Generation of trans-π-pentadienyliron tricarbonyl cations from protonation of (trans,trans-2,4-hexadienal)iron tricarbonyl

Protonation of (trans,trans-2,4-hexadienal)iron tricarbonyl, (V) has been shown to yield two trans-pentadienyliron cations, an s-cis form, (VIa) and an s-trans form (VIb), in an approximately equal ratio. This ratio represents a kinetically controlled product distribution and indicates that the original unprotonated aldehyde complex was present in equally populated s-cis and s-trans forms. Both trans ions, (VIa) and (VIb) isomerize to the cis ion (VIIb), ΔF^ne = 18.5 kcal/mol. From an analysis of the kinetics and NMR line widths it is clear that the barrier to rotation about the C₁C₂ bond in (VI) is at least 15 kcal/mol and probably considerably exceeds this number. At temperatures below −100° the OH protons of (VIa) and (VIb) can be observed owing to slow exchange at this temperature. Chemical shifts of these protons indicate extensive charge delocalization.