Lanthanide Complexes with Tetradentate N,N′,O,O′-Dipyridyl-Based Ligands: Structure, Stability, and Photophysical Properties



A series of lanthanide(III) complexes (Ln = La to Lu) with 2,2′-bipyridyl-6,6′-dicarboxamide ligands were obtained. The X-ray structure of the di(N-ethylanilide) of 2,2′-bipyridyl-6,6′-dicarboxylic acid is reported. The structures of the amides in the gas phase were modeled by DFT calculations. The global minima of the structures on the potential energy surface (PES) strongly depend on the nature of the substituents on the amidic nitrogen atom. The UV/Vis titration results of three diamides of 2,2′-bipyridyl-6,6′-dicarboxylic acid with lanthanide(III) metal nitrates show the composition of the complexes formed and allow us to determine the binding constants. Compounds with a 1:1 metal-to-ligand ratio are formed with the three diamide ligands under study. The binding constants are high, log β1 being higher than 5.5. The values of the constants decrease from LaIII to LuIII. The first structures, obtained by single-crystal X-ray diffraction, for GdIII and TbIII complexes of 2,2′-bipyridyl-6,6′-dicarboxamide ligands are described. Decacoordinated metal ions are bonded by the tetradentate ligand and three bidentate nitrate groups. 2D ROESY experiments with LaIII, LuIII, and EuIII complexes allow us to conclude that the structures of the complexes in solution are the same as those in the solid state. The photophysical properties of these Ln complexes were determined in the solid state at 77 and 300 K. Intense red and green luminescence was observed for both Eu and Tb complexes, and an intrinsic quantum yield of 90 % was determined for the europium complex.