Probing organic phase ligand exchange kinetics of 4f/5f solvent extraction systems with NMR spectroscopy

• NMR spectroscopy improves understanding of ligand exchange reactions in lanthanide-actinide solvent extraction systems.
• Limited information is available in the current chemical literature describing NMR application to solvent extraction systems.
• The exchange rate generally reflects the ligand affinity for the cations – labile complexes correlate with lower thermodynamic stability.
• Associative, dissociative, and interchange mechanisms are operable in f-element solvent extraction systems.
• NMR ligand exchange dynamics studies of solvent extraction system relevant to nuclear fuel reprocessing promise important insights.

In solvent extraction separations, which are integral to lanthanide purification or the recycling of actinide-containing used nuclear fuel, profound changes in cation coordination environments are seen as the aquated ion is transferred from the aqueous medium through the biphasic (aqueous–organic) interface to form the hydrophobic, electroneutral extracted complex. The design and execution of industrial-scale solvent extraction processes is often dictated by the rate and mechanism(s) of the reactions that control this transformation. Depending on the metal ion and the lipophilic complexing agent (extractant), the rate limiting process can occur in any of the three media (aqueous, organic, or interface). This review focuses on the analysis and interpretation of literature reports of studies of the rate and mechanism of ligand exchange reactions in hydrated organic solutions that are relevant to the separations of lanthanide and actinide metal ions. Nuclear magnetic resonance (NMR) spectroscopy has been the principal experimental technique of choice. NMR spectroscopy has been employed (and previously reviewed) in studies of small solvent molecule exchange reactions (H2O, DMF, DMSO). Similar NMR investigations of the stronger, often multidentate, and definitively bulkier ligands used in organic phases of solvent extraction systems are far less common. Both binary (M:L) and ternary (M:X:L) systems have been investigated. The coordination mode of the ligand and the presence of a secondary ligand in the first or second coordination sphere often complicate the assignment of a mechanism. Interestingly, in these systems activation parameters have not always provided definitive guidance in assigning mechanistic details. In the following, results from diverse reports from a number of investigations are correlated and compared to attempt to bring some order to the diverse coordination chemistry of f-element complexes in lipophilic organic media relevant to solvent extraction.