Kinetics and Mechanism of the Substitution of β-Diketonates for Acetylacetonate in Bis(acetylacetonato)(trimethyl phosphate)dioxouranium(VI)

Abstract

Abstract Kinetics of the substitution of β-diketonates for one acetylacetonate in UO2(acac)2tmp (acac=acetylacetonate, tmp=trimethyl phosphate) has been studied in tmp by spectrophotometric methods. The reactions proceed through intermediate complexes UO2(acac)2HB, where HB denotes various β-diketones. The formation of the intermediates are followed by intramolecular proton transfer processes from coordinated HB to one of two acac in UO2(acac)2HB. The proton-transfer step for various HB can be classified into two groups owing to the difference in the values of rate constants k2, i.e. the substitutions of Hbza, Hhpd, and Hdbm and those of Hbta, Htta, and Htfa, k2 of the former group giving the order of 10−3 s−1, while that of the latter the order of 10−1 s−1, at 25 °C, where Hbza=benzoylacetone, Hhpd=3,5-heptanedione, Hdbm=dibezoylmethane, Hbta=benzoyltrifluoroacetone, Htta=2-thenoyltrifluoroacetone, and Htfa=trifluoroacetylacetone. This can be well explained in view of the acidity of β-diketones. The addition of amphiprotic solvents, such as water and alcohols, accelerates the substitution rates, and the observed first-order rate constant kobsd is expressed by kobsd=k2+kc[solvent]. In the case of the addition of alcohols, kc increases in the order: n-BuOH<i-PrOH<n-PrOH<EtOH<MeOH. This is the reverse order of autoprotolysis constants of these alcohols. Kinetic isotope effects kcH/kcD for the substitution of Hbza in tmp in the presence of H2O (or D2O) and CH3OH (or CH3OD) were determined to be 2.5 and 1.8, respectively.