Migratory insertion in N-heterocyclic carbene-containing Fe carbonyl complexes: an experimental and theoretical study

Abstract

The compound [Fe(η-C₅H₅)(CO)₂(Me)] reacts thermally with N-heterocyclic carbenes (L) to give both alkyl, [Fe(η-C₅H₅)(L)(CO)(Me)], and acyl, [Fe(η-C₅H₅)(L)(CO)(COMe)], derivatives. The reaction temperature has been shown to affect the product distribution. The alkyl and acyl derivatives exist in an equilibrium that is more easily perturbed than in the tertiary phosphine analogues. DFT studies on the reactivity of [Fe(η-C₅H₅)(CO)₂(Me)] with PH₃ and dihydroimidazole-2-ylidene (IH) have shown that CO exchange is energetically favoured for IH, and energetically disfavoured for PH₃. The products of CO-induced migratory insertion, [Fe(η-C₅H₅)(L)(CO)(COMe)], are more stable than the parent alkyl, [Fe(η-C₅H₅)(L)(CO)(Me)], compounds. This stabilisation is larger when L = IH than when L = PH₃. Stabilisation of the transition state by agostic interactions was seen in both instances, but this was significantly more pronounced for L = IH.