Kinetics of nucleophilic attack on co-ordinated organic moieties. Part 5. Mechanisms of addition of heterocyclic aromatic species to tricarbonyl(1–5-η-cyclohexadienyl)-iron and -ruthenium cations

Abstract

A kinetic investigation has been made of the addition of heterocyclic aromatic molecules to the cations [M(C₆H₇)( CO)₃][BF₄](1; M = Fe or Ru). For RH = pyrrole and M = Fe or Ru, the equation Rate =k[complex][RH] is observed, while a similar second-order law holds for RH = furan and M = Fe. These results are consistent with [M(C₆H₇)(CO)₃][BF₄]+ RH →[M(C₆H₇R)(CO)₃]+ HBF₄(i) electrophilic attack by the dienyl cations on the heterocyclic aromatics. The absence of primary kinetic-isotope effects for RH = pyrrole and M = Fe or Ru shows that the final proton loss is rapid, and suggests that either π- or σ-complex intermediate formation is rate determining. The relative susceptibility of a wide range of heterocyclic aromatic substrates towards the electrophile [Fe(C₆H₇)(CO)₃]⁺ has been established in both nitromethane and acetonitrile as solvents, showing a difference of 10⁶ between pyrrole and thiophen.