Activating the Aromatic Core of the Water-soluble Complexing Agent PTD

Several solvent extraction processes for separating actinides from irradiated nuclear fuel developed in European research programs make use of 3,3',3'',3'''-(pyridine-2,6-diylbis(1,2,4-triazine-3,5,6- triyl))tetra-benzenesulfonate (SO₃-Ph-BTP) to selectively strip actinides from organic phases loaded with actinides and lanthanides. As a 'CHON' compliant alternative to SO₃-Ph-BTP, 3,3'-(pyridine-2,6- diylbis(¹H-1,2,3-triazole-4,1-diyl))bis-(propan-1-ol) (PTD) was developed. Although performing satisfactorily for selectively stripping actinides, the stability constant of the [Cm(PTD)₃]³⁺ complex is lower by more than one order of magnitude compared to the [Cm(SO₃-Ph-BTP)₃]³⁺ complex, demonstrating its weaker complexation strength. Consequently, PTD-OMe was synthesized, bearing a methoxy group on the central pyridine ring to increase its basicity and hence complexation strength. Time-resolved laser fluorescence spectroscopy (TRLFS) confirms the ligands improved complexation properties (logβ₃(PTD-OMe) = 10.8 ± 0.4 vs. logβ₃(PTD) = 9.9 ± 0.5). However, PTD-OMe proves to be less efficient than PTD in solvent extraction experiments. This is explained by the increased basicity of the ligand (pKa(PTD-OMe) = 2.54 ± 0.08 vs. pKa(PTD) = 2.1). This increased basicity leads to a reduced free ligand concentration under solvent extraction conditions (0.44 mol/L HNO₃), explaining the inferior performance compared to PTD. Both DFT calculations and NMR measurements confirm that protonation occurs at the pyridine nitrogen atom. The lower selectivity of PTD-OMe compared to PTD is explained by the unfavourable changes in polarizabilities of the nitrogen atoms. This work comprehensively demonstrates how activating the aromatic core of a ligand benefits its complexation properties. However, solvent extraction performance is compromised due to more pronounced ligand protonation if the pH of the system is smaller than the pKa value of the ligand. (authors)