Kinetics and Mechanism of Ligand Exchange of Tris(acetylacetonato)-iron(III) in Acetylacetone

Article abstract of DOI:10.1246/bcsj.53.424

Tris(acetylacetonato<2-14C>)iron(III) undergoes ligand exchange in acetylacetone at 4-16 deg C without side reactions.The exchange rate is expressed by Rate = (k_a + k_b)( ca. 0.01 M, ca. 0.05 M), and the rate constant (10.2 deg C) and activation parameters are k_a = (8.9 +/- 1.2) * 10^-4 s^-1, ΔH^<*> = 14.4 +/- 0.9 kcal mol^-1, ΔS^<*> = -22 +/- 3 cal K^-1 mol^-1 and k_b = (2.22 +/- 0.20) * 10^-2 M^-1 s^-1, ΔH^<*> = 16.9 +/- 0.4 kcal mol^-1, ΔS^<*> = -6.1 +/- 1.3 cal K^-1 mol^-1 (1 cal = 4.18 J, 1 M = 1 mol dm^-3).No deuterium isotope effect on the rate.The rate-determining steps seem to correspond to the formation of intermediates containing a one-ended acac^- and an Hacac molecule (the k_a path), and the anion and a water molecule (the k_b path), by the S_N2 mechanism.