Mechanism of addition of 2-ethylpyridine to tricarbonyl(1–5-η-dienyl)iron(II) cations (dienyl = C6H7, 2-MeOC6H6, or C7H9)

Abstract

Detailed kinetic studies of the reactions between the complexes [Fe(CO)₃(1–5-η-dienyl)] BF₄(1; dienyl = C₆H₇, 2-MeOC₆H₆, or C₇H₉) and 2-ethylpyridine in MeCN reveal the rate law, rate =k₁[Fe][amine]+k_–1[Fe]. The observed rate trend C₆H₇ > 2-McOC₆H₆ > C₇H₉(e.g. 46 : 6.4 : 1 at 0 °C) and the low ΔH₁‡ values and the large negative ΔS₁‡ values are consistent with direct addition (k₁) to the dienyl rings of complexes (1). On the other hand, the relatively much higher ΔH_–1‡ values are as expected for bond cleavage in dissociation (k_–1) as is the positive ΔS_–1‡ of +41 ± 13 J K^–1 mol^–1 determined for the addition of 2-ethylpyridine to (1; dienyl = C₆H₇). A plot of ΔH_–1‡versusΔS_–1‡ is linear, the slope of which gives an isokinetic temperature of 208 ± 20 K, indicating entropy control over the dissociative process.