Effects of temperature and pressure on the addition of 4-ethylpyridine to tricarbonyl(η5-dienyl)iron(II) complexes
Abstract
Detailed kinetic studies of the reaction between [Fe(CO)3(η5-dienyl)]BF41(dienyl = C6H7, 2-MeOC6H6 or C7H9) and 4-ethylpyridine in MeCN have been performed as a function of temperature and pressure using stopped-flow techniques. The reactions proceed to completion under pseudo-first-order conditions, i.e. Kobs=k1[4Et-py]. The observed rate sequence C6H7 > 2-MeOC6H6 > C7H9(e.g. 25 : 4 : 1 at 10 °C and 0.1 MPa) and the low ΔH‡ and large negative ΔS‡ and ΔV‡ values are consistent with direct addition (k1) to the dienyl fragments of complexes 1. The results contrast sharply with a recent report on the reversible addition of 4-cyanopyridine to complexes 1(dienyl = C6H7 or C7H9) where application of pressure had no effect on the forward (k1) and reverse (k–1) steps. These results provide the first quantitative information on the applicability of pressure dependences in the mechanistic interpretation of amine additions to dienylium organometallics of type 1.