Catalytic and Photoluminescence Properties of the First- and Second-Sphere Coordination of Lanthanide Complexes

The efficient binding of receptors Lk (Lk = L5-L8) to [Eu(hfac)₃] (H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) allows (i) to gather the basic unit-containing tridentate ligands Lk and the acidic unit-bearing bidentate hfac^- co-ligands into a stable single molecular [LkEu(hfac)₃] adduct working as a catalyst for Michael C─C bond formations, (ii) to enhance the acidity of the N-H units connected to the benzimidazole side arms of the bound ligands L5 and L7, and (iii) to reorganize their geometries into cis-cis conformation for activating Michael substrates 1 and 2 through H-bonding interactions on the second-sphere coordination to provide the target product 3 (up to 92% yield). A maximum enantiomeric excess of 21% could be achieved due to the long distance between the chiral sources and the catalytic sites in the europium complexes. Stepwise distortions from planarity of the terdentate Lk ligands in the yttrium complexes [LkY(hfac)₃] (Lk = L2, L3, and L5-L10) can be tuned by addition of specific substituent into the benzimidazole side arms, thus realizing some control over their dual fluorescence and phosphorescence emission properties. All [LkEu(hfac)₃] (Lk = L2, L3, and L5-L10) complexes display europium-centered photoluminescence properties induced by the antenna effect and modulated by the specific design of the light-harvesting aromatic ligand located in the first coordination sphere.