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Isolation and structural and electronic characterization of salts of the decamethylferrocene dication

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Science  12 Aug 2016:
Vol. 353, Issue 6300, pp. 678-682
DOI: 10.1126/science.aaf6362

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Charging up the iron in ferrocene salts

Ferrocene is the archetype of the sandwich compounds, so called because a metal atom is inserted between two carbon rings. The elucidation of ferrocene's structure was pivotal to the development of organometallic chemistry during the mid-20th century. The ease with which the iron in the center of the molecule can toggle between the +2 and +3 oxidation states has made the compound a common electrochemical standard. Malischewski et al. report the synthesis and isolation of ferrocene salts with iron in the +4 state, which they characterize crystallographically and spectroscopically.

Science, this issue p. 678

Abstract

Ferrocene and its decamethyl derivative [Cp*2Fe] are the most common standards for nonaqueous electrochemical investigations because of their well-defined and only mildly solvent-dependent reversible Fe(II)/Fe(III) redox couple. Higher oxidation states have only rarely been studied. We report the isolation and crystallographic and spectroscopic characterization of surprisingly stable Fe(IV) salts of the [Cp*2Fe]2+ dication, produced by oxidation of [Cp*2Fe] with AsF5, SbF5, or ReF6 in neat sulfur dioxide as well as [XeF](Sb2F11) in neat hydrogen fluoride. The Sb2F11 salt exhibits a metallocene with the expected mutually parallel arrangements of the Cp* rings, whereas the As2F11, AsF6, SbF6, and ReF6 salts manifest tilt angles ranging from 4° to 17°. Both 57Fe Mössbauer spectroscopy and superconducting quantum interference device magnetization studies reveal identical d-orbital splitting with an S = 1, 3E ground state based on the 3d electronic configuration e2g3a1g1 of all [Cp*2Fe]2+ salts.

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