Mechanism of attack by trialkylphosphines on dicarbonyl(dienyl)iodo-iron complexes

Abstract

The reactions of [Fe(1–5-η-C₆H₇)(CO)₂I] and [Fe(1–5-η-C₇H₉)(CO)₂I] with PBuⁿ₃and PPrⁱ₃ have been shown to proceed via the stepwise process: [Fe(dienyl)(CO)₂I]+ PR₃ [graphic omitted] [Fe(R₃P.diene)(CO)₂I] [graphic omitted] [Fe(R₃P.diene)(CO)₂(PR₃)]I Kinetic studies of the rapid first step with PBuⁿ₃ in acetone gave the rate law, Rate =k₁[complex][PBuⁿ₃] for both complexes. The activation parameters {ΔH₁^‡= 32.2 ± 1.3 kJ mol^–1, ΔS₁^‡=–130 ± 3 J K^–1 mol^–1} and the much faster (ca. 80 times) rate for the C₆H₇, complex support a mechanism involving direct addition to the dienyl ligands. The complex rate law, Rate =k₂K₃[complex][PBuⁿ₃]//_{/k–2[I^–]+k3[PBuⁿ3]}, observed in both cases for the much slower second step, indicates a dissociative mechanism for the iodide replacement. ΔH₂^‡= 91.1 ± 4.1 kJ mol^–1 and ΔS₂^‡=–32 ± 8 J K^–1mol^–1 for the iodide dissociation of the C₆H₇ complex.

In contrast to an earlier study, [Fe(1–3-η-C₃H₅)(CO)₃I] has been shown to react with PPh₃ and PPrⁱ₃ at room temperature to give phosphonium salts [PR₃(C₃H₅)]I. Comparison with other related reactions suggests that in general addition to the π-hydrocarbon ligands by trialkylphosphines is kinetically favoured over attack at the metal.