Effect of Heterocyclic Ring on Ln(III) Coordination, Luminescence and Extraction of Diamides of 2,2′-Bipyridyl-6,6′-Dicarboxylic Acid

We have synthesized and examined several complexes of lanthanides with diamides of 2,2′-bipyridyl-6,6′-dicarboxylic acid bearing various hetaryl-based side chains for the elucidation of the effect of the heterocycle on the structure and properties of the ligands. The multigram scale methods for the preparation of various N-alkyl-hetaryls and their diamides were developed. The solid state structure of 6-methyl-2-pyridylamide of 2,2′-bipyridyl-6,6′-dicarboxylic acid possesses a flat structure where the conformation is completely different from that previously observed for N-alkylated 2,2′-bipyridyl-6,6′-dicarboxamides and 2,6-pyridinedicarboxamides. The complexes of new ligands were synthesized and NMR and X-Ray studied their structure in solution and solid state. The results demonstrate that complexes possess the same structures both in solid state and in solution. Stability constants of the complexes were less when comparing with dimethyl-substituted diamides, but higher than for unsubstituted dianilide. Contrarily, the extraction ability of 2-pyridyl-diamide is significantly lower than for corresponding anilide. Specific interaction of extractant with solvent molecules, which is not available for electron-sink pyridine amides, can explain this. The luminescence of new Eu complexes was significantly higher than for all previously 2,2′-bipyridyl-6,6′-dicarboxamides and QY reaches 18%. Asymmetry ratios of Eu complexes were 25% higher when compared other complexes with 2,2′-bipyridyl-6,6′-dicarboxamides, which indicates large deviation from the inversion center.