Regioselective protonation of ferrocene in superacid and formation of a C-H-Fe bond. An experimental and theoretical study of the structure and dynamics of the ferrocenonium ion

Protonation of ferrocene has been suggested to take place on carbon (exo-pr otonation) or iron (endo-protonation). However, experiments have not been c onclusive because of interfering exchange reactions. Now low-temperature pr otonation of ferrocene and [H-2(10)]-ferrocene in superacid and direct obse rvation of the carbocation by H-1 NMR at low temperature shows only primary protonation and that it exclusively takes place in an endo-fashion. Studie s by DFT calculations using B3LYP hybrid functional indicate the presence o f an intramolecular nonlinear C-H-Fe bond and that the proton might be delo calized between carbon and iron. Potential energy barriers for degenerate r earrangements of the hydride bridged carbocation are low, suggesting that t he proton might be delocalized between all 10 carbons and iron. The NMR res ults are consistent with such an interpretation.