A Schiff-Base Bibracchial Lariat Ether Forming a Cryptand-like Cavity for Lanthanide Ions

Abstract

We report here a structural and photophysical study of lanthanide(III) complexes with the di-deprotonated form of the bibracchial lariat ether N,N‘-bis(2-salicylaldiminobenzyl)-1,10-diaza-15-crown-5. The X-ray crystal structures of [Ce(L2-2H)](ClO4)·0.5H2O (2) and [Sm(L2-2H)](ClO4)·C3H8O (5b) show the metal ion being nine-coordinated and deeply buried in the cavity of the dianionic receptor. Thanks to the formation of a pseudomacrocycle through π−π interaction between one of the phenol rings and one of the benzyl rings, the complexes present a cryptand-like structure in the solid state. 1H and 13C NMR studies on the La(III) complex point that the solid state structure is essentially maintained in acetonitrile solution. High-resolution laser-excited emission spectra of the crystalline Eu(III) complex demonstrate the presence of several coordination sites arising from different conformations of the crown moiety. The ligand-to-Eu(III) energy transfer is relatively efficient at low temperature, but back transfer is implied in the deactivation process, especially at room temperature, because the ligand triplet state lies at very low energy. However, the low energy of the 3ππ* state provides an efficient conversion of the visible light absorbed into near-infrared light emitted by the Nd(III) ion.