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).

KEYWORDS:

• dioxouranium nitrate
• TBP
• CMP
• ligand exchange reaction
• reaction kinetics
• nuclear magnetic resonance
• concentration dependence
• nonaqueous solvente