysis of iron(III) in alkaline solution. Perfiliev Y.D., Benko E.M., Pankratov D.A., Sharma V.K., Dedushenko S.K. //Inorganica Chimica Acta. 2007. V.360. №8. P.2789-2791.
Here we report the formation of iron in hexavalent state, FeVIO42- in ozonalysis of iron(III) in alkaline medium. The formation of tetrahedral FeVIO42- ion is confirmed by UV–Visible and Mössbauer spectroscopic techniques. The value of isomer shift - δ, of the tetraoxyanion is consistent with known δ values for various salts of iron(VI) ion.
Iron is the most abundant transition element on Earth and commonly exists in compounds of its +2 and +3 oxidation states. Iron ions in these two oxidation states are generally used in biological electron transfer processes. Iron in higher oxidation states such as +4, +5, and +6 are involved in iron enzymes, organic synthesis, and Fenton chemistry. Examples include capability of Fe(IV) and Fe(V) at enzymatic sites to abstract H and/or to break C–C bond, participation of high-valent nonheme ironoxo species in biometric oxidations, and involvement of aqua oxoiron(IV) in environmental and catalytic chemistry.
Under strong oxidizing environment, higher oxidation states of iron have been obtained. In recent years, iron(VI) FeVIO42- has received much attention because of its potential use in high energy density rechargeable batteries, in cleaner (‘‘greener’’) technology for organic synthesis, and in wastewater treatment. Iron(VI) is generally produced by oxidizing a basic solution of Fe(III) salt by hypochlorite. For the last few years, there is emphasis of green chemistry and alternates to use of chlorine are being sought. In keeping with this goal, in this paper, we attempted ozone as an oxidant to form iron(VI). Ozonation is considered relatively environmentally-friendly process and does not produce chlorinated by-products.
Ozone has strong oxidizing properties: the redox potentials in acid, neutral, and alkaline solutions are as follows:
O3 + 2 H+ + 2e- = O2 + H2O, E0 = +2.075 V (1)
O3 + H2O + 2e- = O2 + 2 OH-, E0 = +1.246 V (2).
Ozone has the great advantage over other oxidants because of its strong oxidizing property. Ozonation is commonly used to prepare transition-metal complexes in which the central atom has a high oxidation state. For example, iron(IV) in acidic solution can be formed by oxidation of iron(II) with ozone Eq. (3). The iron(IV) species in aqueous solution has a half life of about 10 s, thus allowed to measure reaction kinetics of this species with inorganic and organic compounds.
Fe2+•aq + O3 = FeIVO2+ + O2 (3).
The reduction potentials of iron(VI)/iron(III) couples in acid and alkaline solutions are presented by
FeO42- + 8 H+ + 3e- = Fe3+ + 4 H2O, E0 = +2.20 V (4);
FeO42- + 4 H2O + 3e- = Fe(OH)3 + 5 OH-, E0 = +0.72 V (5).
In alkaline solution, the redox potential of the ozone/oxygen couple is higher than that of FeO42-/Fe(OH)3 couple, which suggests that ozone can oxidize iron(III) in alkaline solution to give iron(VI). In the present work, formation of Fe(VI) by ozonolysis of Fe(III) is demonstrated for the first time.
The high-valent iron(VI) was obtained in oxidation of iron(III) by ozone in alkaline medium. This procedure can be used as a simple and environmentally-friendly method to produce FeO42- ion. The UV–Visible and Mössbauer spectra were found consistent with the hexavalent iron state of the ion.