The fluxional character of (η5-C5H5)Fe(CO)2(η 1-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(η ¹-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.