The lanthanide complexes of 2,2′-bipyridyl-6,6′-dicarboxylic dimethylanilides: the influence of a secondary coordination sphere on the stability, structure, luminescence and f-element extraction†
Four of the six possible isomeric 2,2′-bipyridyl-6,6′-dicarboxylic dimethylanilides were studied from the point of view of the impact of a secondary coordination sphere on the formation of complexes with lanthanides in solution, as well as the crystal structure and photophysical properties of the complexes. All ligands form complexes with a 1 : 1 metal-to-ligand ratio with an lg β1 in the range of 6.0–8.8, and strong correlations between secondary coordination sphere modulation and stability of the complexes within the lanthanide series. Although substitution at the o-position of the aromatic ring leads to significant elongation of M–OL bonds in a crystal, this significantly affects the stability of the complexes. The luminescence of the complexes is the most effective for europium complexes. From luminescence measurements of gadolinium complexes, the triplet energy levels of ligands were located as follows: o-methylated ligands show 10% higher levels than other isomers. Also, o-methylation of the phenyl ring increases the lifetime value while m-methylation reduces this value.