In search for mononuclear helical lanthanide building blocks with predetermined properties: Lanthanide complexes with diethyl pyridine-2,6-dicarboxylate

Article abstract of DOI:10.1002/chem.19970031015

The ligand diethyl pyridine-2,6-dicarboxylate (L⁵) reacts with Ln(III) in acetonitrile to successively give the complexes [Ln(L⁵)'1]³⁺ (Ln = La to Lu, i =1-3). Spectroscopic investigations (ES-MS, UV/Vis, NMR) show that the 1:3 complexes [Ln(L⁵)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(L⁵)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(L⁵)₂](TfO)₃ · nH₂O have been prepared. The X-ray crystal structure of [Eu(L⁵)₂(TfO)₂(OH₂)]TfO (1) reveals two meridionally tricoordinated ligands L⁵, 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.