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. K_obs=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.