Electrochemical Behavior of [Cp′₂Fe₂S₄] and [Cp′₂Fe₂S₄](PF₆)₂ (Cp′ = η⁵-C₅Me₅): Structural Change of the Fe₂S₄ Core Accompanied by a Change of Redox State

Bulk electrolysis of Cp′₂Fe₂S₄ in 0.1 M NH₄PF₆ in CH₃CN at a peak potential for a one-electron-oxidation wave (+0.30 V vs SCE) gives a two-electron-oxidation product [Cp′₂Fe₂S₄](PF₆)₂ in high yield, where an Fe₂S₄ core with a bidentate and a doubly bidentate μ-S₂ ligands in the neutral species (A type species) changes to one with two doubly bidentate μ-S₂ ligands in the dicationic species (B type species) during oxidation. Electrochemical and spectroscopic studies on the redox behavior of these neutral and dicationic species have shown that a total of six species, i.e. a neutral complex, a monocation, and a dication with A-type cores and the same with B-type cores, participate in the redox reactions between [Cp′₂Fe₂S₄] and [C′₂Fe₂S₄](PF₆)₂. The one-electron-oxidation potential of [Cp′₂Fe₂S₄] (E_1/2 = +0.25 V vs SCE) is more positive than the one-electron-reduction potential of [Cp′₂Fe₂S₄](PF₆)₂ (E_1/2 = +0.15 V vs SCE), and the interconversion between A and B geometries of the Fe₂S₄ core is fast for monocations. Thus, an ECE-type process operates for the formation of [Cp′₂Fe₂S₄](PF₆)₂ from [Cp′₂Fe₂S₄]. The equilibrium constant for the electron transfer between [Cp′₂Fe₂S₄] and [Cp′₂Fe₂S₄](PF₆)₂ has been estimated.