The Unusual Redox Properties of Fluoroferrocenes Revealed through a Comprehensive Study of the Haloferrocenes

We
report the synthesis and full characterization of the entire
haloferrocene (FcX) and 1,1′-dihaloferrocene (fcX2) series (X = I, Br, Cl, F; Fc = ferrocenyl, fc = ferrocene-1,1′-diyl).
Finalization of this simple, yet intriguing set of compounds has been
delayed by synthetic challenges associated with the incorporation
of fluorine substituents. Successful preparation of fluoroferrocene
(FcF) and 1,1′-difluoroferrocene (fcF2) were ultimately achieved using reactions
between the appropriate lithiated ferrocene species and N-fluorobenzenesulfonimide (NFSI). The crude reaction products, in
addition to those resulting from analogous preparations of chloroferrocene
(FcCl) and 1,1′-dichloroferrocene (fcCl2), were utilized as model systems to probe
the limits of a previously reported “oxidative purification”
methodology. From this investigation and careful solution voltammetry
studies, we find that the fluorinated derivatives exhibit the lowest redox potentials of each of the FcX and fcX2 series. This counterintuitive result is discussed with reference
to the spectroscopic, structural, and first-principles calculations
of these and related materials.