Kinetics of nucleophilic attack on co-ordinated organic moieties. Part 17. Addition of pyridines to [Fe(1–5-η-dienyl)(CO)3]+ cations (dienyl = C6H7, 2-MeOC6H6, or C7H9)

Abstract

Synthetic and kinetic studies of the reactions between [Fe(1–5-η-C₆H₇)(CO)₃]⁺(1) and X-substituted pyridines (X = H, 2-Me, 3-Me, 4-Me, 4-Ph, 2-Cl, 3-CN, 2,5-Me₂, 2,6-Me₂, 3,5-Me₂, or 2,4,6-Me₃) in CH₃CN provide the first quantitative information on the importance of basicity and steric properties in controlling amine nucleophilicity towards co-ordinated π-hydrocarbons. The products are pyridinium adducts of tricarbonyl(hexa-1,3-diene)iron. Similar pyridinium adduct formation occurs with the cations [Fe(1–5-η-2-MeOC₆H₆)(CO)₃]⁺(2) and [Fe(1–5-η-C₇H₉)(CO)₃][BF₄](3). The general rate law rate =k₁[Fe][amine] is observed, except for the equilibrium reaction of (1) with 3-cyanopyridine which gives rate =k₁[Fe][amine]+k_-1[Fe]. The rate trend C₆H₇ > 2-MeOC₆H₆ > C₇H₉ found with several pyridines and the low ΔH₁^‡ and large negative ΔS₁^‡ values are consistent with direct addition to the dienyl rings. For attack of non-sterically crowded pyridines on (1), a Brönsted plot of log k₁versus pK_a of the amine conjugate acid has a high slope α of 1.0, indicating a very marked dependence of rate on amine basicity. Successive blocking of the 2- and 6-positions of pyridine by methyl groups leads to marked non-additive steric retardation.