Kinetics of nucleophilic attack on co-ordinated organic moieties. Part 15. Addition of p-toluidine to tricarbonyl(1–5-η-dienyl)iron cations

Abstract

The reactions of the cations [Fe(η⁵-dienyl)(CO)₃]⁺(1; dienyl = C₆H_7′ C₆H₆OMe-2, or C₇H₉) with p-toluidine in CH₃CN have been shown to give the neutral adducts [Fe(η⁴-diene·NHC₆H₄CH₃)(CO)₃](2) according to the equation (1)+ 2CH₃C₆H₄NH₂→(2)+[CH₃C₆H₄NH₃]⁺. The products have been characterised by analyses, ¹H n.m.r., i.r., and field-desorption mass spectroscopy. The reactions could be reversed by the addition of acid. A stopped-flow kinetic study of the above reactions in CH₃CN yields the general rate law, Rate =k[Fe][CH₃C₆H₄NH₂]. These results are rationalised in terms of a two-step mechanism involving initial reversible addition (k₁, k_–1) of p-toluidine to the dienyl rings to form the cationic intermediates [Fe(η⁴-diene·NH₂C₆H₄CH₃)]⁺(3), followed by rapid proton loss, k₂(either solvent- or amine-assisted). For such a mechanism the k_obs. values refer to k₁, which is consistent with the observed order of reactivity of the cations at 0 °C (C₆H₇ > C₆H₆OMe-2 > C₇H₉, 17 : 2.6 : 1), and the low ΔH‡ and negative ΔS‡ values.