Kinetics and mechanism of addition of tertiary phosphines and phosphites to the dicarbonyl(η5-cyclopentadienyl)-(η-ethene)iron cation

Abstract

Kinetic studies of the addition of a range of tertiary phosphine and phosphite nucleophiles PR₃ to the cation [Fe(cp)(CO)₂(η-C₂H₄)]⁺1(cp =η⁵-C₅H₅)[equation (i)] revealed the general rate law, Rate =k₁[Fe][PR₃]. The second-order rate constants k₁ decrease markedly down the order P(C₆H₄OMe-2)₃ > PBuⁿ₃ > P(C₆H₄OMe-4)₃ > P(C₆H₄Me-4)₃ > P(C₆H₄Me-4)Ph₂ > PPh₃ > P(C₂H₄CN-2)Ph₂ > P(C₂H₄CN-2)₃ > P(C₆H₄Cl-4)₃ > P(OBuⁿ)₃. This reactivity order parallels that of decreasing electron availability at the phosphorus centre, as shown quantitatively by the good correlation between log k₁ and the Tolman Σ_χ values. An excellent fit to the Hammett and Brønsted equations is also observed for reaction (i) with the nucleophiles P(C₆H₄X-4)₃. The moderate Brønsted slope α of [Fe(cp)(CO)₂(η-C₂H₄)]⁺+ PR₃→[Fe(cp)(CO)₂(C₂H₄PR₃)]⁺(i) 0.46 establishes the importance of phosphine basicity in determining nucleophilicity towards the ethene ligand in cation 1. These results, together with the large negative entropy of activation with PPh₃(ΔS^‡₁=–103 J K^–1 mol^–1), are interpreted in terms of direct addition (k₁) of the phosphorus nucleophiles to the ethene ligand in 1 and suggest a transition state in which there is build-up of positive charge on the phosphorus centre and considerable phosphorus–carbon bond formation.