The Fluxional Character of (η5-C5H5)Fe(CO)2(η1-C9H7): Evidence for the [4 + 2] Cycloaddition of a Metal-Substituted Isoindene with Tetracyanoethylene

The variable-temperature NMR spectra of (η5-cyclopentadienyl)Fe(CO)2(η1-indenyl), 1, do not exhibit noticeable line-broadening below 70 °C, at which point decarbonylation to yield benzoferrocene, 8, is rapid. Nevertheless, 2D-EXSY data show clearly that 1 is indeed fluxional; it undergoes successive [1,5] shifts with an activation energy of ∼20 kcal mol-1. Contrary to an earlier report, the intermediate isoindene, 2, is sufficiently long-lived to yield a crystallographically-characterized [4 + 2] cycloadduct, 10, with tetracyanoethylene.