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

Size-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-<bold>b</bold>:7 ',8 '-<bold>j</bold>][1,10]phenanthroline) is investigated, Formation constants (log K-1) 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 K-1 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 K-1 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 K-1 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.