A comparison of the energy barriers to bridge reversal in trisulphur-and triselenium-bridged [3] metallocenophanes

The [3]metallocenophanes with trichalcogen chains as bridging groups, namely [M(C5H4E)2E′] (M = Ru, Os, E = E′ = S; M = Ru; E = E′ = Se), have been shown by NMR spectroscopy to be fluxional by a bridge reversal process. From two-dimensional NMR exchange experiments (2D-EXSY), accurate energy data for the bridge reversal fluxion have been obtained and compared with previous data for analogous [3]ferrocenophanes. The ΔG‡(298 K) values for the process lie in the range 67–93 kJ mol−1. Magnitudes increase in the order Fe < Ru < Os, reflecting the increasing strength of metal-ring bonding, and in the order Se3 < S3 due to the different torsional energies of the chalcogen-chalcogen bridge bonds.