Kinetics and Mechanism of Ligand Exchange in Tetrakis(acetylacetonato)zirconium(IV) in Organic Solvents

Article abstract of DOI:10.1039/DT9950001077

The kinetics of ligand exchange between (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)K_1enol/(1 + K_1enol), where K₁ is the equilibrium constant for the formation of the nine-co-ordinate adduct .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(excit.)/kJ mol^-1 and ΔS(excit.)/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 _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 <(2)H2>acetylacetone in place of Hacac in C6D6.The results are discussed in connection with those of acac exchange in other complexes (M = Hf(4+), Ce(4+), Th(4+) or U(4+)).

Full text of DOI:10.1039/DT9950001077

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