Transformations of ferrates(iv,v,vi) in liquids: Mössbauer spectroscopy of frozen solutions

Abstract

The kinetics and mechanism of ferrate(IV), (V) and (VI) transformations in water and in polar organic solvents (namely ethanol and tetrahydrofuran) have been investigated by the method of ⁵⁷Fe Mössbauer spectroscopy of frozen solutions. Ethanol with a very limited amount of water under an inert atmosphere, significantly slows down the transformation reactions of ferrates(IV and V) and provides direct proof of the existence of intermediate states. Simultaneously, ethanol is oxidized to caboxylates in the close vicinity of the surface of ferrate crystallites as proven by X-ray photoelectron spectroscopy. On the contrary, any transformation of ferrate(VI) in pure ethanol (with a very limited amount of water) was not observed. Mössbauer spectroscopy of frozen solutions enabled us to experimentally identify and quantify intermediates of ferrate(IV) and ferrate(V) transformations for the first time. Sodium ferrate(IV) in its tetrahedral form, Na₄FeO₄, undergoes a two-step charge disproportionation to Fe(III) and Fe(VI) via a Fe(V) intermediate without any evolution of oxygen in polar protic and aprotic solvents, specifically 2Fe(IV) → Fe(III) + Fe(V), and Fe(IV) + Fe(V) → Fe(III) + Fe(VI), i.e. in sum 3Fe(IV) → 2Fe(III) + Fe(VI). Ferrate(V) (K₃FeO₄) transforms to Fe(III) and Fe(VI) without any indication of the Fe(IV) intermediate within the detection limit of the method. In addition to a charge disproportionation reaction proceeding in polar liquids, 3Fe(V) → Fe(III) + 2Fe(VI), a competitive reduction of Fe(V) directly to Fe(III) accompanied by oxygen evolution takes place in water. Oxygen evolution was also measured for ferrate(IV and VI) transformations in water, but to a higher and a smaller extent compared to ferrate(V), respectively. The thermodynamics of the suggested ferrate(IV) and ferrate(V) transformation pathways was examined by DFT calculations.