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