A Nuclear Magnetic Resonance Study on Ligand Exchange Reactions in U(VI) Nitrato Complexes with Ligands Containing Phosphoryl Group in Non-aqueous Solvents

Abstract

The exchange reactions of tributyl phosphate (TBP) and dihexyl-N, N-diethylcarbamoylmethylphosphonate (CMP) in U(VI) nitrato complexes with TBP or CMP (abbreviated as U(VI)-TBP or U(VI)-CMP) have been studied by means of 31P NMR method in CD3COCD3 or CD2Cl2. The number of TBP coordinated to uranyl nitrate was directly determined to be 1 in CD3COCD3 and 2 in CD2Cl2 by the area integrations of 31P NMR signals of free and coordinated TBP. Similarly, the number of coordinated CMP was evaluated as 1 in CD3COCD3. The exchange rate constants of TBP and CMP in U(VI)-TBP and U(VI)-CMP complexes were calculated by the two-site exchange model. The exchange rate of TBP in U(VI)-TBP is independent of the free TBP concentrations ([TBP]f), while that in U(VI)-CMP is dependent on [CMP]f. From these results, the dissociative (D) and interchange (I) mechanisms were proposed for the exchange reactions of TBP in U(VI)-TBP and CMP in U(VI)-CMP, respectively. Kinetic parameters are as follows: rate constant (25°C), ΔH# (kJ·mo1−1), ΔS# (J·K−1·mol−1); 9.9×103s−1, 33.7, −55.4 (U(VI)-TBP system), 2.3×104 m−1·s−1 (apparent second-order), 25.2, −75.9 (U(VI)-CMP system).