Solvent extraction and lanthanide complexation studies with new terdentate ligands containing two 1,3,5-triazine moieties

Abstract

The extracting agent 2,6-bis(4,6-di-pivaloylamino-1,3,5-triazin-2-yl)-pyridine (L⁵) in n-octanol was found, in synergy with 2-bromodecanoic acid, to give D_Am/D_Eu separation factors (SFs) between 2.4 and 3.7 when used to extract the metal ions from 0.02–0.12 M HNO₃. Slightly higher SFs (4–6) were obtained in the absence of the synergist when the ligand was used to extract Am(III) and Eu(III) from 0.98 M HNO₃. In order to investigate the possible nature of the extracted species crystal structures of L⁵ and the complex formed between Yb(III) with 2,6-bis(4,6-di-amino-1,3,5-triazin-2-yl)-pyridine (L⁴) were also determined. The structure of L⁵ shows 3 methanol solvent molecules all of which form 2 or 3 hydrogen bonds with triazine nitrogen atoms, amide nitrogen or oxygen atoms, or pyridine nitrogen atoms. However, L⁵ is relatively unstable in metal complexation reactions and loses amide groups to form the parent tetramine L⁴. The crystal structure of Yb(L⁴)(NO₃)₃ shows ytterbium in a 9-coordinate environment being bonded to three donor atoms of the ligand and three bidentate nitrate ions. The solvent extraction properties of L⁴ and L⁵ are far inferior to those found for the 2,6-bis-(1,2,4-triazin-3-yl)-pyridines (L¹) which have SF values of ca. 140 and theoretical calculations have been made to compare the electronic properties of the ligands. The electronic charge distribution in L⁴ and L⁵ is similar to that found in other terdentate ligands such as terpyridine which have equally poor extraction properties and suggests that the unique properties of L¹ evolve from the presence of two adjacent nitrogen atoms in the triazine rings.