Spectroscopic and Computational Assessment of the Rotational Barrier of a Ferrocenyl-Stabilized Cyclopentadienyl Cation: Evidence for the First Hydroxyfulvalene Ligand

Protonation of 3-ferrocenyl-2,4,5-triphenylcyclopentadienone with trifluoroacetic acid results in the formation of the first hydroxyfulvalene complex. The dramatic deshielding and decoalescence of the 1H and 13C NMR signals in the ferrocenyl region offer evidence for the participation of the metal in the stabilization of the positive charge, which results in restricted rotation of the ferrocenyl group. As determined by NMR spectroscopy, the maximum barrier to rotation in the neutral species is ca. 9 kcal mol-1, whereas the minimum barrier in the cation considerably exceeds 13 kcal mol-1. The difference in rotational barriers for the neutral and protonated species is rationalized by using DFT calculations.