Kinetics and mechanism of ligand exchange in tetrakis(acetylacetonato)zirconium(IV) in organic solvents

Abstract

The kinetics of ligand exchange between [Zr(acac)₄](acac = acetylacetonate) and free acetylacetone (Hacac) in various organic solvents such as CDCl₃, C₆D₆ and CD₃CN has been studied by the ¹H NMR line-broadening method. The observed first-order rate constant k_obs for acac exchange in CDCl₃ and C₆D₆ is expressed as a function of concentration of Hacac in the enol form: k_obs=(k₂+k_3,HA)K₁[Hacac]_enol/(1 +K₁[Hacac]_enol), where K₁ is the equilibrium constant for the formation of the nine-co-ordinate adduct [Zr(acac)₄(Hacac)]. The rate constants k₂ and k_3,HA correspond to those of proton transfer from co-ordinated Hacac to leaving acac and ring opening of acac in the adduct, respectively. Activation parameters ΔH^‡/kJ mol^–1 and ΔS^‡/J K^–1 mol^–1 for the k₂+k_3,HA path are 33.7 ± 2.3, –87.8 ± 8.1 in CDCl₃ and 38.1 ± 0.1, –85.9 ± 0.4 in C₆D₆. The rate constants k_obs in CD₃CN are much smaller than those in CDCl₃ and C₆D₆, and increase linearly with increasing [Hacac]_enol. Addition of water and dimethyl sulfoxide to C₆D₆ solutions results in acceleration and retardation of the exchange rate, respectively. The deuterium kinetic isotope effect was observed by using [²H₂]acetylacetone in place of Hacac in C₆D₆. The results are discussed in connection with those of acac exchange in other [M(acac)₄] complexes (M = Hf⁴⁺, Ce⁴⁺, Th⁴⁺ or U⁴⁺).