Haptotropic Rearrangements in Sandwich (Fluorenyl)(Cyclopentadienyl) Iron and Ruthenium Complexes

Haptotropic rearrangements in sandwich (Cp)M(ηn-Flu) (M = Fe, Ru; Flu = fluorenyl) complexes were investigated by theoretical (DFT) methods and experimental techniques as well in the iron case. The molecular structure of the (Cp)Fe(η5-Flu) isomer (A-Fe) was determined by an X-ray diffraction study. The activation barrier for the η6-Flu (B-Fe, kinetic product) ⇆ η5-Flu (A-Fe, thermodynamic product) process in the iron complex was determined by 1H NMR to be 31.4 ± 2.2 kcal·mol−1 in benzene. Using DFT calculations for the η6 ⇆ η5 process in (Cp)M(ηn-Flu) (M = Fe, Ru), two stable intermediates were found, i.e., η3-“frontside” (C) and η4-“backside” (D) species. The “frontside” pathway was found to be 4–5 kcal·mol−1 more favorable than the “backside” pathway for both the Fe and Ru complexes. The corresponding activation barriers were found to be 27.3 (B3LYP set) and 33.2 (BP86 set) kcal·mol−1 for the Fe complexes and 27.9 kcal·mol−1 (B3LYP set) for the Ru complexes.