A maximally preorganized tetra‐pyridyl ligand. Rigid five‐membered chelate rings enhance selectivity for large metal ions, such as Th(IV), UO22+ and Lanthanides. A DFT and Thermodynamic study

Abstract

AbstractSize‐based selectivity for metal ions based on highly preorganized five‐membered chelate rings is discussed. Metal ion complexation by the tetra‐pyridyl ligand EBIP ((8,9‐dihydro‐diquino[8,7‐b:7′,8′‐j][1,10]phenanthroline) is investigated, Formation constants (log K1) are reported for EBIP with 28 metal ions in 50 % CH3OH/H2O (v/v). The shift in size‐selectivity toward large metal ions and against small metal is demonstrated. Log K1 for the EBIP complexes shows a steady increase from La(III) to Lu(III), with a strong local maximum at Sm(III), and strong local minimum at Gd(III). This difference in log K1 between Sm(III) and Gd(III) for the tetra‐pyridyls is shown to depend largely on the level of preorganization of the ligand, being at a maximum for EBIP and a minimum for quaterpyridine. Log K1 for the Y(III) complex is invariably lower than for the similarly‐sized Ho(III) for all ligands that contain any nitrogen donors. Lower log K values for Y(III) are due to stabilization of the Ln(III) complexes with nitrogen donors by participation of the 5d orbitals, and to a lesser extent the 4 f orbitals, of the Ln(III) ions in M−L bonding. A DFT analysis of selectivity of tetra‐pyridyls for metal ions shows that Y(III) complexes should be less stable than similarly‐sized Ho(III) complexes.