[3]Ferrocenophane bridge reversal barriers

Dynamic NMR studies have yielded accurate energy data for the bridge reversal fluxion of [3]ferrocenophanes with Group VI bridging atoms. This process, whilst appearing very analogous to the chair-to-chair reversal of corresponding 6-membered heterocyclic rings, appears to be a much higher energy process, its associated Δ G ≠ values being in the range 59 to 81 kJ mol −1 depending on the types of Group VI bridging atoms. These data allow estimates to be made for the first time of the relative magnitudes of torsional barriers about single bonds involving like and unlike Group VI atoms. For example, the SS torsion is shown to be 3.9 kJ mol −1 higher in energy than the SSe torsion and 5.8 kJ mol −1 higher than the SeSe torsion. The probable mechanism of the bridge reversal process is discussed.