A nuclear magnetic resonance study of the kinetics of ligand exchange reactions in uranyl complexes. VII. Acetylacetonate exchange in bis(acetylacetonato)( N,N-dimethylformamide)dioxouranium(VI)

Abstract

The exchange reaction of acac(acetylacetonate) in UO 2(acac) 2dmf (dmf= N,N-dimethylformamide) in o-dichlorobenzene has been studied by the NMR line-broadening method. The exchange rate depends on the concentration of the enol isomer of acetylacetone in its low region, and approaches the limiting value in its high region. It is proposed that the exchange reaction proceeds through the mechanism in which the dissociation of one end of the chelated acac is the rate-determining step. The kinetic parameters for this step are as follows: k (25 °C)=5.03 × 10 −3 s −1, Δ H‡=91.6 ± 3.8 kJ mol −1, and Δ S‡ =17.2 ± 10.5 JK −1 mol −1. The exchange rate becomes slower by the addition of free DMF. This may be due to the competition of DMF with the enol isomer of acetylacetone in attacking a four-coordinated intermediate in the equatorial plane.