KINETICS AND MECHANISM OF LIGAND-EXCHANGE IN TETRAKIS(ACETYLACETONATO)ZIRCONIUM(IV) IN ORGANIC-SOLVENTS

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 H-1 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) = (k(2) + k(3,HA))K-1 [Hacac](enol)/(1 + K-1[Hacac](enol)), where K-1 is the equilibrium constant for the formation of the nine-co-ordinate adduct [Zr(acac)(4)(Hacac)]. The rate constants k(2) 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 Delta H-double dagger/kJ mol(-1) and Delta S-double dagger/J K-1 mol(-1) for the k(2) + 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 [H-2(2)]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+).