In Search for Mononuclear Helical Lanthanide Building Blocks with Predetermined Properties: Lanthanide Complexes with Diethyl Pyridine‐2, 6‐Dicarboxylate

The ligand diethyl pyridine‐2, 6‐dicarboxylate ( L 5 ) reacts with Ln III in acetonitrile to successively give the complexes [Ln( L 5 ) i ] 3 + (Ln = La to Lu, i = 1‐3). Spectroscopic investigations (ES‐MS, UV/Vis, NMR) show that the 1:3 complexes [Ln( L 5 ) 3 ] 3 + have poor stability in solution and exist as a mixture of rapidly interconverting conformers. Variable‐temperature NMR data show that the helical P→M interconversion and dynamic on‐off equilibria of the ester side arms both control the observed average structure in solution. Contrary to similar lanthanide building blocks possessing benzimidazole or carboxamide side arms, [Eu( L 5 ) 3 ] 3 + has a sizable quantum yield in anhydrous acetonitrile; this has been attributed to an improved ligand → Eu III energy transfer resulting from a good energetic match between the ligand‐ and metal‐centered excited states. Pure 1:3 complexes cannot be isolated in the solid state, but crystalline 1:2 complexes [Ln( L 5 ) 2 ](TfO) 3 . n H 2 O have been prepared. The X‐ray crystal structure of [Eu( L 5 ) 2 (TfO) 2 (OH 2 )]TfO ( 1 ) reveals two meridionally tricoordinated ligands L 5 , but the long Eu‐O(ester) bonds imply only weak interactions between the carbonyl groups of the ester side arms and Eu III , providing a limited protection of the metallic site. The photophysical studies show that nonacoordinate Eu III in 1 binds an additional water molecule to give a decacoordinate complex in the solid state, thus confirming the accessibility of the metallic site for further complexation.