Density functional study of protonated formylmetallocenes

Protonated formylmetallocenes [M(C5H5)(C5H4-CHOH)]+ (M = Fe, Ru) and their isomers have been studied at the BP86 and B3LYP levels of density functional theory. Oxygen-protonated isomers are the most stable forms in each case, with a plethora of ring- or metal-protonated species at least ca. 14 and 10 kcal/mol higher in energy for M = Fe and Ru, respectively. The computed rotational barriers around the C−C bond connecting the cyclopentadienyl and protonated formyl moieties, ca. 18 kcal/mol, are indicative of substantial conjugation between these moieties. Some of the ring- and iron-protonated species are models for possible intermediates in Friedel–Crafts acylation of ferrocene, and the computations provide further evidence that exo attack is clearly favored over endo attack of the electrophile in this reaction. The structures of the most stable mono- and diprotonated formylferrocenes are corroborated by the good agreement between GIAO-B3LYP-computed and experimental NMR chemical shifts.