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

Ferrocene and its decamethyl derivative [Cp*Fe-2] 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*Fe-2](2+) dication, produced by oxidation of [Cp*Fe-2] 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 degrees to 17 degrees. Both Fe-57 Mossbauer spectroscopy and superconducting quantum interference device magnetization studies reveal identical d-orbital splitting with an S = 1, E-3 ground state based on the 3d electronic configuration e(2g) (3)a(1g)(1) of all [Cp*Fe-2](2+) salts.