Dynamic NMR studies of ring rotation in substituted ferrocenes and ruthenocenes

Abstract

Variable temperature 1H and 13C{ 1H} NMR studies on 1,1′,3,3′-tetra(alkyl)-ferrocenes and -ruthenocenes (alkyl = t-pentyl,t-butyl) have provided accurate barrier energies for restricted rotations of the substituted cyclopentadienyl rings. Energies (Δ G† (298.15 K)) are in the range 40–57 kJ mol −1 and are dependent on the ‘sandwich’ metal (Fe ⋙ Ru) and alkyl substituent (t-pentyl < t-butyl). Ring rotation in 1,1′,3.3′-tetra(phenyl)ferrocene was too rapid for measurement even at 173 K. Spectral changes were analysed primarily on the basis of exchange between a mirror pair of structures having both 5-membered rings eclipsed and the alkyl substituents staggered. In the case of 1,1′,3,3′-tetra(t-butyl)ferrocene a pair of staggered ring rotamers also contributed to the observed NMR bandshape changes.