Add time:08/21/2019         Source:sciencedirect.com

The kinetics of the ligand exchange reaction between tetrakis(acetylacetonato)hafnium(IV), Hf(acac)4, and free acetylacetone (Hacac) in C6D6 have been studied by the 1H NMR line-broadening method. The observed first- order rate constant kobs is expressed as a function of [Hacac]enol: kobs = (k2 + k3, HA,)K1, HA[Hacac]enol/ (1 + K1,HA[Hacac]enol), where [Hacac]enol signifies the concentration of Hacac in the enol form and K1, HA is the equilibrium constant for the formation of a nine-coordinate adduct complex Hf(acac)4, Hacac. The rate constants k2 and k3, HA correspond to those of proton transfer from coordinated Hacac to leaving acac and the ring opening of acac in the adduct complex, respectively. The existence of the k3, HA path was evidenced by the deuterium isotope effect on the rate. Activation parameters for the (k2+k3, HA) path were obtained from the temperature dependence of the (k2+k3, HA) value: ΔH≠=41±2 kJ mol−1 and ΔS≠=−81±5 J K−1 mol−1 The exchange reaction was accelerated by addition of H2O to the sample solution. These results are discussed in connection with those of the acac exchange in analogous quadrivalent metal complexes.

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