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 reversible arrow 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) . nH(2)O have been prepared, The X-ray crystal structure of [Eu(L-5)(2)(TfO)(2)(OH2)]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.